361
FXUS21 KWNC 081820
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT May 08 2024

SYNOPSIS: Early in week-2, a transient area of mid-level low pressure forecast,
along with continued surface return flow from the Gulf of Mexico sustains the
risk of heavy precipitation for many parts of the southeastern contiguous U.S.
(CONUS).  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, and Southeast, Thu-Fri, May 16-17.

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.

DETAILED SUMMARY

FOR SATURDAY MAY 11 - WEDNESDAY MAY 15:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR THURSDAY MAY 16 - WEDNESDAY MAY 22: A fairly muted 500-hPa height anomaly
pattern is featured in both the GEFS and ECMWF total week-2 ensemble means,
generally consisting of above-normal heights throughout the West and the
southern tier, with near to weakly below normal heights mostly east of the
Mississippi. Analysis of the daily 500-hPa height fields show that the
anomalous mid-level troughing is strongest early in the period over the Plains
and Mississippi Valley, and gradually deamplifies while lifting out over the
eastern CONUS towards the end of next week. Along with return flow favored at
the surface, much of the southeastern CONUS looks to remain wetter than normal
through mid-May, with the potential for locally heavy precipitation over parts
of the Lower Mississippi Valley and the Southeast. Surface prognosis charts
from the Weather Prediction Center (WPC) continue to feature a warm frontal
boundary draped across the Gulf of Mexico coastline and extending into the
western Atlantic late in week-1. With this forcing expected to remain
established heading into week-2, there is fair agreement between the raw GEFS
and ECMWF mean ensemble depicting enhanced precipitation amounts, with the
latter being wetter, over the central and eastern Gulf Coast states through day
9 (May 17). Therefore, a slight risk of heavy precipitation is continued for
May 16-17 where the Probabilistic Extremes Tool (PETs) guidance shows at least
a 20% chance for 3-day amounts exceeding the 85th percentile and 1 inch.



Upstream, the anomalous mid-level height departures initially featured over the
West Coast at the start of week-2 are favored to shift eastward, where there
are hints of trough development in the ensembles over the eastern Pacific and
West Coast later in the period. This is expected to bring cooler conditions to
an otherwise hot start of the period for parts of the Pacific Northwest and
California (particularly in the northern portions of the Central California
Valley). However, both the GEFS and ECMWF ensembles depict the maximum height
center amplifying over the lower Four Corners and Southern Plains. Given the
position and strength of the ridge axis in the latest ensemble guidance,
above-normal temperatures are favored to return, and renew the risk for
excessive heat across the south-central CONUS later in May. This is reflected
in the PET guidance indicating increased chances (20-30%) for maximum daytime
temperatures exceeding the 85th percentile and 100 degrees F along the Rio
Grande during the middle of week-2 and  a slight risk for excessive heat is
posted for the region. 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 consistently 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 ensembles depicting an anomalous
surface high near the Front Range of Colorado and a deepening thermal low over
the Desert SW to 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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