388
FXUS02 KWNH 220627
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
226 AM 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...

A northern stream compact closed low and southern stream shortwave
moving from the West to Central U.S. will be the main drivers for
heavy rainfall potential across parts of the Middle Mississippi
Valley to Ohio Valley 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 good agreement on the
overall large scale upper level pattern through the medium range
period, but there are some key differences still in the details
especially the second half of the period. There was good enough
agreement the first half to warrant a general blend of the
deterministic guidance. After this, the 12z/18z guidance suite
continued to advertise a faster moving southern stream shortwave
ahead of the northern stream upper low over the West, which would
limit interactions between the two and brings a weaker system into
the East early to mid next week. The 12z/May 21 ECMWF was the
slowest with the shortwave and showed energy interactions with the
next low behind it resulting in a stronger system through the Great
Lakes and East. There is support for this from the AI/ML guidance
though so this will need to continue to be watched. The ECMWF was
also a lot stronger with shortwave energy into Western Canada late
period resulting in much less amplified ridging over the Western
U.S. next week, which does not have much/if any support from the
rest of the guidance. The late period blend for the WPC forecast
leaned more on the ensemble means with some modest inclusion of the
GFS and CMC to help maintain some system definition and details.


...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 is 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. For now, introduced a broad marginal risk across this region
on the Day 5/Sunday ERO.

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


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





$$