159
FXUS21 KWNC 231917
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT April 23 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 episodes of heavy rainfall across the central Contiguous United
States (CONUS) through the middle of week-2, 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 Middle Mississippi Valley early in
the period, thus a moderate risk is designated. The forecast episodes of heavy
rain are expected to occur over an area expecting very heavy precipitation
prior to week-2, supporting an increased risk of  flooding across parts of the
central and southern Great Plains and much of the Mississippi Valley.

HAZARDS

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

Slight risk of heavy precipitation for much of the northern Rockies and High
Plains, Great Plains, the Mississippi Valley, the Upper Midwest, and the Lower
Ohio Valley, Wed-Sun, May 1-5.

Slight risk of high winds for much of the Southwest, Rockies, and Great Plains,
Wed-Sun, May 1-5.

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

DETAILED SUMMARY

FOR FRIDAY APRIL 26 - TUESDAY APRIL 30:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR WEDNESDAY MAY 01 - TUESDAY MAY 07: 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, but has shifted
toward the solutions of the GEFS and ECMWF ensemble means, reloading the
mid-level trough near the West Coast late week-2 as the initial mid-level
trough moves into the central CONUS and dissipates. This boosts confidence in
the favored solution relative to yesterday. Unlike yesterday, both the 0Z ECMWF
ensemble mean and 6Z GEFS mean show weakening features late in the period, but
hinting that conditions may re-amplify beyond the week-2 period.



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. This quasi-stationary pattern
is expected to change little until weakening later week-2, keeping a slight
risk of heavy precipitation in place through the early and middle parts of the
period, May 1-5. Probabilistic Extremes Tools (PETs) from the GEFS and the
European ensembles have come into better agreement today, with both showing
enhanced odds for 3-day precipitation totals above both the one-inch and
85th-percentile thresholds in a swath from central Texas northeastward into
western parts of the Middle Mississippi Valley during the first 3 days of
week-2 (Wed-Fri May 1-3), supporting a moderate risk for heavy precipitation.



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. 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, this includes much
of  the south-central Great Plains 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 central Great Plains has a slight risk for one or more episodes of high
winds for May 1-5 until conditions ease later 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: Rich Tinker

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