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000
FXUS21 KWNC 241826
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT April 24 2024

SYNOPSIS: In week-2, heavy precipitation remains a concern across the central
contiguous U.S. (CONUS) with mid-level low pressure forecast across the West
and mid-level high pressure likely across the eastern CONUS. This will allow
for a continued active pattern leading to chances for heavy rain and
thunderstorms across the Plains and Mississippi Valley. The strongest chances
for heavy rain have shifted north relative to the prior forecasts. Heavy rain
is also expected in week-1, and additional precipitation in week-2 could cause
localized flooding over portions of the Plains, Mississippi Valley, and western
Great Lakes. Associated with mid-level low pressure over the West, chances for
periodic high winds are possible for much of week-2 over much of the
southwestern CONUS, Rockies, and Plains.

HAZARDS

Moderate risk of heavy precipitation for portions of the Central Plains and
Middle and Upper Mississippi Valley, Thu-Fri, May 2-3.

Slight risk of heavy precipitation for portions of the Northern Rockies,
Plains, Mississippi Valley, and western Great Lakes, Thu-Sun, May 2-5.

Slight risk of high winds for much of the Southwest, Rockies, and Great Plains,
Thu-Sun, May 2-5.

Flooding possible for parts of the eastern Central and Southern Plains,
Mississippi Valley, and parts of southwestern Great Lakes.

DETAILED SUMMARY

FOR SATURDAY APRIL 27 - WEDNESDAY MAY 01:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR THURSDAY MAY 02 - WEDNESDAY MAY 08: The GEFS and ECMWF ensemble means are
in good agreement indicating a fairly stagnant pattern featuring a
moderately-amplified mean 500-hPa long-wave trough over or near western North
America, and a downstream mid-level ridge with an axis over the eastern CONUS.
The western trough looks likely to weaken and diminish by the middle of the
week-2 period. However, a trough across Alaska may help to reestablish negative
anomalies across the eastern Pacific Ocean at the end of the period.



The persistent mid-level trough in the West favors southerly surface flow
bringing unusually moist air northward into the central CONUS, where
cyclonically-curved divergent flow aloft is expected. This combination puts a
large part of the central CONUS at increased risk for one or more episodes of
heavy precipitation and localized thunderstorms. This quasi-stationary pattern
is expected to persist into the middle of week-2 before weakening later in the
period, keeping a slight risk of heavy precipitation in place through the early
and middle parts of the period, May 2-5. Probabilistic Extremes Tools (PETs)
from the GEFS and the ECENS are in less agreement relative to yesterday. The
GEFS has lower precipitation amounts relative to the ECENS and this increases
the uncertainty. Further, the heaviest precipitation totals are in areas that
are currently experiencing moderate to severe long term drought. Nevertheless,
raw tools from the GEFS and ECENS forecast at least a 30% chance of 1 day
precipitation amounts exceeding 0.5 inches on days 8 and 9. Therefore, a
moderate risk of heavy precipitation remains posted for May 2-3 across portions
of the Central Plains and Middle and Upper Mississippi Valley. The risk has
shifted north relative to yesterday to capture the strongest probabilities
among the raw forecast guidance and the ECENS PET.



The heavy precipitation and thunderstorm activity anticipated during week-2 is
not expected to be as intense as conditions in the short-term, but will
exacerbate impacts related to excessive precipitation in week-1. The combined
effects of two weeks of excessive precipitation support a flood risk near and
downstream from areas where the greatest rainfall totals are expected including
eastern portions of Kansas and Oklahoma and much of the Mississippi Valley
according to the GEFS and ECMWF ensemble means. Runoff from heavy rainfall is
expected to enhance the flood risk farther to the south and east (downstream)
across the Lower Mississippi Valley.



The anticipated long-wave pattern also favors cyclogenesis near or to the lee
of the Rockies, with higher surface pressures forecast farther east and west.
The timing and strength of any surface low pressure system that forms is
uncertain, so a large area from the Southwest and the Rockies eastward through
the Great Plains has a slight risk for one or more episodes of high winds for
May 2-5 until the mid-level trough across the West weakens by the end of week-2.



Farther north, models generally favor below normal 500-hPa heights across
Alaska, with a trough axis near or west of the Mainland that favors stormy
weather near the coast of south-central and southeastern Alaska. This pattern
may support one or more periods of enhanced precipitation and winds, but
conditions are not expected to reach hazardous criteria precluding associated
hazards from being designated.

FORECASTER: Ryan Bolt

$$