802
FXUS21 KWNC 091909
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT May 09 2024

SYNOPSIS: Early in week-2, a transient area of forecast mid-level low pressure
sustains the risk of heavy precipitation and possible flooding for many parts
of the southeastern contiguous U.S. (CONUS) and Mid-Atlantic.  Upstream,
building mid-level high pressure over the lower Four Corners is expected to
bring unseasonably warm temperatures, renewing the risk of excessive heat
conditions for parts of the Rio Grande Valley. Combined with drier than normal
conditions and strengthening surface pressure, there is an increased risk of
high winds which may elevate wildfire potential over the southwestern CONUS.
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 Lower Mississippi and
Tennessee Valleys, Southeast, Appalachians, and Mid-Atlantic, Fri-Sat, May
17-18.

Slight risk for excessive heat for parts of the Southern Plains, Sat-Tue, May
18-21.

Slight risk of high winds for parts of the Four Corners and western High
Plains, Fri-Tue, May 17-21.

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

DETAILED SUMMARY

FOR SUNDAY MAY 12 - THURSDAY MAY 16:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR FRIDAY MAY 17 - THURSDAY MAY 23: Since yesterday, the week-2 hazards
perspective mostly remains on track, though there are a few new developments
and discrepancies in the latest model guidance adding to more uncertainty in
the updated outlook.  Late in week-1, both the GEFS and ECMWF ensembles
continue to feature anomalous 500-hPa ridging extending from the eastern
Pacific into the western CONUS, with a fairly large coverage of anomalous
troughing east of the Rockies. Within this negative height anomaly coverage,
two shortwave disturbances have become more evident in the GEFS and ECMWF
ensembles; one situated over the Tennessee Valley, and a secondary vorticity
maximum associated with the possible remnants of a cutoff low upstream over
northern Mexico. In the wake of a frontal system that remains favored to bring
enhanced precipitation over the Southeast and Mid-Atlantic towards the middle
of next week, this secondary feature is expected to renew the risk of heavy
precipitation over parts of the southern and eastern CONUS heading into week-2.
The return of potentially heavy precipitation is supported in both the
uncalibrated and calibrated Probabilistic Extreme Tool (PET) guidance, thus the
corresponding slight risk area is continued over southeastern CONUS, and
extended through May 18. This area is expanded northward further into the
Mid-Atlantic in the updated outlook where the uncalibrated ECMWF depicts
greater than a 20% chance for 3-day precipitation amounts exceeding an inch.



A possible flooding hazard is also added in the outlook extending from eastern
Texas and Louisiana (where some areas have registered more than 10 inches of
rainfall in the past two weeks), eastward into parts of western Georgia and the
Florida Panhandle where heavy precipitation is favored during week-1. Any
additional precipitation during week-2 may trigger flash flooding as well as
river flooding within the highlighted region, even in areas where precipitation
deficits are currently registered.



As the secondary shortwave weakens and lifts out, both the GEFS and ECMWF
favorincreasing mid-level heights over the Interior West and Great Plains, with
a maximum height center amplifying over the lower Four Corners and northern
Mexico. However, the ensembles are divided in regards to the pattern evolution
both upstream and downstream. While the GEFS shows a sharper trend towards
developing a trough over the eastern Pacific and West Coast, the ECMWF is
virtually absent of this mid-level feature and maintains a widespread coverage
of positive height departures which spread northward across the West Coast and
into southwestern Canada. As a result of the stronger ridging over western
North America, the ECMWF shows anomalous troughing redeveloping over the
Midwest by the middle of the period, translating into predominantly cooler
conditions than the GEFS over many parts of the central and eastern CONUS later
in week-2. Notably, the ECMWF PET and uncalibrated tools depict the possible
return of heavy precipitation over the Central Plains and Middle Mississippi
Valley tied to the deepening trough aloft. No corresponding hazards are posted
given no support in the GEFS PET, and significant differences among the models
at this lead, but this troughing potential will continue to be monitored moving
forward.



Despite these differences in the evolution of the height pattern, however, the
potential for excessive heat still remains a concern across the southwestern
and south-central CONUS given model consensus of the amplifying ridge center
over the lower latitudes of North America.  There is fair agreement in GEFS and
ECMWF PETs indicating 20-30% chances for maximum temperatures exceeding the
85th percentile and 100 degrees F along the Rio Grande during the middle of
week-2 , therefore a slight risk for excessive heat remains posted for the
region (May 18-21). Similar heat signals are also evident further west across
the Sonoran Desert of California and southern Arizona, though maximum
temperatures are not expected to exceed hazard criteria over this part of the
country for this time of year. The excessive heat potential will continue to be
closely monitored over the Desert Southwest, lower Four Corners, and Southern
Plains as these regions were plagued with excessive heat conditions for much of
the warm season last year.  With warmer and drier than normal conditions
favored, a slight risk of high winds is also posted for parts of the Desert
Southwest, Southern Rockies, and Southern Plains (May 17-21). This is supported
in the GEFS and ECMWF PETs and raw ensembles depicting an anomalous surface
high near the Front Range of Colorado which could induce episodes of high winds
and elevate the wildfire risk.



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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