Written by Sig Silber
Updated at 7:56 pm EDT April 16 to reflect the new Week 3 – 4 weather forecast used to do our visual conistency test on the Early Outlook forecast for May
Here is the April 15, 2021, NOAA four-season forecast for CONUS and Alaska including the Early Outlook for May 2021. It covers a period that begins one month later than the forecast last month and extends one month longer. Also included is the updated three-month drought forecast. In two or three days we will publish the comparison with the JAMSTEC forecast. There is a lot of uncertainty about next Fall and Winter. Next month I think they change the definition of climatology (normal) and this will make the temperature maps look considerably different.
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A. Focus on the NOAA Update
A note about terminology; the deviations from climatology/normal are color-coded but also labeled “A” for more than (above) normal and “B” for less than (below) normal. The area designated EC means Equal Chances of being more or less than normal. In my comments for the shorter-term maps, NOAA uses “N” instead of “EC” as they believe they can be more definitive for shorter time frames. So the words “warm”, “cool”, “wet”, “dry” should be generally interpreted as being relative to climatology/normal for that location and time of year.
First, we will take a look at the NOAA Early Outlook for May 2021. It is called the Early Outlook because it will be updated at the end of May. Only the May Outlook will be updated at that time.
Temperature
Precipitation
We have nothing to compare these maps with as NOAA does not provide in their Update a forecast for each of the subsequent two months separate from their three-month forecast. Thus I do not have forecast maps for May from the previous NOAA Report to compare against. And the current month is not over so we can not really compare the forecast for next month against the actual for this month. It is probably best to just try to understand what NOAA is trying to convey about May*. For Temperature, CONUS is warm except for the Northern Tier which is mostly EC. Most of Alaska including the Panhandle is cool with the Western Aleutians and Northern Alaska being warm. With respect to precipitation, much of CONUS is EC with a large dry anomaly centered on New Mexico and Texas and a large eastern wet anomaly that does not include the Northeast. Western Alaska is dry and Eastern Alaska and the Panhandle are wet.
* compared to normal/climatology for the indicated time of the year.
Visual Consistency Testing.
It is useful to see how the month forecast fits with the set of shorter forecasts that we have. The first two maps cover only April with the second map extending to April 29, 2021, and the third map as of today covers through May 7. Thus only the third of the three maps is at all useful for this purpose. The Seasonal Outlook was issued early this month since the third Thursday fell on April 15. The earliest possible issue of the forecast is on the 15th of the month. We are publishing on a Thursday so we do not have the updated Week 3-4 forecast. 7 days is not an adequate basis for doing this visual consistency testing but it is worth looking at. There are 31 days in May.
We have now updated this article and now have 14 days of short-term forecasts which is marginally adequate for our purposes.
First Temperature
And then Precipitation
Sometimes it is useful to look at the Week 3-4 Discussion. We will add that when it is issued Friday afternoon April 16, 2021.
La Nina conditions continued but weakened across the Pacific Ocean. Equatorial sea surface temperatures (SSTs) have trended closer to average across the west-central equatorial Pacific Ocean, but enhanced upper level westerly wind anomalies persist across the eastern tropical Pacific. The RMM index depicts an eastward propagation of the MJO over the West Pacific during mid-April. Dynamical models continue to favor the eastward propagation of the MJO over the Pacific and the Western Hemisphere during the remainder of April. The Week 3-4 temperature and precipitation outlooks are based primarily on dynamical model forecasts from the NCEP CFS, ECMWF, and JMA, and the Subseasonal Experiment (SubX) multi-model ensemble (MME) of experimental and operational ensemble prediction systems with additional considerations for MJO, La Nina, and long-term trends, as well as the predicted evolution of the pattern from Week-2 forecasts.
The forecast circulations are reasonably consistent among model guidance regarding the anomalous 500-hPa height patterns over the week 3-4 period. Dynamical model 500hPa height anomaly forecasts during week 3-4 show a fairly consistent evolution from the forecast state during Week-2. Most dynamical models feature anomalous ridging with above normal 500-hPa heights over the West Coast and southwest CONUS, while anomalous trouging is forecast across the north-central CONUS. Above normal 500-hPa heights are predicted over most of Mainland Alaska. Near normal 500-hpa heights are likely over Hawaii.
Below normal temperatures are favored over parts of the Northern Rockies, the Northern Plains and the Upper Mississippi Valley due to below normal 500-hpa heights. Above normal temperatures are more likely across the remainder of the CONUS, under predicted above normal 500-hPa heights. The highest confidence for above-normal temperatures lies across Arizona and New Mexico (greater than 60%) tied to the forecast position of the anomalous 500-hPa ridge axis. Predicted ridging and above normal 500-hPa heights lead to above normal temperatures favored over Alaska.
The dynamical model guidance is in reasonably good agreement on the spatial pattern of anomalous precipitation during the Week 3-4 period. Near to below normal precipitation is favored throughout most of the western CONUS, under the near to above normal 500-hPa heights, while predicted troughing and below normal 500-hPa heights increase the chances of above normal precipitation over the Great Lakes, Ohio Valley, and Mid-Atlantic, consistent with dynamical and statistical guidance. Anomalous ridging and above normal 500-hPa heights lead to enhanced probabilities of below normal precipitation over Alaska.
Most dynamical model guidance, including the experimental SubX models, depict enhanced chances for above normal temperatures for Hawaii. Dynamical model precipitation forecasts generally favor below normal precipitation over the southeastern Hawaiian islands.
Now we consider the three-month Outlook.
Notice that the three-month periods are abbreviated e.g May/June/July is shown as MJJ. You will see such abbreviations often in this report.
Prior Temperature Outlook for MJJ 2021
New Temperature Outlook for MJJ 2021
Now Precipitation.
Prior forecast for MJJ 2021
New Precipitation Outlook for MJJ 2021
I can make the comparison easier by showing the prior forecast and the current forecast side by side.
Prior Forecast | New Forecast | |
Temperature | ||
Precipitation |
Now let us focus on the long-term situation.
First Temperature
Prior 14 Month Temperature Outlook: MJJ 2021 – AMJ 2022
New 14 Month Temperature Outlook: JJA 2021 – MJJ 2022
To compare maps from one release to another, one needs to remember that the new release drops one three-month period and adds a later one. So to make the comparisons one has to shift the new maps to the right one position and that makes the map on the right drop-down to become the left-most map in the next level. I do not have a computer software tool for doing that for you so you have to do it mentally. When I do the comparison, I print the two sets of maps and put them side by side and number the same three-month maps 1, 2, 3,…..,11 in both sets of maps to make it easier for me to easily compare the same three-month period in the new with the previous forecast.
I am now showing them side by side in addition to one after another. It is easier to make the comparison this way but to do it I have to make the maps smaller and readers have to remember how to compare two sets of maps when one starts a month later than the other. Here goes:
Forecast Issued Last Month | Forecast Issued This Month |
Based on this procedure, I conclude that:
Now Precipitation
Prior 14 Month Precipitation Outlook: MJJ 2021 – AMJ 2022
New 14 Month Precipitation Outlook: JJA 2021 – MJJ 2022
Because there are a few significant changes, I am also going to show the two graphics side by side. It is tricky because the array for the current month starts one month later. So you have to follow the instructions I provided with the temperature array of maps. So you can compare the forecast maps by looking at the large ones which are shown with the prior month forecast followed by this month forecast or you can look at them side by side.
Last Month | This Month |
If you want larger versions of each newly issued map (temperature and precipitation) you can find them here. And then each of those maps can be clicked on to further enlarge them.
Sometimes it is useful to compare the three-month outlook to the forecast for the first of the three months. It shows how much the pattern changes over the three months.
One can mentally subtract the First-Month Outlook from the Three-month Outlook and create the Outlook for the last two months in the three months.
*The concept is that the probabilities of a deviation from climatology in the First Month and the combined Month Two-Three forecast that one derives must average out to the probabilities shown in the three-month maps.
Below is the NOAA Discussion (slightly reorganized) released by NOAA on April 15, 2021. Headings that are “Initial Cap” only rather than all caps were added by the Author of this Update Report for clarity. Also, we have organized the sequence of the sections of NOAA Discussion to first present the Atmospheric and Oceanic Conditions and then the Initial Month (May), the NOAA Summary for MJJ, and finally the remainder of the 15-Month Forecast.
CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS
La Nina conditions continued but weakened during the last month in the equatorial Pacific Ocean as shown by both oceanic and atmospheric indicators. Equatorial sea surface temperature (SST) anomalies became less negative over the central equatorial Pacific during the last month. The most recent weekly value of the Nino3.4 SST index is -0.5 degrees C. SSTs and atmospheric conditions are likely to be modulated by an ongoing MJO event over the next few weeks. Below the surface, the depth and magnitude of negative ocean temperature anomalies decreased and currently extend in a shallow layer to a depth of 50 meters from 175 E to 110 W. Positive subsurface oceanic heat content anomalies are present below 50 meters to depths of 200 meters or more and have expanded eastward in the western and east-central Pacific Ocean.
Suppressed convection persists across the equatorial Pacific surrounding the Date Line, while enhanced convection weakened over the western Pacific but remains over parts of Indonesia and the Philippines. Recent monthly average 850-hPa winds continue to show enhanced easterly trade winds across the western and east-central equatorial Pacific. Westerly wind anomalies continued at 200-hPa over much of the equatorial Pacific Ocean. These ocean and atmosphere indicators show the coupled ocean-atmosphere system is consistent with weakening La Nina conditions.
PROGNOSTIC DISCUSSION OF SST FORECASTS
Forecasts of the Nino3.4 SST index from the NMME show considerable spread in ensemble members both across the modeling center forecasts and within each modeling center ensemble by the start of summer. Most guidance favors a continued decrease in the magnitude of negative SST anomalies over the next three months, and the official consensus calls for ENSO-neutral with a probability of 80 percent. Forecasts begin to diverge during the summer months with some guidance favoring a decrease in Nino3.4 sea surface temperatures, suggesting a return to La Nina conditions by autumn 2021. The CA and Markov statistical models in the CPC Nino3.4 SST consolidated forecast indicate a trajectory towards ENSO-neutral in the autumn, while the CCA predicts a steady rise in Nino3.4 SST towards El Nino conditions over the summer and autumn months.
30-DAY OUTLOOK DISCUSSION FOR MAY 2021
The May 2021 outlooks are issued against the backdrop of a weakening La Nina and an ongoing strong MJO event. The Nino 3.4 index currently stands at -0.5C, which represents a significant weakening relative to the value during much of the past winter when the average index value was near -1.0C. The oceanic La Nina signal is generally forecast to weaken further in May and ENSO Neutral conditions are expected to become the most likely scenario as we progress deeper into Spring. Weakening La Ninas of this magnitude and evolution tend to result in below normal heights at high latitudes during the month of May, including most of Alaska. Farther to the south, above normal heights typically dominate most of the CONUS under such conditions. However, complicating matters is an ongoing strong MJO event in the Western Pacific which would tend to counteract this ENSO signal across much of the forecast domain. Specifically, composites from strong (defined in this case as a Wheeler-Hendon Index amplitude greater than 1.0) Western Pacific MJO events occurring this time of year indicate increased troughing across much of the western CONUS and less troughing across Alaska during the month of May relative to the underlying ENSO signal. It should be noted, however, that the sample size of these MJO composites is relatively small, thus, reducing the reliability of the signal. Analysis of the most recent monthly and subseasonal dynamical model guidance, recent trends , as well as contributions from current environmental conditions, including soil moisture, snow cover and depth, SSTs, and sea ice extent, were also utilized in the production of these outlooks. These additional inputs were collectively used to resolve the competing background ENSO and MJO signals .
Temperature
Above normal temperatures are favored for the southern two-thirds of the CONUS and for much of the Northeast consistent with ENSO composites, trends , and dynamical model guidance. However, the ongoing MJO event would temper probabilities, resulting in generally weaker above normal temperature probabilities from the Rockies westward. Probabilities of warmer than normal conditions are the greatest (exceeding 60 percent) across the Southern Plains where warm signals from ENSO composites, dynamical model guidance, soil moisture, and trends converge with the greatest confidence. Confidence in above normal temperatures diminish farther to the north as the current strong MJO event supports a cold signal across much of the northern tier, which also coincides with areas where recent trends toward warmer than normal conditions are at its weakest or absent altogether. This cold MJO signal directly conflicts with dynamical model guidance, which favors above normal temperatures across almost the entire CONUS. For these reasons, equal chances (or EC) of above, near, or below normal temperatures are indicated across most of the northern tier of the CONUS. Enhanced probabilities of above normal temperatures are also absent for much of Alaska. Deep snow cover, lingering sea ice, and cold ENSO signals will likely combine to counteract recent trends toward above normal temperatures across much of the state. Enhanced probabilities of below normal temperatures are indicated for much of southern mainland Alaska and the Alaska Panhandle where cold signals from increased snow depth, sea ice, ENSO, and dynamical model guidance from the NMME, C3S, and ECMWF converge. Above normal temperatures are favored for parts of northern Alaska where trends are the strongest. Above normal temperatures are also favored for the Aleutians where SSTs are near to above normal and trends tilt toward warmer than normal conditions.
Precipitation
The potential for enhanced north to south temperature gradients is likely to promote an active pattern across parts of the central and eastern CONUS. Specifically, dynamical model guidance from the CFSv2, combined ENSO/MJO composites, and trends support a wet signal stretching from parts of the Corn Belt eastward across the Ohio Valley to the Mid-Atlantic. Therefore, above normal precipitation is favored for these areas. Enhanced above normal precipitation probabilities also extend southward to include much of the Southeast due to support from ENSO/MJO composites and trends . Conversely, below normal precipitation is favored across the Southern Plains where dry signals from the NMME, C3S, ECMWF, soil moisture, and trends converge. Farther to the west, uncertainty is high as the dry pattern forecast by dynamical model guidance directly conflicts with a strong wet signal produced by MJO composites from northern California to parts of the Central and Northern Rockies. Additionally, trends are not particularly strong across this region. For these reasons, equal chances of below, near, and above normal precipitation are indicated across most the western CONUS. Confidence is somewhat higher across Alaska as dynamical model guidance and ENSO composites generally agree on a dry signal across the southwestern Mainland and the Alaska Peninsula. Conversely, above normal precipitation is favored across parts of the Alaska Panhandle and eastern mainland Alaska due to a combination of wet signals from ENSO composites, trends , and dynamical model guidance.
SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS (Focus on May/June/July)
La Nina conditions continue across the equatorial Pacific Ocean as indicated by oceanic and atmospheric conditions, while indications of a potential transition to ENSO-neutral during the May through July period are apparent. The chance of a return to La Nina increases over the summer, such that La Nina becomes slightly more favored than ENSO-neutral in autumn. In addition to the forecast ENSO evolution described above, current anomalous soil moisture along with numerical and statistical model guidance inform the seasonal temperature and precipitation outlooks.
Temperature
The May-June-July (MJJ) 2021 temperature outlook favors above-normal seasonal mean temperatures for nearly all the contiguous U.S. and for Mainland Alaska and the Aleutian Islands. Equal chances (EC) of above-normal and below-normal temperatures are predicted for a region from far southeastern Mainland Alaska to parts of the Pacific Northwest, including northwestern Oregon and western Washington. Above-normal temperatures are most likely for the entire forecast domain from the summer into the beginning of autumn.
Precipitation
The MJJ 2021 precipitation outlook favors above-normal seasonal total precipitation amounts for central and eastern areas surrounding the Great Lakes, the Midwest, the Mid-Atlantic, and most of the Northeast, except for northern Maine, as well as southward along the Eastern Seaboard and the eastern Gulf Coast. Above-normal precipitation is also favored for northwestern areas of Alaska, including the north-central coast. Below-normal precipitation is most likely for a region from the West coast across the Rockies to the central and southern Great Plains, excluding areas of the Southwest that are climatologically drier at this time of year. The area of likely below-normal precipitation is forecast to slowly shift northward and eastward during the summer seasons while above-normal precipitation continues to be favored along the Eastern Seaboard and expands westward along the Gulf Coast over the same period. An area of elevated probabilities for above-normal precipitation is introduced for parts of the Southwest during the monsoon season.
Equal chances (EC) are indicated for areas where seasonal mean temperatures and seasonal total precipitation amounts are expected to be similar to climatological probabilities.
BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS
PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS
Although equatorial Pacific SSTs continue to rise towards climatological normal values and are forecast to indicate ENSO-neutral oceanic conditions for MJJ 2021, typical impacts of La Nina are considered in preparing the MJJ 2021 seasonal outlook, as some impacts are likely to result at the start of the
season. Tools used to inform the outlooks include regressions of temperature and precipitation on the Nino3.4 SST consolidation forecast and the CBaM forecast tool, that statistically “bridges” dynamical model Nino3.4 SST anomaly forecasts from the NMME with observed temperature and precipitation using a Bayesian methodology.
Current anomalous soil moisture and snow cover are considered and contributed to the outlook in some locations for the MJJ 2021 season. Dynamical model forecasts from the NMME and Copernicus (C3S) multi-model ensemble systems are used extensively for the first six leads when they are available, as was the objective, historical skill weighted consolidation, that combines both dynamical and statistical forecast tools. Due to the frequency of back-to-back La Nina events in the historical record and the consensus ENSO outlook slightly favoring La Nina over ENSO neutral, potential impacts of redevelopment of La Nina is considered in the autumn and winter seasons. Decadal timescale trends in temperature and precipitation are utilized extensively to inform the seasonal outlooks at all leads.
PROGNOSTIC DISCUSSION OF OUTLOOKS – MJJ 2021 TO MJJ 2022
TEMPERATURE
The MJJ 2021 temperature outlook favors above-normal seasonal mean temperatures for nearly all the contiguous U.S. (CONUS) and for the Alaska Mainland and the Aleutian Islands. The greatest probabilities for above-normal temperatures are located over the Southwest in the Four Corners region, exceeding 70 percent, due to multiple factors including very consistent dynamical model guidance, low soil moisture conditions, positive decadal temperature trends , and the support of statistical forecast tools. Modestly elevated probabilities for above-normal temperatures extend eastward across the Southeast and northward along the Eastern Seaboard to New England, supported by model guidance and positive decadal trends . Greater uncertainty, related to the early impacts of La Nina and an MJO event, lower historical forecast skill in model guidance, and weaker decadal temperature trends decrease probabilities for above-normal temperatures in the north-central CONUS. Consistent with possible lingering La Nina impacts as indicated by ENSO regressions and hybrid statistical-dynamical “bridging” tools, equal chances of above-normal and below-normal temperatures are forecast for southeastern areas of Alaska and parts of the Pacific Northwest, and the probabilities of above-normal temperatures have been decreased for the southern Alaska Mainland. Negative trends in sea ice coverage and thickness and positive decadal temperature trends favor above-normal temperatures for northern areas of Alaska.
Dynamical model guidance and statistical forecast tools, including those derived from forecasts of the enhanced probabilities of ENSO-neutral or La Nina conditions and decadal timescale trends , favor continued elevated probabilities for above-normal temperatures for most of the forecast domain through the SON 2021 season with the greatest chances indicated for the western U.S. and along the eastern seaboard with slightly lower probabilities for the north-central CONUS.
Beginning in OND 2021 and continuing through FMA 2022, there is a gradual increase in the area designated as EC, with an increased chance for La Nina redevelopment with potential impacts over much of the northern tier of the CONUS from the Pacific Northwest to the western Great Lakes, and later southeast Alaska. Other areas of the forecast domain continue to favor likely above normal temperatures, primarily due to positive decadal temperature trends .
PRECIPITATION
The MJJ 2021 precipitation outlook favors above-normal seasonal total precipitation amounts for central and eastern areas of the Great Lakes region, the Ohio Valley, the Mid-Atlantic, and most of the Eastern Seaboard, excluding northern Maine, based on the continued influence of La Nina early in the season, positive precipitation decadal trends , other statistical forecast tools including the constructed analog from soil moisture conditions, and some dynamical model forecasts from the NMME and C3S ensemble systems. There is considerably greater agreement among dynamical models , however, for a region of likely below-normal precipitation from the central and northern Pacific coast across the Rockies into the Central and Southern Plains. Higher probabilities for below-normal over the Northern Rockies, exceeding 50 percent in some areas, are supported by typical La Nina impacts and dynamical model forecasts.
The area of likely below-normal precipitation is forecast to shift slowly northward and eastward during the summer and southward into the west-central CONUS during the autumn, consistent with dynamical model guidance. Above-normal precipitation is predicted to persist along much of the Eastern Seaboard through the summer months, and an area of likely above-normal precipitation continues for parts of the southeast Atlantic coast and the eastern Gulf Coast into the early autumn, consistent with the impacts of the likely La Nina or ENSO-neutral climate conditions, with very low probability of El Nino conditions over the tropics.
Below-normal soil moisture in the Southwest and southern Rockies may allow for more efficient heating of the land mass and potentially an enhanced monsoon circulation. Elevated probabilities for above-normal precipitation are predicted for parts of the southwest from JJA through ASO 2021, supported in part by dynamical model forecasts.
Dynamical model guidance supports above-normal precipitation for parts of the northwestern coast of Alaska from MJJ through ASO 2021, while decadal precipitation trends related to positive trends in SSTs support above-normal precipitation for the northern coast through OND 2021. The outlooks from OND 2021 through MJJ 2022 are primarily based on decadal precipitation trends .
We will discuss this more when we compare the JAMSTEC to the NOAA forecast. But it is useful to look at the JAMSTEC discussion especially as it applies to Spring and Summer since their raising the issue that this is somewhat of a La Nina Mokoki raises questions about the use of statistical analysis in these forecasts. We will address this in more detail when we publish our NOAA v JAMSTEC comparison. NOAA hardly recognizes an El Nino Modoki let alone a La Nina Modoki. Fortunately, the major impact of a La Nina Modoki is seen in the Western Pacific as it impacts the track of cyclones headed west. But what heads west frequently turns north and then returns east at higher latitudes and impacts North America in that way but perhaps in a more subtle way. Below is the JAMSTEC discussion. Their website is down for maintenance but we have arranged to have access to their forecasts.
ENSO forecast:
As predicted earlier, observation shows that the La Nina Modoki-like state is decaying from boreal spring. The SINTEX-F predicts that a weak La Nina state will persist in boreal summer and autumn.
Indian Ocean forecast:
Observation shows that the tropical Indian Ocean is a neutral state. The SINTEX-F predicts that a negative Indian Ocean Dipole will occur in boreal autumn. However, there is a large uncertainty in the prediction.
Observation also shows the occurrence of Ningaloo Nino off the west coast of Australia, and the prediction suggests that it will decay from late austral autumn (boreal spring).
Regional forecast:
On a seasonal scale, the SINTEX-F predicts that most part of the globe will experience a hotter-than-normal condition in boreal summer (austral winter) except for Alaska, central Canada, northern part of the South American Continent, India, some parts of Africa, some parts of Europe, some parts of Russia. In boreal autumn (austral spring), the model still predicts most part of the globe will experience a warmer-than-normal condition, except for India, most part of Africa, Saudi Arabia, eastern Europe, and western Russia.
As regards to the seasonally averaged rainfall in boreal summer (austral winter), a drier-than-normal condition is predicted for western Canada, eastern U.S.A., Colombia, Ecuador, La Plata Basin, Philippines, western Africa, most part of Europe, central Russia, northeastern China, and Korean Peninsula. In contrast, Alaska, Indonesia, India will experience a wetter-than-normal condition. In boreal autumn (austral spring), a wetter-than-normal condition is predicted for eastern Australia, central Africa, Philippines, Indochina Peninsula, Indonesia, India, and southwestern China. In contrast, eastern U.S.A., some part of La Plata Basin, eastern China, western Africa, South Africa, and most part of Europe will experience a drier-than-normal condition.
The model predicts most part of Japan will experience a hotter-than-normal condition in summer and autumn as a seasonal average. As regards to the seasonally averaged rainfall, most part of Japan will experience a drier-than-normal condition in summer. In autumn, most part of Japan (except for Hokkaido) will experience a slightly drier-than-normal condition.
Drought Forecasts
These two forecasts (Monthly and Season) are issued by the same agency but to obtain them you need to access two different links here (for the single month forecast which comes out at the end of the month and here for the forecast issued with the Seasonal Outlook.
Here is the Three-Month Drought Forecast that was issued on April 15, 2021
We can compare the new forecast to the prior forecast recognizing that the new forecast is for a three-month period that begins one month later.
Prior Forecast | New Forecast |
There are two versions of the discussion of the seasonal drought outlook: long and short and this month I am providing the long version.
Tools used in the U.S. Seasonal Drought Outlook (SDO) included the Climate Prediction Center (CPC) temperature and precipitation outlooks for May and May through July 2021 (MJJ), various short- and medium-range forecasts and models such as the 7-day quantitative precipitation forecast (QPF) totals from the Weather Prediction Center (WPC), the 6-10 day and 8-14 day CPC extended-range forecasts (ERFs), the Week 3-4 outlooks and tools from CPC, dynamical models at the monthly and seasonal time scales, the 384-hour total precipitation forecasts from several runs of the GFS, climatology for the MJJ season, and initial conditions such as soil moisture. The April 13 USDM was used for initial drought conditions. ENSO-neutral conditions are likely during this outlook period.
Long-term drought remains entrenched throughout a majority of the West with D2 (severe) to exceptional (D4) drought designated for parts of Oregon, California, and the Southwest. As of mid-April, snow water equivalent remains below normal for California, the Great Basin, and the Four Corners region. Water-year-to-date (Oct 1, 2020 to Apr 12, 2021) averaged less than 75 percent of normal for these same areas. Farther to the north across Washington, short-term drought expanded due to a lack of precipitation during the past 30 to 60 days. Persistence is likely for California as May-June-July is a dry time of year. The Southwest is also entering a dry time of year until monsoon rainfall begins later in July. Therefore, persistence is forecast for the Southwest and any potential improvements related to monsoon rainfall are not expected until late July or August. Drought is expected to expand northward across parts of the Pacific Northwest and northern Rockies based on: below normal precipitation during the past 30 to 60 days, low soil moisture conditions, and elevated probabilities of below normal precipitation and above normal temperatures for MJJ. Also, longer term precipitation deficits dating back a year or more across eastern Oregon and Washington is another factor to support development for these areas. Based on the seasonal precipitation and temperature tools, ongoing drought is likely to persist across Montana and potentially intensify if spring rainfall averages below normal.
Forecast confidence for the Western Region is high for persistence and moderate for development.
Widespread, heavy precipitation (2 to 8 inches, liquid equivalent) during March resulted in 1 to 3-class improvements to drought conditions across Colorado, Kansas, Nebraska, and Wyoming. The eastern two-thirds of Kansas and Nebraska are currently drought-free with soil moisture above the 70th percentile. Soil moisture declines rapidly to the north across northern South Dakota and North Dakota where drought continues to intensify after a lack of snowfall this past winter. According to the USDA’s National Agricultural Statistics Service for the week ending on April 11, topsoil moisture is rated as short to very short across 83 percent of North Dakota. Based on a dry signal among precipitation tools from week-2 through the seasonal time scale along with the likelihood of above normal temperatures, persistence is forecast for a majority of the long-term drought across the central to northern Plains. However, this region is entering their wettest time of year when 40 to 65 percent of its annual precipitation occurs from May through July. This wet climatology reduces forecast confidence of persistence. The most likely area for improvement or removal is across eastern North Dakota where the seasonal outlook calls for equal chances of below, near, or above normal precipitation. Although additional improvements are expected across Colorado during April, the MJJ outlook favors below normal precipitation and above normal temperatures. Therefore, persistence is forecast for long-term drought across Colorado and Wyoming with development favored for areas designated with abnormal dryness (D0).
Forecast confidence is low for the High Plains Region.
Drought varies in intensity across Oklahoma and Texas with the lowest soil moisture (below the 5th percentile) affecting western and southern Texas. According to the USDA’s National Agricultural Statistics Service for the week ending on April 11, topsoil moisture is rated as short to very short across 77 percent of Texas. Although rainfall during mid to late April may prompt minor improvements across central Texas, broad persistence is most likely as seasonal tools support elevated probabilities of below normal precipitation and above normal temperatures. Development is favored in parts of west-central Texas due in part to the large probabilities (above 60 percent) of above normal temperatures forecast in the MJJ outlook. Since May is one of the wettest months of the year across the southern Plains and the dynamical model guidance has a strong dry signal for May, development is also forecast for parts of Oklahoma and northwest Texas where 60-day precipitation deficits (more than 2 inches) exist. The southern Plains will be closely monitored in the monthly drought outlooks through the summer. Moderate drought recently developed across northeast Louisiana due to inadequate precipitation during the past 30 to 60 days along with periods of above normal temperatures. Since this is a short-term drought area, heavy rainfall (1 to 3 inches) during the next week and the lack of a dry signal at the seasonal time scale favors drought removal for northeast Louisiana. Removal is also forecast for parts of the short-term drought area in southeast Texas, based on heavy rainfall forecast during mid to late April. Recent heavy rainfall and saturated soils make drought development unlikely throughout the lower Mississippi Valley and Tennessee Valley.
Forecast confidence is low for the Southern Region.
The most severe long-term drought across the Midwest remains centered across northwest Iowa, based on 6 to 12-month SPI values. Given the long-term duration of this drought area, persistence is most likely but forecast confidence is low since the spring and early summer are a relatively wet time of year. Removal of moderate drought is more likely across northwest Minnesota based on: recent heavy rainfall, the absence of a dry signal at the seasonal time scale, and a wet climatology. Since mid-March, changes to coverage of abnormal dryness (D0) and moderate drought (D1) have varied throughout the Corn Belt, upper Mississippi Valley, and Great Lakes. Although below normal precipitation is favored for the Midwest during the next two weeks, cooler-than-normal temperatures are expected to limit any additional development this month. Based on elevated probabilities of above normal precipitation in the MJJ outlook, removal is forecast for ongoing drought areas across the Great Lakes. The Corn Belt will be closely monitored in the monthly drought outlooks through the summer.
Forecast confidence is low for the Midwest Region.
Only small areas of abnormal dryness (D0) and short-term moderate drought (D1) are designated for parts of Alabama, Georgia, and Florida. Due to the lack of any dry signal among seasonal tools, development is unlikely for the Southeast. Recent heavy rainfall, a wet pattern during the next two weeks, and the onset of the convective season during June strongly favor removal of short-term drought across south Florida.
Forecast confidence is high for the Southeast Region.
Moderate drought (D1) is ongoing across parts of southern New England, the central Appalachians, and eastern Great Lakes where soil moisture and 28-day average streamflows are below the 20th percentile. 30 to 60-day precipitation deficits exceed 2 or 4 inches across these D1 areas. During the latter half of April, an amplified 500-hPa trough over eastern North America favors above normal precipitation. Therefore, drought may begin to ease during the next couple of weeks. Since the seasonal precipitation outlook for MJJ calls for elevated probabilities of above normal precipitation for much of the Northeast, drought removal is forecast by the end of July.
Forecast confidence is moderate for the Northeast Region.
Due to a lack of snowfall this past winter, abnormal dryness (D0) is designated for parts of northern and eastern Mainland Alaska. Based on an increasingly wet climatology later in the spring and early summer, this D0 area is unlikely to worsen to moderate drought (D1). Alaska is likely to remain drought-free through July.
Forecast confidence is high for Alaska.
Although heavy to excessive rainfall during March resulted in 1 to 3-category improvements to drought conditions across Hawaii, abnormal dryness (D0) and moderate drought (D1) exists across the leeward slopes of the Big Island. Drought is forecast to develop for leeward parts of the Big Island of Hawaii with increasing chances of development across Lanai and Molokai later in the outlook period.
Forecast confidence is moderate for Hawaii.
Drought coverage continues to decrease across Puerto Rico, due to above normal rainfall during the past month. Inputs to the North American Multi-Model Ensemble feature large differences in their seasonal precipitation outlooks. Since May is one of the wetter months of the year, drought removal is favored.
Forecast confidence is moderate for Puerto Rico.
ENSO Considerations
We have covered some of this when we reported on the IRI/CPC analysis on April 10, 2021. We may discuss it again when we do the NOAA/JAMSTEC Comparison.
Now let us look at the NOAA recent ENSO Update. This happens every month on the second Thursday. I show this because it is the information that NOAA works from in producing the Four-Season Forecast.
The Southern Oscillation Index or SOI being above 7 confirms that the Atmosphere is in tune with the Sea Surface Temperatures in the Pacific along the Equator. At least temporarily the SOI Index is in neutral territory and is not confirming the La Nina. This is another graphic that we have not frozen.
Below is the graphic above but also one issued five days later. (I believe these images are five-day averages centered on the date shown.)
Prior Version | Updated Version |
Here is the BOM Nino3.4 tracker.
Here is their current forecast and I will let that one auto-update also. But it only updates every two weeks.
And here is the NOAA proprietary model that it seems they have decided to ignore.
B. Conclusion
From the perspective of NOAA, we are still in a La Nina at least through May. There may be an issue as to how long we remain in that condition. The tools NOAA uses to forecast the state of ENSO has introduced the concept of a possible double-dip La Nina. That is not unusual. It would be devastating for the West.