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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 Wed Mar 31 2021


30-DAY OUTLOOK DISCUSSION FOR APRIL 2021

The updated April 2020 outlook is based on the April outlook released in
mid-March adjusted using the current WPC outlooks for week 1 and CPC outlooks
for the extended range and week 3-4, while considering current climate
conditions, including current La Nina conditions, and following the most recent
subseasonal and monthly integrated dynamical model guidance. Although La Nina
conditions persist over the tropical Pacific, the weekly Nino 3.4 sea surface
temperature (SST) anomaly was observed to be about -0.5 degree C, and
conditions are likely to transition to ENSO-neutral during spring. The Madden
Julian Oscillation (MJO) active phase is currently over the Indian Ocean as
determined by the RMM indices, and forecast by dynamical models to progress
into the Western Pacific in the next couple weeks. The impacts of the current
MJO and ENSO state and the decadal trends are mostly consistent with current
dynamical model forecasts for temperature and precipitation for April.

The updated April temperature outlook has only small changes relative to the
prior outlook. Most notably, the area of likely below normal temperatures in
the Pacific Northwest increased, while the area of probable above normal
temperatures over the West decreased. Probabilities of above normal
temperatures over parts of the Northern Plains, western Great Lakes region, and
northern New England increased to be over 60 percent. Dynamical models predict
ridging generally over the CONUS during April, with some eastward progression
from the western to the eastern CONUS during the month. While above normal
temperatures are expected for most of the West, except for near coastal areas,
early in the month (based on WPC outlooks), near to below normal temperatures
are favored from the Pacific coast to the Rocky Mountains in the recent CPC
6-10 day and week 2 outlooks, leading to an increased probability for below
normal temperatures for much of the Pacific Northwest in the updated April
outlook, and equal chances (EC) of above, near and below normal temperatures
for much of the remaining areas west of the Rockies. Enhanced probabilities of
above normal temperatures remain for interior parts of Southern California,
eastern Nevada, the Northern Rockies, and the rest of the CONUS to the east
coast, in the updated April outlook. Confident outlooks for above normal
temperatures for eastern areas of the Northern Plains and western areas of the
Great Lakes region, throughout the month of April, and for northern areas in
the Northeast from the 6-10 day to the week 3-4 periods, led to an increase in
the probabilities of above normal temperatures in the updated April outlook.
Expected below normal temperatures early in the month of April for much of the
Southeast, offset by likely above normal temperatures in the CPC 6-10 day, week
2, and week 3-4 outlooks, led to a decrease in the probabilities of above
normal temperatures for this region in the monthly update. Below normal
temperatures are favored in both the half-month lead and updated monthly
outlook for southern Mainland Alaska and the Alaska Panhandle, with a
persistent circulation pattern and troughing over the state. Above normal
temperatures continue to be favored for northern areas of the state of Alaska,
supported by decadal temperature trends .

The updated April precipitation outlook indicates some persistence of the
relatively dry pattern forecast over the CONUS at a half-month lead, with some
notable changes. The WPC forecasts precipitation for Northern California and
western Oregon in the first few days of April, which are near climatological
normal amounts for these areas. The CPC 6-10 day outlook calls for near normal
precipitation, and the week 3-4 outlook indicates equal chances of above and
below median precipitation for much of the West. These outlooks are consistent
with recent CFS precipitation forecasts for April that predict near to above
normal precipitation for much of the West. Therefore, the chances of below
normal precipitation have decreased relative to the half-month lead April
outlook for southern Oregon, western Idaho, Northern California, and western
Nevada, and equal chances of above, near and below normal precipitation are
indicated in the monthly update. Uncertainty has increased and probabilities of
below normal precipitation have decreased for west Texas and parts of the
eastern Southern Plains states, due to weak probabilities for above normal
precipitation in the extended range. Probabilities for above normal
precipitation have increased for parts of the Northern Plains into the far
western Great Lakes region, as outlooks across all lead times during April
predict slightly enhanced probabilities of above normal precipitation. The area
of likely above normal precipitation for western areas of Mainland Alaska have
expanded into parts of the Alaska Interior, with persistent enhanced westerly
flow in model forecasts.

******************************************************************************
********** Previous forecast discussion from mid-March is below ************
******************************************************************************

The April 2021 monthly temperature and precipitation outlooks are based
primarily on dynamical model forecast guidance in the context of persistent La
Nina conditions that are expected to transition to ENSO neutral conditions
later in spring. However, La Nina conditions are expected to persist into April
and play a role in climate conditions over the forecast domain. In addition,
some areas of the forecast domain consider current land surface soil moisture
and nearby ocean surface temperatures.

The MJO was considered but played a lesser role in the outlook, as there is
substantial uncertainty whether the MJO signal currently over Africa will
persist and propagate eastward during April and lead to significant subtropical
impacts. This uncertainty is due in part to expected interaction of any
eastward propagating MJO convective signal with suppressed convection over the
west-central equatorial Pacific, related to La Nina. Also, statistically
significant impacts on higher latitudes from the MJO are smaller in spring,
compared to winter months.

The April 2021 temperature outlook favors above-normal monthly mean
temperatures for nearly all of the CONUS, with the exception of the Pacific
Northwest, eastward into parts of the northern Intermountain West, and
southward along much of the immediate coastline of California. This outlook is
supported largely by dynamical model forecast guidance, from the models of the
NMME and IMME, which are in very good agreement over much of the CONUS. Decadal
temperature trends contribute to higher probabilities for above-normal
temperatures across the southern tier of the CONUS and into the Northeast
region. A soil moisture deficit across much of the western CONUS may contribute
to increased probabilities of above-normal temperatures in some regions of the
West, particularly the Southwest, where model guidance leads to probabilities
exceeding 60 percent in some areas of southeastern New Mexico and western
Texas. A much larger area of probabilities for above-normal temperatures
exceeding 50 percent over the West is supported by the consensus forecast of
the NMME. Typical temperatures observed during April under La Nina conditions
were considered, and support increased chances of below-normal temperatures
over parts of the Pacific Northwest. Dynamical model guidance, as well as
persistent impacts of La Nina, favor increased probabilities for below-normal
temperatures across southern Mainland Alaska into the Alaska Panhandle, while
decadal temperature trends are the primary forcing leading to likely
above-normal temperatures across northwestern coastal areas of Mainland Alaska
and the North Slope.

For precipitation, consistent dynamical model forecast guidance from models of
the NMME and typical La Nina impacts in spring favor below-normal precipitation
stretching from California and southern Oregon eastward across the Central and
Southern Rockies into the Central and Southern Plains, the Lower Mississippi
Valley, and much of the Southeast region. Increased precipitation variability
and decreased predictability in dynamical model forecasts leads to a lower
probability for below-normal precipitation for some areas of the Southern
Plains. Equal Chances (EC) of above, near and below-normal precipitation are
indicated for remaining areas of the CONUS, including drier areas of the
Southwest. Inconsistency between the typical above-normal precipitation signal
due to La Nina and dynamical model forecasts of below-normal precipitation for
parts of the eastern Central Plains leads to a forecast of EC for this area in
the April outlook.

The consensus of dynamical model forecast guidance favors above-normal monthly
total precipitation amounts for parts of the southern Alaska Panhandle,
supported by typical impacts of La Nina conditions. La Nina conditions support
above-normal precipitation for western Mainland Alaska, as does the consensus
of dynamical model guidance. EC is forecast for remaining areas of Alaska,
where climate signals are weak and dynamical model forecasts are inconsistent.

FORECASTER: Dan Collins

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 May ... will be issued on Thu Apr 15 2021

These outlooks are based on departures from the 1981-2010 base period.
$$

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