Prognostic Discussion for Monthly Outlook 
NWS Climate Prediction Center College Park MD 
300 PM EDT Fri Apr 30 2021 
 
 
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. 
 
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********** Previous forecast discussion from mid-April is below ************ 
****************************************************************************** 
 
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 cas e 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 . 
 
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. 
 
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. 
 
FORECASTER: Scott Handel 
 
The climatic normals are based on conditions between 1981 and 2010, following 
the World Meteorological Organization convention of using the most recent 3 
complete decades as the climate reference period.  The probability anomalies 
for temperature and precipitation based on these new normals better represent 
shorter term climatic anomalies than the forecasts based on older normals. 
 
The next monthly outlook...for Jun ... will be issued on Thu May 20 2021 
 
These outlooks are based on departures from the 1981-2010 base period. 
$$