101
FXUS02 KWBC 231848
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Thu May 23 2024

Valid 12Z Sun May 26 2024 - 12Z Thu May 30 2024


...Heavy Rainfall/Flooding possible from the Midwest to the
Northeast Sunday and Monday...

...Hazardous heat will continue through Memorial Day weekend for
South Texas, the Central Gulf Coast, and South Florida...


...Overview...

An upper trough (which may be a combination of different energy
streams) will traverse the CONUS through the medium range period.
This will serve as the main driver for heavy rainfall potential
early in the period from the Midwest/Mid-South into the East. After
Monday, the overall pattern should turn more amplified as an upper
ridge builds over the interior West, causing the downstream trough
to deepen and slow as it moves over the East early next week.
Upstream, a deep upper low anchored off the West Coast should
eventually send energy into the Northwest and northern tier around
the middle of next week. Meanwhile, a strong ridge to the south
will maintain its influence over Mexico and will continue a
hazardous heat threat from South Texas through the Gulf Coast and
for South Florida through Monday. A cold front will drop south
into the northern Gulf mid-next week and bring some much needed
relief from the heat.


...Guidance/Predictability Assessment...

Model guidance shows good agreement on the overall large scale
upper level pattern through the medium range period, but there are
some small/mid scale differences between models and runs that will
affect how upper lows and shortwaves interact. Some of the highest
uncertainty surrounds the timing and strength of a shortwave that
will move over the northern Rockies and Central Plains on Sunday
before interacting with a deep upper low over the Great Lakes
region. The interactions between these two features will influence
the timing and position of several successive frontal systems that
will impact the eastern third of the nation next week.

Shortwave energy should move into the Pacific Northwest and cross
the northern tier of the nation mid-next week, but there is still a
significant amount of uncertainty surrounding the speed and
amplitude of this feature. Recent runs of the GFS and ECMWF have
shown poor run-to-run consistency, so this will be a feature to
keep an eye on in future forecast updates.

The afternoon WPC forecast used a blend of the deterministic
GFS/ECMWF/CMC/UKMET along with the previous WPC forecast blend
through Tuesday, with the heaviest weight placed on the ECMWF and
GFS. This method aims to find a middle ground solution given the
differences in the latest model runs and lack of run-to-run
consistency. After Tuesday, the WPC blend used a combination of the
deterministic GFS/ECMWF/CMC and ensemble means from the GEFS/ECE to
help smooth out model differences. This also maintained good
continuity with the previous WPC forecast for Days 3-6.


...Weather/Hazards Highlights...

A surface low and associated fronts will be the focus for strong
to severe thunderstorms and heavy rainfall/flooding potential
Sunday and Monday from parts of the Midwest to the East. A Marginal
Risk of Excessive Rainfall (level 1/4) remains in place across
much of the Ohio Valley and into the Tennessee Valley on Sunday
(Day 4), shifting into the Northeast and Mid-Atlantic on Monday
(Day 5). The guidance continues to show elevated QPF totals, but
still with a lot of spread in exactly where. It is possible that
the guidance signal will become more defined and an embedded Slight
Risk (level 2/4) may be needed in the Midwest on Sunday.

The heaviest rainfall should exit the East Coast after Monday with
the cold front, but renewed troughing across the region will allow
for shower chances across the Great Lakes into the
Appalachians/Northeast through mid-next week. Elsewhere,
precipitation chances should increase across the Northwest and
northern/central Plains Tuesday through Thursday while additional
showers and thunderstorms expand across the southern Plains.

Expect hazardous heat to continue through Memorial Day weekend
from South Texas through the Central Gulf Coast and South Florida.
Daytime highs are forecast to be as much as 10-20 degrees above
average with widespread heat index values above 100 degrees, and
max heat index values may exceed 115 degrees in parts of South
Texas. Highs near 100 degrees could stretch farther north across
the southern High Plains on Sunday 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 and South Florida. Temperatures should trend closer to
normal on Tuesday across much of the South following a cold front
passage, but South Florida will likely remain warm through mid-
next week.

The forecast pattern will favor below average highs over the
Northwest and northern Plains on Sunday, but above normal
temperatures will build again across the West early next week
underneath of an amplified upper ridge. Above average temperatures
along the eastern seaboard will moderate back to near normal next
week with some chance for below normal highs around mid-next week.


Dolan/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



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