Written by Sig Silber
At the end of every month, NOAA issues an update of their Early Outlook for the following month which, as usual, was issued on the Third Thursday of the month. Today, fifteen days later, NOAA has issued their Updated Outlook for May. We are seeing the transition from La Nina to ENSO Neutral but the transition is taking place slowly. The changes from the Early Outlook forecast issued on April 15, 2021, are less significant than we might have expected given that the Early Outlook for May was issued fifteen days ago. The drought continues.
Some housekeeping: On April 15, 2021, we published Part I of our analysis of the NOAA and JAMSTEC Three -to Four -Season Outlook and that can be accessed here. In that report, the May 2021 Early Outlook was issued. This article presents the NOAA update of their Early temperature and precipitation outlooks for May and the most recent Drought Forecast update for the month of May.
Please share this article – Go to the very top of the page, right-hand side for social media buttons. Also, feel free to send this article to anyone you feel will benefit from it. You can find the latest version of all our weather articles by consulting the Directory by clicking here and then clicking on the latest version of the article which is of interest to you.
First, we will compare the NOAA Early Outlook for May 2021 with the newly issued update.
Early Outlook Temperature
Updated Temperature Forecast
Early Outlook Precipitation
Updated Precipitation Forecast
Here is the discussion released with the forecast.
30-DAY OUTLOOK DISCUSSION FOR MAY 2021
During the latter half of April, negative SST anomalies in the tropical Pacific have continued to weaken. The Nino 3.4 index currently stands at -0.3C, representing a 0.2C decrease in magnitude since mid-April as ENSO Neutral conditions are becoming increasingly likely as we progress deeper into Spring. This evolving ENSO pattern would tend to favor below normal 500-hPa heights across much of the higher latitudes and anomalous ridging farther to the south across the western CONUS. Additionally, the continued weakening of the oceanic La Nina signal would tend to favor ridge development over the Bering Sea and an increasingly incoherent pattern across the eastern CONUS during the month of May. Compounding this uncertainty is the strengthening of an already strong ongoing MJO event, which recently transitioned from Phase 7 to Phase 8 in the Wheeler-Hendon classification. This strong MJO event would act to counteract the ENSO signal, increasing uncertainty and the likelihood of a variable pattern across much of the forecast domain. WPC short term and CPC week-2 outlooks, dynamical model guidance, trends , and analysis of antecedent boundary layer conditions were used to resolve competing background ENSO and MJO signals where applicable.
Above normal temperatures are favored for most of the southern half of the CONUS, consistent with CFSv2 dynamical model guidance and trends . However, the ongoing MJO event is increasingly favoring cold across much of the CONUS which is coinciding with diminishing ENSO influences, particularly across the eastern CONUS. Therefore, probabilities of above normal temperatures were reduced across the eastern CONUS relative to the previous May outlook. A small region of favored below normal temperatures was introduced for parts of the Great Lakes Region and Upper Mississippi Valley, where short term WPC and week-2 CPC outlooks indicate a fairly persistent cold signal. The highest probabilities of above normal temperatures are indicated from the Southern Plains to the Southwest, where a coherent warm signal remains from ENSO composites. Also, these elevated probabilities are consistent with low soil moisture (below the 10th percentile). The temperature outlook for Alaska is mostly similar to the previous May outlook as below normal temperatures remain favored for southwestern Mainland Alaska and parts of the Panhandle, consistent with ENSO composites. Confidence in the cold signal is relatively higher for southwestern Mainland Alaska relative to the Panhandle as the former has dynamical model support from the CFSv2. However, due in part to rapid snow melt, the area of favored below normal temperatures has been reduced across Alaska relative to the previous outlook. Above normal temperatures are likely for northern Mainland Alaska and the Aleutians, consistent with dynamical model guidance and recent trends .
Wetter than normal conditions are likely for much of the east-central CONUS, as a storm system early in the month will likely result in increased rainfall for much of this region. This area of enhanced above normal precipitation probabilities is mostly consistent with the previous May outlook. Exceptions are eastern portions of the Southern Plains and the western Gulf Coast Region, as the storm system currently affecting much of Texas is expected to linger into the weekend. Drier than normal conditions are likely farther to the west, from the southern High Plains to parts of the West Coast, consistent with dynamical model guidance and trends. A small area of enhanced probabilities of above normal precipitation was introduced to parts of the northern High Plains, consistent with CFSv2 dynamical model guidance. The precipitation outlook for Alaska is consistent with ENSO composites, which maintain a wet signal for the Alaska Panhandle and a dry signal for the southwestern Mainland. However, probabilities of below normal precipitation were reduced across southwestern mainland Alaska relative to the previous May outlook due to lack of dynamical model support.
So I am wondering if NOAA is providing us with the straight scoop here with respect to where we stand re ENSO. So I have added two additional graphics to those issued by NOAA today. Here is the first.
Back to reviewing what NOAA issued today. Here we compare the prior forecast to the new forecast as above but in a more compact format. This is simply the maps already presented organized in a different way to make comparison easier. Later, we show almost the entire month broken into four parts.
April 15, 2021 Forecast for May 2021 | April 30, 2021, Forecast for May 2021 | |
Temperature | ||
Precipitation |
Visual Consistency Testing.
It is useful to see how the full-month forecast fits with the set of partial-month forecasts that we have. These will continue to update in this article. It is important to remember that with the partial-month forecasts, we generally have about 25 days of the subsequent month to look at when we do this at the end of the month as we are doing now. It depends on what day of the week the month ends. So this month, which ended on a Friday, we have short-term forecasts that cover 28 of the 31 days in May so we are in very good shape to see if the whole is equal to the sum of the parts.
First Temperature
And Precipitation
I decided that even though the monthly and the Week 3 – 4 forecasts both were issued today, it is likely that the Week 3-4 discussion is more current than the discussion issued with the full month forecast. It probably goes through fewer review steps and it is for precipitation still an experimental tool. So I decided to show that discussion because of some lack of total consistency between the 28 days of short-term forecasts and the full month forecast. The lack of consistency is not major and there are three days at the end of the month for which we do not have forecasts. Nevertheless, I concluded that showing the Week 3-4 Discussion was useful so here it is.
Week 3-4 Forecast Discussion Valid Sat May 15 2021-Fri May 28 2021
La Nina conditions continued but weakened across the Pacific Ocean. Equatorial sea surface temperatures (SSTs) are below average from the west-central to eastern Pacific Ocean, as enhanced upper level westerly wind anomalies persist across most of the equatorial Pacific Ocean. The RMM index depicts an eastward propagation of the MJO from mid-March through early April, when the MJO became less coherent during mid-to-late April. The GEFS and ECMWF are in good agreement that the MJO resumes its eastward propagation over the Western Hemisphere during week-1, but uncertainty on its evolution increases later in week-2. The Week 3-4 temperature and precipitation outlooks are based primarily on dynamical guidance including the GEFS, CFS, Canadian, ECMWF, JMA, the Subseasonal Experiment (SubX) multi-model ensemble (MME) of experimental and operational ensemble prediction systems, with additional considerations for MJO, La Nina, decadal climate trends, and soil moisture conditions, 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. The CFS and ECMWF models feature near to above normal 500-hPa heights over the CONUS, while the JMA model favors above normal 500-hPa heights over the southern CONUS and near to below normal 500-hPa heights across the northern CONUS. The CFS and ECMWF models indicate anomalous ridging with above normal 500-hPa heights over Mainland Alaska, the Alaska Panhandle and the Aleutians. Near normal 500-hPa heights are likely over Hawaii.
Below normal temperatures are favored over parts of the Northern Plains, consistent with the CFS/ECWMF/JMA Equal Weighted and Manual Blend tools. Above normal temperatures are more likely across the remainder of the CONUS, under predicted near to above normal 500-hPa heights. The highest confidence for above-normal temperatures lies across the Southwest (greater than 70%), supported by most dynamical tools. Predicted ridging and above normal 500-hPa heights lead to predicted above normal temperatures over Alaska.
The dynamical model guidance is in reasonably good agreement on the spatial pattern of anomalous precipitation during the Week 3-4 period. Above normal precipitation is favored over parts of the Upper and Lower Great Lakes as well as the Ohio Valley, consistent with the CFS/ECWMF/JMA and CFS/ECWMF Equal Weighted as well as Manual Blend tools. Near to below normal precipitation is favored over the remainder of the CONUS, under the near to above normal 500-hPa heights, consistent with dynamical guidance. Anomalous ridging and above normal 500-hPa heights lead to enhanced probabilities of below normal precipitation over southern Mainland Alaska, the Alaska Panhandle, and the Aleutians.
Most dynamical model guidance, including the experimental SubX models, depict enhanced chances for above normal temperatures over the northwestern Hawaiian islands (Lihue and Honolulu). Equal chances of below or above normal precipitation are forecast for Hawaii, based on little to no signal among precipitation tools.
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-month period.
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-month period.
And with respect to drought, this was also issued on April 30, 2021
Latest Monthly Assessment – Heavy precipitation during late April and into the beginning of May is likely to result in either drought removal or improvement across northeast Colorado along with central to eastern Texas. Persistence is likely for western Texas and also throughout the long-term drought areas of the West. Recent dryness and elevated probabilities of below normal precipitation and above normal temperatures during May favor a return of drought to northwest Oklahoma and parts of the Texas Panhandle. Development is also favored to occur across southern Idaho and eastern Oregon by the end of May.
Broad persistence is favored for much of the northern to central Great Plains due to the long-term drought conditions and varying precipitation forecasts throughout the month. The first week of May is likely to be mostly dry, while the week-2 precipitation outlook on April 30 calls for elevated probabilities of above normal precipitation. Forecast confidence is low for the northern to central Great Plains since May is one of the wetter months of the year. Persistence is also forecast for Iowa, Wisconsin, and most of Michigan. Prospects for drought removal increase farther to the south across Illinois, Indiana, and Ohio due to rainfall predicted during the first week of May and larger probabilities (above 40 percent) for above normal precipitation in the updated May outlook.
Development for the eastern U.S. is most likely across parts of the Carolinas due to increasing short-term precipitation deficits and a relatively dry start to the month. Any lingering drought across southwest Florida is likely to end during May or shortly thereafter. Removal is also forecast for northwest Pennsylvania and western New York, while persistence is more likely for New England.
Drought persistence is forecast for the Big Island of Hawaii and development remains favored during the early summer. Alaska is forecast to remain drought-free through the end of May. Due to a recent drying trend and the latest precipitation forecasts, persistence or development are favored for Puerto Rico.
Looking back on April 2021 to relate the forecast for May to the actuals in April.
First April 2021 Temperature (29 out of 30 days).
And then April Precipitation (29 out of 30 days).
We then show the new forecast and the prior month’s actuals (less one day) side by side.
Prior Month (usually missing one day) | Forecast for current Month | |
Temperature | ||
Precipitation |
Conclusion
The purpose of this article was to present the updated Outlook for May 2021 and compare it with the Early Outlook. It is not to critique the updated Outlook for May based on our opinions but we point out possible inconsistencies if we find them and we have. We have assessed the extent to which the monthly outlook seems to fit with the other forecasts provided by NOAA. In reality, all forecasts are obsolete when issued as the situation changes day by day or six hours by six hours i.e. each new model run. In our other articles, we continually examine the current forecasts from NOAA including Day 1- 5, 6-10, 8-14, and Week 3 and 4. So we are continually looking at approximately 25 days into the future.
On May 13, 2021, IRI/CBC will issue their ENSO forecast and we will present that and critique it. What they present on May 13 is likely to begin to impact the forecasts for the next three months and should shed some light on this Summer. It is an ongoing process and we attempt to provide our readers with the best current assessment of the next approximately 25 days (except when we cover the Four-Season Analysis or medium-frequency cycles such as ENSO in which cases we are looking farther out than 25 days), so our analysis is pretty much ongoing. We also publish a “LIVE” Weather Article which updates in real-time to reflect NOAA Weather Headlines and for those interested provides detailed information down to current warnings in place. All of our reports including past versions of these reports can be located via the Directory/Archive which can be accessed here.