Written by Sig Silber
Here is the December 17, 2020, NOAA four-season forecast for CONUS and Alaska including the Early Outlook for January 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. There is not a lot of change in the forecast from the one issued a month ago. NOAA is still not ready to forecast the Monsoon for this summer. JAMSTEC is and we will cover that Sunday Night most likely.
Please share this article – Go to the very top of the page, right-hand side for social media buttons. Please feel free to share this report with anyone you feel will find it useful.
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 January 2021. It is called the Early Outlook because it will be updated at the end of December. Only the January 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 January 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 January*. For Temperature, it is warm for most of CONUS and Northern Alaska and the Aleutian Islands but Southern Alaska and the Panhandle are cool and this extends into the Greater Northwest with a fairly large EC area being a buffer between the cool area and the large warm anomaly that covers a large part of CONUS. With respect to precipitation, the Southern Tier is dry and there are two wet anomalies one in the Northwest and another somewhat triangle shaped that extends from Maine to Eastern Minnesota and south to just beyond where the Ohio River merges with the Mississippi. Northwest Alaska is also 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 December with the second map extending to January 2, 2021 and the third map as of today covers thirteen additional days in January. Thus only the third of the three maps (to a much lesser extent the second map) is useful for this purpose. The Seasonal Outlook was issued fairly early this month since the third Thursday fell on December 17, 2020. The earliest possible issue of the forecast is on the 15th of the month. We are publishing on a Friday so we have the newly issued Week 3-4 forecast. Fifteen days (really thirteen as the second map only contains two days in January blended with five days of December) of shorter forecasts is not a great basis for doing this visual consistency testing but it is worth looking at. There are thirty-one days in January.
First Temperature
 |  |  |
| The new monthly forecast is to the right. Does it appear to be reflective of the parts of the month for which we have forecasts? → |
| ↑ ← It is too soon to do this analysis and be very confident in the analysis. The Week 3-4 forecast fits with the full month forecast except in the Northwest. So the question is does NOAA see a cool second half of the month for the Northwest? We will revisit the forecast at the end of December when the January forecast is updated. |
And then Precipitation
 |  |  |
| The new monthly forecast is to the right. Does it appear to be reflective of the parts of the month for which we have forecasts? → |
| ↑ ←It is too soon to do this analysis and be very confident in the analysis. But the Week 3 – 4 forecast fits pretty well with the full month forecast but it is warmer in Alaska and the Northern Great Plains than the monthly forecast. So the question is will the second half of the month adjust the results so that the full-month forecast is correct or is the first half of the month more representative of what the full month will be like? We will revisit the forecast at the end of December when the January forecast is updated. |
Sometimes it is useful to look at the Week 3-4 Discussion.
Week 3-4 Forecast Discussion Valid Sat Jan 02 2021-Fri Jan 15 2021
La Nina continues to play a major role in the Tropical Pacific with sea surface temperature (SST) anomalies in the Nino 3.4 region observed just below -1 degree C. The Madden-Julian Oscillation (MJO) is currently weak and is not expected to re-emerge in the next two weeks. As such, this week’s forecast is largely based on the dynamical model guidance in addition to considerations for La Nina-related impacts.
There is good agreement among the dynamical models (CFS, ECMWF, JMA and the SubX multi-model ensemble) regarding the anomalous 500-hPa height pattern over the Week 3-4 period. Models all indicate troughing over Alaska, though the CFS is shifted slightly westward. Positive height anomalies are favored over the Pacific, extending east over the western CONUS. Anomalous heights over the central CONUS are weak (CFS, ECMWF) to slightly negative (JMA, SubX). Positive height anomalies are also consistently forecast over the Northeast, related to an anomalous high pressure south of Greenland.
The temperature forecast tilts toward above-normal probabilities throughout the Lower-48. The highest probabilities are expected in the northeast, favoring a 60-70% chance of above normal temperature. Similar probabilities are also forecast over the Northern Plains, where despite the weak forecasted height anomalies, the abnormal lack of snow cover may lead to above normal temperatures. Below normal temperatures are favored in western Alaska, associated with troughing in that region and equal chances in the eastern part of the state and the panhandle.
Conditions for east coast Nor’easter cyclones continue to be favorable throughout the forecast period and are indicated by increased chances for above-normal precipitation throughout the Northeast, extending down to the Ohio Valley and Mid-Atlantic. Below normal precipitation is favored throughout the southern tier of the US, a pattern commonly observed during La Nina and consistently predicted in dynamical and statistical guidance. Above normal precipitation odds are also increased in the Pacific Northwest and Alaska panhandle related to the aforementioned troughing and potential above normal atmospheric river activity. The rest of Alaska is expected to see dry conditions, with highest probabilities of below normal precipitation in the western part of the state.
Hawaii is surrounded by slightly positive SST anomalies, lending to an increased chance of above normal temperatures throughout the forecast period. Despite the warm SST, dynamical guidance suggests slightly drier conditions throughout the state.
Now we consider the three-month Outlook.
Notice that the three-month periods are abbreviated e.g.. January/February/March is shown as JFM. You will see such abbreviations often in this report.
Prior Temperature Outlook for JFM 2021
New Temperature Outlook for JFM 2021
Now Precipitation.
Prior forecast for JFM 2021
New Precipitation Outlook for JFM 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: JFM 2021 – DJF 2021/2022
New 14 Month Temperature Outlook: FMA 2021 – JFM 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. One uses the same procedure to compare the precipitation maps. Based on this procedure, I conclude that:
Now Precipitation
Prior 14 Month Precipitation Outlook: JFM 2021 – DJF 2021-2022
New 14 Month Precipitation Outlook: FMA 2021 – JFM 2022
If you want larger versions of each 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 and 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 December 17, 2020. 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 (January), the NOAA Summary for JFM, and finally the remainder of the 15-Month Forecast.
BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS
CURRENT ATMOSPHERIC AND OCEANIC CONDITIONS
Tropical oceanic and atmospheric observations reflect ongoing La Nina conditions. During the past 30-days, a Pacific equatorial cold tongue with negative sea-surface temperature (SST) anomalies was located from about 160 deg E across the Date Line to about 100 deg W. SST anomalies for the period ranged between 0.5 deg C to 1.5 deg C over most of this region. Subsurface temperatures were colder-than-average for the same region, down to a depth of about 175 meters. The Oceanic Nino 3.4 Index (ONI) for the latest observed season (SON 2020) is -1.2 deg C, which qualifies as a moderate La Nina at this time. Atmospheric observations feature enhanced easterly trade winds at 850-hPa from the western to the east-central equatorial Pacific, westerly wind anomalies at 200-hPa over most of the Pacific, and suppressed tropical convection over the western and central Pacific.
PROGNOSTIC DISCUSSION OF SST FORECASTS
The CPC Nino3.4 SST consolidation depicts negative SST anomalies increasing slightly in amplitude to about -1.3 or -1.4 deg C by the anticipated NDJ 2020 peak of this La Nina event, before slowly recovering to -1.0 deg C by JFM 2021. The consolidation forecast predicts the ONI value will cross the -0.5 deg C threshold into ENSO- neutral territory during MAM 2021, and reach the zero anomaly line by MJJ 2021. The NMME ensemble mean SST plume for the Nino 3.4 region reaches the -0.5 deg C threshold by April 2021, and the C3S (Copernicus) SST plume reaches the ENSO- neutral threshold slightly later in the spring. The CPC-IRI consensus predicts the likelihood of La Nina will drop to the same likelihood of ENSO-neutral by about AMJ 2021. The official ENSO forecast calls for approximately a 95% chance for La Nina to continue through Northern Hemisphere winter, with a potential transition during the spring 2021 (~50% chance of Neutral during AMJ 2021).
PROGNOSTIC TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS
Given a very high likelihood of the persistence of La Nina conditions, the seasonal outlooks utilized canonical impacts during past observed La Nina events as guidance for many areas of the forecast domain through at least MAM 2021. This guidance included regressions of temperature and precipitation relative to the CPC consolidation forecasts of the Nino3.4 region and via “bridging” techniques utilizing statistical relationships between dynamical model forecasts of the Nino 3.4 index and observed temperature and precipitation. Dynamical model guidance from the North American Multi-Model Ensemble (NMME) and the model suite from the Copernicus program are a significant component of guidance for the temperature and precipitation outlooks through MJJ 2021. Beyond MJJ 2021, the consolidation of various statistical tools, including decadal trends , was the primary basis for the outlooks, with little remaining influence from ENSO or other reliable large-scale signals of climate variability for these forecast leads. Also, based on current and potential drought conditions in many areas across the southern tier of the CONUS, low soil moisture conditions influenced the temperature outlooks for the spring and early summer 2021. At later leads, decadal trends in temperature and precipitation were the primary tool used in creating the seasonal outlooks.
30-DAY OUTLOOK DISCUSSION FOR JANUARY 2021
The outlook for January 2021 is based on a number of factors. The first is the footprint of the ongoing La Nina as determined from composites. La Nina is likely to continue through winter 2021 before potentially transitioning in spring. Most La Nina composites and correlations are run on a seasonal basis, so signals in those tools will have a portion of signal in January.
Other factors considered are current and likely land surface conditions with respect to snow cover and frost depth, the predicted state of intraseasonal oscillations in the tropics, and model guidance from the NMME suite and constituent models. The MJO is not forecast to be a significant contributor to the variability in January as mature La Ninas can inhibit the eastward propagation of MJO related winds of the Pacific Ocean. Snowpack and frost depth are below normal across much of the northern plains but are likely to end the month above normal in the Northeast (if the latest short-term outlooks verify well).
Temperature
Model guidance from the NMME suite favors broadly above normal temperatures from the Rockies eastward, with little to no shift in temperatures favored for the Pacific northwest and much of California. Below normal temperatures are favored for much of Alaska in many of the model tools, though there are weak signals for above normal temperatures along the west coast of Alaska and for the Aleutians. The outlook for January temperatures largely reflects the model guidance, but with some modifications. The odds for below normal temperatures in the Pacific Northwest are higher than model guidance, attributed the likely impact of La Nina. Odds for above normal temperatures are slightly lower than model guidance over the Northern Plains, as that area has high variance and La Nina composites and trends would favor below normal temperatures. However, below normal snowpack and shallow frost depths, aligned with predicted above normal temperatures in the next two weeks would favor above normal temperatures in this region. The upcoming major snowstorm for the northeast may set down a blanketing and lasting snowpack, a factor in moderating the odds for above normal temperatures through the Northeast.
Precipitation
The precipitation outlook is also largely model based, with some minor modifications to reflect the likely impact of La Nina during the month. The biggest areas of uncertainty in the precipitation outlook are over Northern California, the Northern Plains, and the Alaska Panhandle. Over California, EC is indicated, but some tools have above normal precipitation while others have below normal as far north as Eureka and Tahoe. The uncertainty for the Northern Plains is present in the model solutions and La Nina composites, though La Nina would have a slight tilt toward above normal precipitation on the seasonal time period for Dec-Jan-Feb, that signal isn’t strong so it’s not coming through on the monthly outlook. La Ninas are correlated with ridging over the eastern North Pacific, but if the eastern edge of the ridge is slightly further west or north, storms may not be steered into the Alaska Panhandle. Composites favor below normal precipitation on the seasonal time periods but model solutions have only a weak signal from the Kenai Peninsula to the very northern portions of the Alaska Panhandle, with more storms pushing into western Alaska.
BASIS AND SUMMARY OF THE CURRENT LONG-LEAD OUTLOOKS
SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS (Focus on JFM 2021)
Moderate La Nina conditions are present across the equatorial Pacific Ocean, as indicated by current oceanic and atmospheric observations. La Nina is expected to continue through the Northern Hemisphere winter, with a potential transition to ENSO-neutral (~50% chance) during Spring 2021. The January-February-March (JFM) temperature outlook favors below normal seasonal mean temperatures for approximately the southeastern quarter of Mainland Alaska and the Alaska Panhandle, and from the Pacific Northwest eastward across the Northern Rockies to the Northern Plains. Maximum probabilities for below normal temperatures exceed 50% over the extreme southeast portion of Mainland Alaska, the Alaska Panhandle, and Washington state. Above normal temperatures are favored across parts of northern and western Alaska, and for approximately the southern one-half of the CONUS, extending northward across the Ohio Valley, Central and Northern Appalachians, Mid-Atlantic, and Northeast. Maximum probabilities for above normal temperatures exceed 60% across parts of the Southwest, Southern Rockies and Southern Great Plains. The JFM precipitation outlook favors above normal precipitation for most of the northern tier of the CONUS, extending southward from the Great Lakes across eastern portions of the Middle Mississippi Valley, the Ohio Valley, and the Central and Northern Appalachians. Above normal precipitation is also favored for northern and western portions of Alaska. Maximum probabilities for above normal precipitation exceed 50% over the Lower Ohio Valley. There are elevated odds of below normal precipitation for approximately the southern one-third to one-half of the CONUS, and along the southern Alaska coast from the eastern Alaska Peninsula to the southern Panhandle region. Maximum probabilities of below normal precipitation exceed 60% from southeastern Arizona to the Big Bend area of Texas, and over parts of northern Florida and southern Georgia.
Equal Chances (EC) are forecast for areas where probabilities for each category of seasonal mean temperatures or seasonal total precipitation amounts are predicted to be similar to climatological probabilities.
PROGNOSTIC DISCUSSION OF OUTLOOKS – JFM 2021 TO JFM 2022
TEMPERATURE
The temperature outlook for JFM 2021 favors below normal seasonal mean temperatures for approximately the southeastern quarter of Mainland Alaska and the Alaska Panhandle, based largely on La Nina composites/regressions and climate model forecasts. Over the Lower 48 states, below normal temperatures are also favored for the Pacific Northwest, the Northern Rockies, and much of the Northern Great Plains, which is also characteristic of a mature cold season La Nina. Perhaps the area of greatest uncertainty regarding below normal temperatures is over South Dakota and portions of the Upper Mississippi Valley, where there are significant differences between the tools. For example, tools such as the Final CON and CBaM support anomalous cold across this region, whereas the uncalibrated NMME, C3S(Copernicus) and CFS favor anomalous warmth. Parts of this region are in drought, and are expected to be in drought in the early spring. Any lack of snow cover would also support relative warmth during JFM (please refer to CPC’s Monthly Outlook discussion for more information). It was decided that EC was the best bet for the temperature pattern across most of South Dakota and much of the Upper Mississippi Valley during JFM. Regarding the odds for below normal temperatures over Alaska and the CONUS, maximum probabilities exceed 50% over the extreme southeast portion of Mainland Alaska, the Alaska Panhandle, and western Washington state. Above normal temperatures are favored across portions of northern and western Alaska, and for approximately the southern one-half and eastern one-third of the CONUS. Maximum probabilities for above normal temperatures exceed 60% across parts of the Southwest, Southern Rockies and Southern Great Plains. The above normal (upper-tercile) temperatures favored in northwestern Alaska is consistent with many of the objective tools, and temperature trends during the last 15 years. The upper-tercile temperatures predicted over the southern, and much of the eastern, CONUS, are supported by La Nina composites, most of the NMME inputs, and the ENSO-OCN tool which combines ENSO information with trends .
The subsequent temperature outlooks for FMA and MAM are similar to the JFM temperature outlook, which is reasonable given the intensity and anticipated duration of the La Nina. As the region of favored below normal temperatures fades across the northern tier states (from the Pacific Northwest to the Northern Plains), the probabilities and spatial coverage for above normal temperatures slowly increases across the eastern CONUS. In Alaska, the chances of above normal temperatures increases to over 50% in MAM, based in part on the earlier climatological breakup of sea ice, and longer-term temperature trends . By AMJ 2021, the odds of La Nina and ENSO-neutral are roughly equal near 50%, with a potential transition from La Nina to ENSO-neutral. From MJJ through SON, warmer-than-normal temperatures are favored across most of the CONUS and Alaska, supported by dynamical models , the statistical consolidation (Stat-CON), and trends . From OND onwards, uncertainty (and therefore, EC) gradually expands across the northwestern and north-central CONUS, while temperature trends favor above normal temperatures in Alaska.
PRECIPITATION
The JFM 2021 precipitation outlook favors above normal precipitation amounts from much of the northwestern quarter of the CONUS eastward across the Great Lakes and interior Northeast, extending southward over the eastern CONUS across the Ohio, Tennessee, and much of the Mississippi Valleys, and portions of the Appalachians.. Above normal precipitation is also favored for northern and western portions of Alaska. These areas of favored wetter-than-normal conditions are consistent with what is typically observed during mature, cold season La Nina’s, and are well supported by a majority of climate models. Maximum probabilities of above normal precipitation exceed 50% over the Lower Ohio Valley. There are elevated odds of below normal precipitation for approximately the southern one-third to one-half of the CONUS, and along the southern Alaska coast from about Kodiak Island and Cook Inlet southeastward across the Alaska Panhandle. These favored regions of below normal precipitation are also consistent with classic La Nina wintertime setups. Maximum probabilities of below normal precipitation exceed 60% over far southern New Mexico and adjacent parts of southwestern Texas, and over parts of northern Florida and southern Georgia. Unfortunately, much of the southern CONUS is also experiencing drought, after a lackluster summer monsoon season and relatively dry autumn.
La Nina is the primary physical driver behind the subsequent FMA and MAM precipitation outlooks, so only minor adjustments were needed to the corresponding outlooks made last month. In MAM, the amplitude of La Nina and its associated precipitation impacts are expected to begin to decline.
In AMJ 2021, below normal precipitation amounts are forecast to diminish in spatial coverage across the southern CONUS, and be centered over most of the southwestern quarter of the CONUS before the signal disappears completely in MJJ. Over much of the eastern CONUS during AMJ and MJJ, above normal precipitation is favored, based in large part on the consolidation tool, precipitation trends , the CFS, and the Constructed Analog on Pacific SSTs. In JJA and JAS 2021, trends favor the development of a dry signal in the Northwest CONUS. Trends also favor the reduction in coverage of the wet signal in the East, beginning in JJA and continuing through OND. The final three leads (NDJ 2021-JFM 2022) feature a favored area of above normal precipitation in the vicinity of the Northern Great Plains, which expands eastward with time towards the Ohio Valley. The Alaska precipitation outlooks from AMJ 2021 through JFM 2022 indicate EC, based on the predicted conclusion of La Nina during the springtime and weak and/or inconsistent precipitation signals among the tools/trends thereafter.
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 Winter and Spring 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.
Dec. 17, 2020 Prediction from 1st Dec., 2020
ENSO forecast:
As predicted earlier, observation now shows a La Niña Modoki-like state. The SINTEX-F predicts that this La Niña Modoki-like state will persist in this boreal winter. Then, it will start to decay from the boreal spring of 2021. We need to be careful of its impact as it may be different from that of a canonical La Niña.
Indian Ocean forecast:
Observation shows that the tropical Indian Ocean is a neutral state. The ensemble mean prediction suggests that the present condition will persist in the first half of 2021.
Regional forecast:
On a seasonal scale, the SINTEX-F predicts that most part of the globe will experience a warmer-than-normal condition in boreal winter except for Alaska, northern Brazil, southwestern Australia, southern Saudi Arabia, Indochina Peninsula, and India. In boreal spring, the model predicts persistence of a similar condition.
As regards to the seasonally averaged rainfall in boreal winter, a drier-than-normal condition is predicted for southern U.S.A., eastern Brazil, La Plata Basin, southwestern Africa, southern Europe facing the Mediterranean Sea, some part of the Middle East, eastern China, and Indonesia. In contrast, Canada, most part of Brazil, western/eastern Australia, eastern part of South Africa, Philippines, and northern Europe will experience a wetter-than-normal condition. In boreal spring, a wetter-than-normal condition is predicted for most part of Canada, Mexico, northwestern Brazil, northern Australia, Indochina Peninsula, Philippines, West Africa, northern Europe, and eastern Russia. In contrast, most part of U.S.A (including California), southern Europe, southeastern China, and Indonesia will experience a drier-than-normal condition.
The model predicts most part of Japan will experience warmer-than-normal condition in winter and spring as a seasonal average. As regards to the seasonally averaged rainfall, most part of Japan (except for Hokkaido) will experience a drier-than-normal condition in winter. In spring, western part of Japan will experience slightly drier-than-normal conditions.
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 which was issued on December 17, 2020
Tools used in the U.S. Seasonal Drought Outlook (SDO) included the Climate Prediction Center (CPC) temperature and precipitation outlooks for January (Jan) 2021 and January through March (JFM) 2021, various short- and medium-range forecasts and models such as the 7-day quantitative precipitation forecast (QPF) 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 Jan and JFM, and initial conditions for parameters such as soil moisture. Existing drought areas on the forecast are based on the December 15 U.S. Drought Monitor. Conditions currently reflect the presence of a moderate La Niña, with a 95% chance for La Niña to persist through the Northern Hemisphere winter, and forecaster consensus indicated the potential for a strong cold event given the robust ocean-atmospheric coupling already in place.
Mostly dry weather and above-normal temperatures did little to alleviate drought conditions across most of the West, but recent storminess over the Pacific Northwest brought some relief to ongoing drought conditions there during the past several weeks. The short-term pattern looks similar, with storm systems bringing additional coastal rain and mountain snows to the Cascades, northern Rockies, and the northern Sierra Nevadas, with accumulations tapering off toward the south. Both the CPC Jan and JFM outlooks reflect climate anomalies associated with La Niña, favoring above-median precipitation for the Northwest and northern Rockies, below-median precipitation for central and southern California through the southern Four Corners region, and equal chances for near-, below-, and above-median precipitation straddling northern California eastward to most of Utah and Colorado. JFM is a normally a wet time of year for the Far West, and winter moisture is critical to building mountain snowpacks that feed reservoirs from river snowmelt during the typically drier late spring and summer months. Therefore, the wet signal forecast for the Northwest favors further drought reductions across the Northwest and northern Rockies, with drought elimination possible for much of Washington. In contrast, drought persistence is favored for the remainder of the West, with drought expansion possible across southern California due to good odds for below normal precipitation and above normal temperatures during JFM.
Forecast confidence is high for the Western region.
Across the High Plains, drought conditions worsened during the past several weeks with degradations observed in most states, although some improvements did occur but were fairly localized. The La Niña favored anomalous precipitation dipole is reflected in the seasonal outlook, where above-median precipitation is favored for North Dakota and northwestern Wyoming, while below-median precipitation is favored for much of Colorado, Kansas, and Nebraska. JFM is a dry, cold time of year for the Plains, and any anomalous snowfall during the winter months across eastern Montana and the Dakotas will likely remain above the frozen soils until the spring thaw beyond the JFM period. Therefore, little change in the incipient drought conditions is likely during the outlook period. Further south, despite the normally dry time of year, the potential for subnormal rainfall and above-normal temperatures favors development of drought conditions across Kansas.
Forecast confidence is moderate for the High Plains region.
The most substantial drought development over the past month occurred in Texas due to abnormal dryness and near- to above-normal temperatures. The increasing dryness in southern Texas was interrupted by swaths of heavy rainfall, which brought some relief to the areas. La Niña winters tend to be warm and dry across most of the southern CONUS, and this is reflected in the probabilistic monthly and seasonal outlooks. Therefore, drought development is likely for the rest of Texas and Oklahoma. In addition, the surplus rainfall this summer and fall from several tropical systems affecting the Southeast are gradually becoming a faded memory from southeastern Louisiana eastward across southern sections of Mississippi, Alabama, Georgia, South Carolina, and northern Florida. With this region accumulating growing precipitation deficits out to 60 days and a JFM outlook for subnormal precipitation and above normal temperatures, drought development is likely. However, short-term drought removal is favored for northern Mississippi and western Tennessee due to short-term forecasts of above normal rainfall and favorable time for soil moisture recharge.
Forecast confidence is moderate to high for the South and Southeast regions.
Small pockets of drought currently extend across parts of northern Missouri, central Illinois, and northern Indiana as these areas missed surplus precipitation that overspread portions of the Ohio River Valley for the past month. Additionally, unusually long-lasting drought has persisted across central Pennsylvania, upstate New York, and coastal New England, though recent storminess brought relief to much of this area. During La Niña winters, the climatological winter storm track typically shifts to the west, favoring the Ohio Valley and interior New England with enhanced precipitation. This is reflected by elevated chances for above-median precipitation in the JFM seasonal outlook. Therefore, drought removal is favored for the eastern half of the Corn Belt, and additional drought improvements are expected across the Northeast. While enhanced chances for above-median JFM precipitation do not extend to coastal New England (Equal Chances-EC), climatology favors frequent coastal storms during the winter, and these storms should be sufficient to erode the ongoing drought. In contrast, with no tilt of odds (EC) for JFM precipitation and with a drier and colder climatology than eastern regions, drought across parts of western Iowa, northwestern Missouri, northern and western Minnesota should persist as the ground will most-likely remain frozen well into March.
Forecast confidence is high for the Midwest and moderate to high for the Northeast.
No drought is currently in place or anticipated to develop across Alaska or Puerto Rico. Persistent dryness and above-average temperatures promoted widespread drought development in Hawaii, with nearly 23% of the state currently experiencing drought conditions. With the start of the wet season in January, as well as enhanced trade winds associated with the La Niña response, above-average rainfall is favored, particularly across the leeward sides of the islands. Therefore, drought improvement or removal is favored during JFM.
Forecast confidence is high for Hawaii.
ENSO Considerations
We have covered some of this when we reported on the IRI/CPC analysis on December 10, 2020. We may discuss it again when we do the NOAA/JAMSTEC Comparison.
This is the early December analysis which is based on a survey of meteorologists.
NOAA has their own proprietary model which they rarely use. It is not exactly clear why they shun their own model.
In most cases, I freeze the models as of the date of publication but for this one, I am going to just let the above model run so if you refer to the article in the future, the values in the above may not relate well to the discussion. But I am doing that so if you refer to this article in say two weeks, you will see if there has been any change in the forecast from this model.
Comparison models would include JAMSTEC
And the JAMSTEC Modoki Index
And the Australian BOM
I am going to introduce another graphic which is the current view of the subsurface along the Equator.
| Last Month | This Month |
 |
|
B. Conclusion
We are now in a La Nina. There may be an issue as to how westerly oriented this La Nina will be and what if any will be the impacts of it having some characteristics of a Modoki as JAMSTEC suggests.
