Written by Sig Silber
Edited at 2:10 p.m. EST November 20 to reflect the Week 3-4 forecast that confirms the continuing drought for December but a somewhat wet Northern Tier.
Here is the November 19, 2020, NOAA four-season forecast for CONUS and Alaska including the Early Outlook for December 2020. 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.
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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 December 2020. It is called the Early Outlook because it will be updated at the end of November. Only the December 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 December 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 December*. For Temperature, it is warm for most of CONUS and Alaska but southern Alaska and the Panhandle is cool and this extends into the Greater Northwest. With respect to precipitation, the Southern Tier is dry and there are two wet anomalies one in the Northwest and another that extends SE from Maine to Kentucky. Northwest Alaska is wet and Southeast Alaska and most of the Panhandle is dry.
* 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 November and the third map as of today only covers eleven days in December. Thus only the third of the three maps is at all useful for this purpose. The Seasonal Outlook was issued fairly late this month since the third Thursday fell on November 19, 2020. The earliest possible issue of the forecast is on the 15th of the month. Eleven days of shorter forecasts is not a good basis for doing this visual consistency testing. There are thirty-one days in December. Tomorrow, Friday, the Week 3 – 4 forecast will be updated and then we will have 18 days of December to work with and can begin to perform a visual test of consistency. We may update this report tomorrow but whether we update it or not, all three of the partial forecasts will update with the Week 3 – 4 forecast updating by one week. So readers will be able to begin to draw conclusions on the validity of the Early Outlook for December. [Yes, we have updated the discussion below now that we have the Week 3 – 4 Forecast and have short term forecasts for 18 days of December.]
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? → |
| ↑ ← One day after initial publishing we now have 18 days of short-term forecasts and the full-month forecast seems fairly reasonable with the caveat that how far cool Canadian air will intrude in the Northwest is very difficult to predict as per the discussion provided below in the newly issued week 3 – 4 forecast. We will also revisit the forecast at the end of November when the December 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? → |
| ↑ ←One day after initial publishing we now have 18 days of short-term forecasts and the full-month forecast seems fairly reasonable with the caveat that how far wet Canadian air masses will intrude in the Northwest and Northeast is very difficult to predict as per the discussion provided below in the newly issued week 3 – 4 forecast. We will also revisit the forecast at the end of November when the December forecast is updated. |
Sometimes it is useful to look at the Week 3-4 Discussion. We will add that tomorrow when the Week 3 – 4 forecast is updated.
Week 3-4 Forecast Discussion Valid Sat Dec 05 2020-Fri Dec 18 2020
Substantial tropical contributions to the midlatitude pattern during the Week 3-4 period are likely, with both an active La Nina and MJO signal in place at this time. The atmospheric response to La Nina continues to be robust, with enhanced trades continuing across the tropical Pacific, and anomalous high pressure aloft north of Hawaii. The MJO active phase is currently over the Indian Ocean, which teleconnects well with above-average mid-level heights over the central and eastern CONUS by Week-3 and may counteract the La Nina favored cold signal along the northern tier. Due to these active tropical signals, the multiple linear regression statistical tool based on ENSO, MJO, and long term trend exhibits fairly high probabilities in its temperature and precipitation outlooks, and the pattern is similar to the dynamical model forecasts.
Dynamical model 500-hPa height anomaly forecasts exhibit remarkable consistency among the various ensembles. The CFS, JMA, ECMWF, and Canadian all depict strong troughing in the vicinity of Alaska during Week 3-4, with the trough axis shifting slightly eastward of its forecast position in Week-2. Downstream over the CONUS, the models all favor above-average height anomalies. Overall, the pattern is indicative of a positive AO phase with strong Pacific flow into western Canada, which limits the potential for any significant arctic airmass development over North America. Therefore, while minor differences in the forecast height pattern result in variations among the tools regarding where the highest potential for above-normal temperatures sets up, all of the models depict a warm scenario for most of the CONUS.
The temperature and precipitation outlooks are based on a consensus of the dynamical and statistical models, which are fairly consistent with the canonical response to tropical forcing. Despite the potential for deep troughing over Alaska, the pattern appears to be transient, with southerly flow early in the period gradually transitioning to northerly flow. Despite the potential for northerly flow, the anticipated warmth early in the period and a lack of significant sea ice across the Chukchi Sea presents a limiting factor for substantial cold during the Week 3-4 period. Therefore, equal chances for above- and below-normal temperatures are maintained. Across the CONUS, equal chances are maintained for the Northwest, where dynamical model probabilities for above-normal temperatures are the weakest, and the La Nina pattern suggests a potential for below-normal temperatures. Above-normal temperatures are favored across the rest of the CONUS, with the highest probabilities extending across the northern Plains, upper-Midwest, and the Northeast.
The precipitation outlook is more complex. The CFS, JMA, and Canadian all favor above-average precipitation across the Pacific Northwest and Northern Rockies, while the ECMWF keeps the storm track to the north over western Canada and the Alaska Panhandle. Differences are also apparent over the Northeast, where most tools favor above-average precipitation, but the ECMWF again depicts dryness. Given the agreement among most of the guidance, above-average precipitation is favored for Washington State and the northern Rockies as well as New England, but probabilities are kept low in deference to the ECMWF. Elsewhere, the guidance fairly consistently favors below-median precipitation, with the highest probabilities along the southern tier, which is consistent with La Nina conditions as well as the lagged response to a robust Indian Ocean MJO. Below-median precipitation is also favored across the central CONUS, consistent with the anticipated above-average heights.
The outlook favors above normal temperatures for the Hawaiian Islands due to positive SST anomalies in the vicinity and ridging to the north. Guidance from the Subseasonal Experiment (SubX) multi-model ensemble suggests enhanced probabilities for below-median precipitation, though some of the individual model ensembles show enhanced rainfall.
Now we consider the three-month Outlook.
Notice that the three-month periods are abbreviated e.g. December/January/February is shown as DJF. You will see such abbreviations often in this report.
Prior Temperature Outlook for DJF 2020-2021
New Temperature Outlook for NDJ 2020-2021
Now Precipitation.
Prior forecast for DJF 2020-2021
New Precipitation Outlook for NDJ 2020-2021
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: DJF 2020/2021 – NDJ 2021/2022
T
New 14 Month Temperature Outlook: JFM 2021 – DJF 2021 – 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: DJF 2020-2021 – NDJ 2021-2022
New 14 Month Precipitation Outlook: JFM 2021 – DJF 2021/2022
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 November 19, 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 (December), the new NOAA Summary for DJF, 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. The latest monthly mean Sea Surface Temperatures (SSTs) through November 11, 2020 continue to indicate a Pacific equatorial cold tongue with negative SST anomalies on or near the equator stretching from west of the Date Line to the coast of South America. Negative anomalies are as large as -2.0 degrees C in some areas. The latest weekly Nino3.4 index value is -1.0 degrees C. Negative subsurface ocean temperature anomalies persist to depths of approximately 100-200 meters from just west of the Date Line to 80W.
Suppressed convection continues over part of the western and central Pacific while enhanced convection continues over parts of the Maritime Continent. Trade Winds are enhanced over the past 30 days over parts of the western and east-central Pacific Ocean.
PROGNOSTIC DISCUSSION OF SST FORECASTS
The CPC Nino3.4 SST consolidation forecast depicts median negative anomalies increasing in magnitude to near -1.5 degrees C by DJF 2020-2021, followed by a gradual warming to about -0.5 degrees C around MAM 2021. The North American Multi-Model Ensemble (NMME) ensemble mean forecast for the Nino3.4 SST anomaly has a similar evolution during the next several months through MJJ 2021. Based on the latest observations and model forecasts as of early November, the official CPC/IRI ENSO outlook indicates high confidence in the persistence of La Nina conditions through the Northern Hemisphere winter (probabilities exceeding 90 percent through JFM 2021) with La Nina conditions now likely to extend through MAM 2021 (probabilities exceeding 60 percent).
30-DAY OUTLOOK DISCUSSION FOR DECEMBER 2020
The monthly temperature and precipitation outlooks for December 2020 are based on statistical and dynamical model guidance, the latest Week 3-4 CFS and ECMWF forecasts, La Nina composites, recent observations, coastal sea surface temperatures (SSTs), and consideration of the current and predicted states of the Madden Julian Oscillation (MJO).
The latest ENSO Diagnostic Discussion (issued on November 12th) indicates a La Nina advisory remains in effect. La Nina is likely to continue through the Northern Hemisphere winter 2020-21 (~95% during January through March) and into spring 2021 (~65% chance during March through May). The cold event strengthened during October 2020, with below average near-equatorial SSTs extending from near the Date Line eastward to near the South American coast. A moderate to strong La Nina is predicted in December, with enhanced trade winds over most of the tropical Pacific, enhanced convection over Indonesia, suppressed convection near the Date Line, and a deep reservoir of relatively cold subsurface water in the central and eastern near-equatorial Pacific.
The MJO crossed the Western Hemisphere during early November, favoring the development and rapid intensification of Hurricanes Eta and Iota in the Atlantic basin. At present, the enhanced convective envelope of the MJO is located over the western Indian Ocean. Widely divergent model forecasts indicate much uncertainty in the evolution of the MJO during the next two weeks, but current thinking favors a slow eastward progression and gradual decay of the subseasonal signal. Typically, the MJO crossing the Indian Ocean results in an anomalous wavetrain that is associated downstream with above normal temperatures focused over the Great Lakes region 3-4 weeks afterward, and a slight tilt toward below normal temperatures over the western CONUS. However, it remains to be seen if the tropical convection can force such a response, given the uncertainty regarding the future state of the MJO.
Temperature
The temperature outlook for December 2020 favors above normal mean temperatures across most of the Lower 48 states, with the greatest probabilities (50%+) over eastern and southern sections of the CONUS. This is supported by the Week 3-4 CFS and ECMWF forecasts for the first half of December, the C3S model suite (Copernicus) and nearly all the constituent models that comprise the NMME, including the CFS, for the month of December. Above normal mean temperatures over approximately the southern half of the CONUS are broadly supported by an ENSO regression technique, where historical temperature anomaly data for the time of year is regressed onto the standardized Nino 3.4 index. Above normal temperatures are also favored over northern and western sections of Alaska, based on dynamical guidance, long-term trends and the proximity of relatively warm coastal SSTs. The greatest odds for anomalous warmth (50%+) are predicted for the northwest corner of the state. For the southeastern Mainland of Alaska and the adjacent Panhandle region, below normal temperatures are slightly favored, consistent with some of the dynamical model guidance and historical La Nina composites. Below normal temperatures are also favored, for similar reasons, across the Pacific Northwest and the Northern Rockies. Elsewhere, Equal Chances (EC) of below, near, and above normal mean temperatures are predicted.
Precipitation
The precipitation outlook for December 2020 favors above normal precipitation amounts over western Alaska, portions of the northwestern CONUS, and from the Ohio Valley northeastward across northern New England. These anomalies are commonly observed during cold season La Ninas, and are also well supported by the C3S Copernicus suite, and some of the dynamical models that go into the NMME. These precipitation anomalies appear to be underdone in the calibrated version of the NMME (the PAC), with only the Northern Rockies wet signal getting past the skill mask (which incorporates historical skill information). Below normal precipitation amounts are favored over approximately the southern one-third to two-thirds of the CONUS, which is predicted by many of the dynamical models, though with variations on the overall spatial pattern. Below normal precipitation is also consistent with cold season La Ninas across much of the South, and is likely to exacerbate drought in the Southwest, especially after the paltry monsoon this past summer. Probabilities of below normal precipitation exceed 50% for portions of the Southwest, Southern Rockies, Southern Great Plains, and into Louisiana. Near the southern Alaska coast, from south-central Alaska to the Northern Panhandle region, drier-than-normal conditions are possible, though the tilt towards below normal precipitation is weak. This signal occurs frequently enough to show up in historical La Nina composites and has some support from the C3S and NMME dynamical model suites. The Alaska precipitation outlook for December is also consistent with the idea of persistent high pressure over the Gulf of Alaska, with onshore flow (wetter) in western Alaska, and offshore flow (drier) in southeastern Alaska. Elsewhere, Equal Chances (EC) of below, near, and above normal precipitation amounts are forecast.
SUMMARY OF THE OUTLOOK FOR NON-TECHNICAL USERS (focus on DJF)
La Nina conditions are present across the equatorial Pacific Ocean, as indicated by current oceanic and atmospheric observations. La Nina conditions are likely to continue through the Northern Hemisphere winter and into Spring 2021.
Temperature
The December-January-February (DJF) 2020-2021 temperature outlook favors above-normal seasonal mean temperatures for a majority of the CONUS and for northwestern areas of the Alaska mainland and Aleutian Islands. The greatest probabilities (larger than 60 percent) are forecast for parts of the Southwest and the Rio Grande Valley. Below-normal temperatures are most likely for southeastern Alaska, parts of the Pacific Northwest, the far Northern Rockies, and the Northern Plains.
Precipitation
The DJF 2020-2021 precipitation outlook depicts enhanced probabilities for above-normal seasonal total precipitation amounts for northwestern areas of Alaska, as well as across the northern tier of the contiguous U.S., including parts of the Pacific Northwest, the Northern Rockies, the northern Great Plains, the Ohio Valley, and the Great Lakes Region. Below-normal precipitation is most likely for Central and Southern California, stretching eastward to include the Southwest, the southern and central Great Plains, the Lower Mississippi Valley, and the Southeast.
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 TOOLS USED FOR U.S. TEMPERATURE AND PRECIPITATION OUTLOOKS
With an increasing likelihood of the persistence of La Nina conditions, the seasonal outlooks utilized typical 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 Nino3.4 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 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.
PROGNOSTIC DISCUSSION OF OUTLOOKS – DJF 2020 TO DJF 2021
TEMPERATURE
The December-January-February (DJF) 2020-2021 outlook favors above-normal seasonal mean temperatures for a majority of the CONUS based on the impacts of a La Nina climate state along with consistent calibrated dynamical model guidance from the NMME models and the consolidated multi-model ensemble mean. Highest probabilities are forecast for the Southwest and Rio Grande Valley where decadal trends are strongly positive and dynamical model probabilities are the greatest. There is less forecast confidence and lower probabilities of above-normal temperatures for the Ohio Valley and Mid-Atlantic, due to uncertainties in the phase of the Arctic Oscillation / North Atlantic Oscillation and other mid-latitude variability, especially during the months of December 2020 and January 2021. There is a slight increase in the probabilities of below-normal temperatures for parts of the Pacific Northwest, far Northern Rockies, and the Northern Plains, consistent with impacts from La Nina conditions, especially later in the winter season. Above normal sea surface temperatures and decadal temperature trends favor above-normal temperatures for areas in northern and western Alaska.
Expected impacts associated with La Nina conditions, however, favor below-normal temperatures for southeastern Alaska. Model forecasts predict a transition from likely above normal temperatures to a greater probability of below normal temperatures and, as a result, there is high uncertainty in the DJF temperature forecasts for parts of the western Midwest and Great Lakes regions. A forecast of Equal-Chances (EC) of below-, near-, or above-normal seasonal mean temperatures for this region is indicated for DJF 2020-2021.
The evolution of the temperature outlooks from DJF 2020-2021 through MAM 2021 are consistent with typical impacts associated with La Nina conditions spanning the winter and spring seasons indicated by statistical, dynamical and hybrid (combined statistical and dynamical) model guidance products. This typical historical evolution is augmented by the latest dynamical model guidance and consideration of decadal trends . Outlooks show a gradual expansion of likely below-normal temperatures for areas along the northern tier of the CONUS, primarily for parts of the Northern Plains and western Great Lakes as well as portions of the Pacific Northwest and southern Alaska, through early spring 2021. Later in Spring 2021, probabilities for above-normal temperatures across parts of the Southwest and southern Great Plains are enhanced by potential worsening or developing low soil moisture conditions and its warming influence on surface temperatures during the warm season, following likely below-normal precipitation associated with La Nina.
PRECIPITATION
Similar to the DJF 2020-2021 temperature outlook, the DJF 2020-2021 outlook for seasonal total precipitation amounts are strongly influenced by typical impacts associated with a predicted La Nina climate state during the winter and spring seasons. The greatest confidence in the precipitation outlooks for the DJF 2020-2021 and JFM 2021 seasons are for elevated probabilities, greater than 60 percent, for below-normal precipitation for a region extending from southeastern Arizona to western Texas. Probabilities exceeding 50 percent for below-normal precipitation during DJF 2020-2021 extend from Southern California across parts of the Southwest, into the Southern Plains, along the Gulf Coast, and across the Florida Peninsula. From JFM through MAM 2021, the area of probable below normal precipitation evolves to extend further north into the Great Plains, while decreasing in extent over the Southeast, such that eventually only a small area of likely below normal precipitation remains over the Southwest in AMJ 2021.
Above-normal precipitation is most likely for parts of the Pacific Northwest extending eastward across the northern Rockies and northern Plains to include the Great Lakes, the Ohio Valley, and parts of the Northeast region during the DJF 2020-2021 through FMA 2021 seasons. This favored region of above-normal precipitation continues over a smaller area of the Pacific Northwest into MAM 2021. Above-normal precipitation is favored for western Alaska during similar seasons while below-normal precipitation is favored over the southeastern Alaska Mainland and the Alaska Panhandle, consistent with typical La Nina impacts.
The precipitation outlooks in subsequent seasons are driven primarily by decadal trends , predicting below normal precipitation for parts of the northwestern CONUS during the warm seasons, MJJ through JAS 2021, and above normal precipitation for parts of the central and eastern CONUS from AMJ 2021 through ASO 2021, continuing for parts of the Northeast for SON and OND 2021. Areas labelled EC indicate a forecast of climatological probabilities for below-, near- or above-normal seasonal total precipitation amounts.
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.
Nov 12. 15, 2020 Prediction from 1st Oct., 2020 ENSO forecast:
ENSO forecast:
Observation shows that the SST anomalies associated with the developing La Niña have covered most part of the tropical Pacific now. The SINTEX-F predicts that this La Niña will be more La Niña Modoki-like and will persist in this year. 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 warmer-than-normal at present. The ensemble mean prediction suggests that the present condition will return to a neutral state in the boreal spring 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, western Canada, Greenland, northern Brazil, Indochina Peninsula, and India. In boreal spring, the model predicts the persistence of a similar condition.
As regards to the seasonally averaged rainfall in boreal winter, a drier-than-normal condition is predicted for southern Alaska, southern U.S.A., eastern Brazil, La Plata Basin, southern Africa, southern Europe facing the Mediterranean Sea, eastern China, and Indonesia. In contrast, western Canada, most part of Brazil, northern Australia, Philippines, northern Europe, and northern Eurasia will experience a wetter-than-normal condition.
In boreal spring, a wetter-than-normal condition is predicted for Alaska, Canada, Mexico, Brazil, northern Australia, Indochina Peninsula, Philippines, central Africa, northern Europe, northeastern China, eastern Russia, and the Korean Peninsula. In contrast, western part of U.S.A (including California), some part of southern Africa, 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, southern part of Japan will experience a drier-than-normal condition in winter. In spring, northern and southern parts of Japan will experience slightly wetter-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 November 19, 2020
Discussion for the Seasonal Drought Outlook
Tools used in the U.S. Seasonal Drought Outlook (SDO) included the Climate Prediction Center (CPC) temperature and precipitation outlooks for December (Dec) 2020 and December through February (DJF) 2020-21, 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 Dec and DJF, and initial conditions for parameters such as soil moisture. Existing drought areas on the forecast are based on the November 19th U.S. Drought Monitor. Conditions currently reflect the presence of La Niña, with a 95% chance for La Niña to persist through the Northern Hemisphere winter, and forecaster consensus indicates 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 California, the Great Basin, and the Four Corners region, but recent storminess over the Northwest and northern Rockies brought some relief to ongoing drought conditions during the past few 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 towards the south. Both the CPC Dec and DJF 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 the California Oregon border eastward to southern Idaho and northern Utah. DJF is a very wet time of year for the western U.S., and winter moisture is critical to build mountain snowpacks that feed reservoirs during the dry season months. Therefore, the wet signal favored for the Northwest favors further drought reductions across the Northwest and northern Rockies, with drought elimination possible for Washington state. In contrast, drought persistence is favored for the southern portions of the Western Region, with drought expansion possible across southern California. Above-normal temperatures favored for DJF may exacerbate the drought conditions by keeping mountain snow levels abnormally high.
Forecast confidence is high for the Western region.
Drought changes across the High Plains were chaotic during the past four weeks, with degradations observed across North Dakota, and improvements or degradations fairly localized in other states. The La Niña favored anomalous precipitation dipole is reflected in the seasonal outlook, where above-median precipitation is favored for the Dakotas and northern Wyoming, while below-median precipitation is favored for southern Colorado and most of Kansas and Nebraska. DJF 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 DJF period. Therefore, little change in the incipient drought conditions is likely during the outlook period despite the tilt towards above-median precipitation. Further west, some drought improvement is possible across the higher elevations of northwestern Wyoming due to a potential for above-average snowpack building. Further south, despite the dry time of year, a potential for above-normal temperatures and continued dryness favors eastward expansion 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. Further east, a patchwork of increasing dryness along the Gulf Coast States and Southeast was interrupted by swaths of heavy rainfall from tropical cyclones, including Hurricane Delta, which impacted the central Gulf Coast, and Tropical Storm Eta, which brought flooding rains to parts of the Florida peninsula. La Niña winters tend to be warm and dry across the south-central and southeastern CONUS, and this is reflected in the probabilistic monthly and seasonal outlooks. Therefore, drought development is likely for the rest of Texas (except extreme eastern Texas), and along a swath extending from southeastern Louisiana eastward across southern Alabama, Georgia, South Carolina, and northern Florida, as these areas have growing precipitation deficits out to 60 days. In contrast, areas that received heavy rainfall from tropical cyclones are favored to begin drying out during DJF, but widespread drought development is unlikely.
Forecast confidence is high for the Southern and Southeastern regions.
Small pockets of drought currently extend across parts of northern Missouri, central Illinois, and northern Indiana, as these areas missed out on heavy precipitation that overspread parts of the Ohio River Valley in association with a strong midlatitude cyclone and the remnants of Hurricane Delta. Additionally, unusually long-lasting drought conditions also persist across central Pennsylvania, upstate New York, and much of New England, though recent storminess brought some relief to these regions. During La Niña winters, the climatological winter storm track shifts to the west, favoring the Ohio Valley through interior New England. This is reflected by enhanced chances for above-median precipitation in the DJF seasonal outlook. Therefore, drought removal is favored for the Corn Belt, and additional drought reductions are the favored outcome across the Northeast. While enhanced chances for above-median precipitation do not extend to coastal New England, climatology favors frequent coastal storms during the winter, and a few such storms should be sufficient to erode the ongoing drought.
Forecast confidence is high for the Midwestern region and moderate for the Northeastern region.
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 three-quarters of the state currently experiencing drought conditions. The start of the wet season in January, as well as enhanced trade winds associated with the La Niña response, favor above-average rainfall, particularly across the leeward sides of the islands. Therefore, drought improvement or removal is favored during DJF.
Forecast confidence is high for Hawaii.
ENSO Considerations
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.
