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HOME> Outlook Maps>Seasonal Forecast Discussion
 
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.

******************************************************************************
********** 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.
$$

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