507
FXUS21 KWNC 101806
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT May 10 2024

SYNOPSIS: There is significant disagreement among the dynamical models reducing
confidence in the week-2 outlook. Late in week-1, forecast low pressure at the
mid-levels and at the surface is expected to increase the risk of heavy
precipitation for parts of the Mid-Atlantic and Northeast continuing into the
outset of week-2. Near to above-normal precipitation predominately favored from
the Southern Plains to the Southeast may trigger localized flooding across
parts of the Gulf states during week-2 following rounds of heavy precipitation
forecast during week-1. Over Alaska, cooler than normal conditions are forecast
as snowmelt and river ice breakup season is underway.

HAZARDS

Slight risk for heavy precipitation for portions of the Northeast,
Appalachians, and Mid-Atlantic, Sat, May 18.

Flooding possible for parts of the Southern Plains, Lower Mississippi Valley
and Southeast.

DETAILED SUMMARY

FOR MONDAY MAY 13 - FRIDAY MAY 17:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR SATURDAY MAY 18 - FRIDAY MAY 24: For what appears to be firmly in-line with
the challenges of springtime weather prediction, the latest runs of the GEFS
and ECMWF ensemble mean 500-hPa height fields continue to show poor run-to-run
continuity, with significant disagreements in the overall evolution of the
height pattern contributing to large uncertainty in the outlook.
Notwithstanding, model consensus is at its highest leading into the week-2
period, where both ensembles continue to feature some form of shortwave
activity lifting out across the Plains and Mississippi Valley. Compared to
yesterday, the 0z and 6z GEFS feature a much more amplified trough aloft over
the eastern CONUS, with a stronger accompanying surface low that deepens over
the Great Lakes while bringing increased precipitation amounts over many parts
of Eastern Seaboard at the start of week-2. The ECMWF has become considerably
weaker with this mid-level feature, but uncalibrated guidance maintains a
modest threat of heavy precipitation over many parts the Mid-Atlantic and the
Northeast associated with the mean surface low, as several ensemble members
also are beginning to point to a secondary cyclonic surface feature forming
offshore in the Atlantic. Although Probabilistic Extremes Tools (PETs) have
become less supportive of the heavy precipitation potential early in week-2, a
slight risk of heavy precipitation is posted (May 18) based on the uncalibrated
guidance and continuity with previous hazard outlooks.



A possible flooding hazard remains issued in the outlook extending from eastern
Texas eastward into parts of western Georgia and the Florida Panhandle where
heavy precipitation is favored based on the Weather Prediction Center`s (WPCs)
week-1 QPF outlook. Despite more uncertainty with the risk of heavy
precipitation returning to the southeastern CONUS later in week-2, the
continuation of above-normal precipitation predominately favored in the region
may trigger flash flooding as well as river flooding within the highlighted
region, even in areas where precipitation deficits are currently registered.



Beyond days 8 and 9, mean ensemble solutions vary wildly between the GEFS and
ECMWF leading to the discontinuation of a pair of hazards previously posted in
the southern CONUS. In regards to excessive heat potential across the Southern
Plains, the ECMWF maintains the amplification of an anomalous 500-hPa ridge
center over the lower latitudes of North America, with increased chances for
maximum temperatures exceeding the 85th percentile in its PET over the Southern
Plains and lower Four Corners by next weekend. However, the aforementioned
deeper troughing favored in the latest GEFS establishes more northwesterly flow
and colder air overspreading many parts of the Plains and Mississippi Valley
early in the period. Even after this troughing weakens and lifts out after next
weekend, the mid-level flow becomes appreciably more zonal than the ECMWF,
reducing confidence for the renewal of excessive heat potential later in May.
Similarly, pressure gradients in the mean surface fields appear more neutral,
lowering the risk of elevated winds, as the potential for anomalously cold air
in the GEFS would also help to quell any associated wildfire risks over the
Southern Plains.



No hazards are issued for Alaska. Snowmelt season is underway and frozen rivers
are beginning to break up, leading to the potential for river flooding related
to ice-jams. Currently, there are no indications of impending major river
break-ups or serious threat of ice-jamming so no flooding-related hazards are
posted at this time. Caution will continue to be exercised as river break-up
can be unpredictable and local conditions can change quickly.

FORECASTER: Nick Novella

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