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000
FXUS21 KWNC 261833
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT April 26 2024

SYNOPSIS: Early 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. 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. By the middle of the period the
longwave height pattern may break down as more zonal flow is forecast, reducing
risks for hazardous weather. Rapid onset drought may become a concern in parts
of the Florida peninsula over the next few weeks.

HAZARDS

Slight risk of heavy precipitation for portions of the Plains, Mississippi
Valley, and western Great Lakes, Sat-Sun, May 4-5.

Slight risk of high winds for much of the Southwest, Rockies, and Great Plains,
Sat-Sun, May 4-5.

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

Rapid Onset Drought risk for portions of central Florida.

DETAILED SUMMARY

FOR MONDAY APRIL 29 - FRIDAY MAY 03:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR SATURDAY MAY 04 - FRIDAY MAY 10: The GEFS and ECMWF ensemble means are in
good agreement across the eastern CONUS indicating a mid-level ridge early in
the period. But upstream, the ensemble means disagree on the strength of a
reloading mid-level trough near and along the West Coast. The ECENS maintains a
much weaker trough at the end of week-1 and into week-2, whereas the GEFS has a
much more pronounced trough for nearly the same period . However, this does not
seem to have too big of an impact on the chances for hazardous weather across
the CONUS at this time. A trough across Alaska early in week-2 is forecast to
bring a renewed chance for negative 500hPa height anomalies and troughing
across the West by the end of the period. At the same time, negative mid-level
height departures are forecast to develop along and near the Northeast coast.



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 through the early part of week-2 before weakening later
in the period, prompting a slight risk of heavy precipitation in place through
the early part of the period, May 4-5. Probabilistic Extremes Tools (PETs) from
the GEFS and the ECENS are in better agreement relative to yesterday, which
both  highlight a similar area across the Plains and Mississippi Valley with a
20% chance for precipitation to exceed the 85th climatological percentile or 1
inch of precipitation. Raw tools from the GEFS and ECENS are also in more
agreement over this positioning with the ECENS remaining slightly more robust
in its probabilities for exceeding an inch over a 3-day period..



The heavy precipitation and thunderstorm activity anticipated during early in
week-2 may exacerbate impacts related to excessive precipitation in week-1. The
combined effects of two weeks of excessive precipitation support a flooding
possible risk near and downstream from areas where the greatest rainfall totals
are expected including eastern portions of Kansas, 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 early in the period 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 4-5 until the mid-level trough across the West
weakens by the end of week-2. Some of the raw ensemble model guidance is also
indicating increased chances for high winds to become reestablished by the
middle to end of the week-2 period. But uncertainty in the overall height
pattern reduces confidence and no wind hazard is posted for later in the period
at this time. This potential will continue to be monitored in upcoming outlooks.



In the Southeast, above-normal temperatures and near- to below-normal
precipitation is forecast during weeks-1 & 2. In parts of the central Florida
peninsula and the North Carolina piedmont soil moisture levels are declining
due to limited precipitation in recent weeks and months. Recent weeks have seen
areas of abnormal dryness (D0) developing and expanding across both of these
regions. As we approach the middle of May, increasing sun angles and
temperatures in the 80’s to 90’s deg F. could lead to rapid onset drought (ROD)
across these regions. Therefore, an area of ROD has been added to parts of
central Florida. However, in North Carolina, near-normal precipitation is
favored in the week-2 forecast, therefore, no ROD hazard has been posted for
this region at this time but interests should remain vigilant.



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 the
wind is likely to be largely maritime across the Gulf of Alaska and land
conditions are not expected to reach hazardous criteria. During week-2, river
breakup will have commenced across many rivers across Alaska leading to chances
for flooding hazards but uncertainty in the location of any potential impacts
precludes any hazards at this time.

FORECASTER: Ryan Bolt

$$