933
FXUS02 KWBC 221846
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
246 PM EDT Wed May 22 2024

Valid 12Z Sat May 25 2024 - 12Z Wed May 29 2024


...Heavy Rainfall/Flooding Threats for the Mid-Mississippi,
Tennessee, and Ohio Valleys through the weekend...

...South Texas to the Central Gulf Coast and South Florida Heat
Threat...


...Overview...

It remains generally the case that northern stream upper trough/compact
closed low and southern stream shortwave/low energies and
instabilities working from the West to Central U.S. will be the
main drivers for heavy rainfall potential across parts of the
Middle Mississippi Valley through the Tennessee/Ohio Valleys this
weekend. By Monday, the overall pattern across the CONUS should
turn more amplified as an upper ridge builds over the interior West
and a large closed low anchors off the West Coast. This allows the
downstream troughing to deepen and slow as it reaching the East
early next week. A strong upper ridge over Mexico will keep
hazardous heat a threat from south Texas to the Gulf Coast and
southern Florida through at least the weekend with dangerously high
heat indices coupled with anomalously warm nighttime lows.

...Guidance/Predictability Assessment...

Models and ensembles continue to show reasonably good agreement on
the overall mid-larger scale upper level pattern through the
medium range period, but small-mid scale system differences and
cycle to cycle variances and especially heavier convective QPF foci
remain more problematic. Accordingly, the WPC medium range product
suite was primarily derived from best clustered guidance from the
06 UTC GFS/GEFS and 00 UTC ECMWF/ECMWF ensemble mean that seems to
provide detail consistent with individual system predictability.
This also maintains good WPC product continuity. Newer 12 UTC
guidance continues to exhibit these types of warm season variances.

...Weather/Hazards Highlights...

A deepening surface low and associated warm front will be the
focus for strong to severe thunderstorms and heavy
rainfall/flooding potential this weekend across parts of the Mid-
Mississippi Valley to Ohio Valley. A marginal risk of excessive
rainfall remains in place from eastern Kansas to the lower Ohio
Valley for Saturday as a frontal boundary stalls and lifts north
through the region. An organized surface low moving into the
Midwest later in the weekend should help tap additional moisture
and instability for heavy rainfall development farther east on
Sunday into the Ohio and Tennessee Valley. Much of the guidance is
showing some elevated QPF totals, but still with a lot of spread in
exactly where. Opted to maintain continuity with a broad marginal
risk across this region on the Day 5/Sunday ERO, albeit with some
expectation a more welll defined guidance signal closer to the
event will likely prompt issuance of a Slight Risk area.

Elsewhere, showers will move from the Northern Rockies into the
Plains associated with the northern stream system this weekend, and
then a strong cold front into the East should bring increased rain
chances across the East and South early next week.

Expect hazardous heat to expand from South Texas through the
Central Gulf Coast and South Florida during the period with daytime
highs as much as 10-15 degrees above normal with max heat index
values possibly reaching 115 degrees, especially for South Texas.
Highs near 100 degrees could stretch farther north across the
southern High Plains this weekend as well. The combination of
dangerous heat indices and lack of overnight recovery/cooling will
allow for a major to extreme heat risk across South Texas, with
widespread major heat risk also extending into the Central Gulf
Coast region as well as southern Florida. Temperatures should trend
closer to normal next week across the South following passage of a
cold front. The forecast pattern will favor below average highs
over the Northwest to northern Plains for this weekend but above
normal temperatures will build again across the West early next
week underneath of amplified upper ridging. Above average
temperatures along the eastern seaboard will moderate back to
normal next week.

Santorelli/Schichtel


Additional 3-7 Day Hazard information can be found on the WPC medium
range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw







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