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000
FXUS21 KWNC 222009
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT April 22 2024

SYNOPSIS: A persistent pattern featuring a long-wave mid-level low pressure
over western North America and mid-level high pressure farther east is expected
to support a slight risk of episodes of heavy rainfall across the central
Contiguous United States (CONUS) throughout the week-2 period, with a slight
risk for episodes of high winds for much of the Southwest, Rockies, and Great
Plains. The timing and magnitude of individual episodes of heavy rain and high
wind are uncertain at this time, but the greatest risk for heavy precipitation
is anticipated for parts of the Southern and Central Plains and Lower
Mississippi Valley around the middle of week-2, thus a moderate risk is
designated. The episodes of heavy rainfall are expected to support increased
risk of  flooding across parts of the central and southern Great Plains and
Lower Mississippi Valley.

HAZARDS

Moderate risk of heavy precipitation for portions of the Southern and Central
Plains and Lower Mississippi Valley , Wed-Fri, May 1-3.

Slight risk of heavy precipitation for much of the northern High Plains, Great
Plains, the Middle and Lower Mississippi Valley, the Upper Midwest, and the
Lower Ohio Valley, Tue-Mon, Apr 30-May 6.

Slight risk of high winds for much of the Southwest, Rockies, and Great Plains,
Tue-Mon, Apr 30-May 6.

Flooding possible for parts of the Central and Southern Plains, Mississippi
Valley, and Ohio Valley.

DETAILED SUMMARY

FOR THURSDAY APRIL 25 - MONDAY APRIL 29:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR TUESDAY APRIL 30 - MONDAY MAY 06: 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 Canadian ensemble mean shows a more progressive pattern, moving the western
CONUS mid-level trough axis eastward into the central CONUS by the end of the
period. This is not the favored solution currently, but it does increase
uncertainty in the outlook, and this uncertainty is further increased by the
lack of consensus on the timing and strength of individual surface lows
expected to eject eastward out of the Rockies.



The persistent mid-level trough in the West favors a 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 Since the pattern is expected
to be quasi-stationary, the risk extends through the entire period, but the
lack of model consensus on the timing and strength of individual features makes
it difficult to identify smaller areas and time frames that might be at higher
risk than others. Currently, a moderate risk of heavy precipitation is posted
for days 9 through 11 (Wed-Fri May 1-3) for  portions of the Southern and
Central Plains and Lower Mississippi Valley. This delineation is based on the
highest odds for 3-day rainfall totals to exceed 2 inches in the uncalibrated
ECMWF ensembles. Given the significant model uncertainties, and the fact that
more than one system is expected to eject from the western mid-level trough
eastward through the central CONUS, a slight risk of heavy precipitation is
posted for this broad region for the entirety of week-2.



Anticipated heavy rain combined with expected antecedent heavy precipitation
during week-1, supports a possible flood risk near and downstream from areas
where the greatest rainfall totals are expected over the next two weeks. The
GEFS, Canadian ensemble mean, and ECMWF ensemble mean all show 2.5 to locally 5
inches of rain falling during the last week of April and the first week of May
on a region extending from central and eastern Oklahoma and adjacent Texas
northeastward across eastern Kansas, northern and western Missouri, much of
Iowa, and some adjacent areas.



The anticipated long-wave pattern also favors cyclogenesis to the lee of the
Rockies, but the timing and strength of any surface low pressure system that
forms is highly uncertain at this time, so a large area from the Rockies
eastward into the central Great Plains has a slight risk for one or more
episodes of high winds throughout the week-2 period.



Farther north, models generally favor slightly below normal 500-hPa heights
across Alaska, with a trough axis 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: Rich Tinker

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