239
FXUS02 KWBC 071900
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Tue May 7 2024

Valid 12Z Fri May 10 2024 - 12Z Tue May 14 2024


...Overview with Guidance/Predictability Assessment...

Model guidance this morning shows very good agreement for mean
troughing to prevail over the eastern U.S. while a modest Rex
block/split flow pattern will take its time to break down or de-
amplify over the western U.S. through the medium-range period. A
number of shortwave troughs are forecast to rotate around the mean
upper trough in the East, and they presented some issues as to
when and where the shortwaves may interact and/or amplify with one
another. As for the Rex block in the West, models generally show a
gradual flatting of the pattern with time, with emerging
differences on whether a southern stream low-amplitude trough will
eject out into the southern Plains or remain slow-moving over the
Desert Southwest by early next week. Despite these timing
differences, there appears to be good model agreement for moisture
to return to the Gulf Coast region by early next perhaps as early
as Monday.

Overall, a general model consensus appears to offer a good starting
point out through Day 7. The 06Z GFS appears too aggressive in
developing a coastal storm off the New England coast on Saturday as
the 12Z GFS has backed off. The general model trend has been for
the mean upper trough to sag farther south down into the eastern
U.S. which would tend to eject the coastal low off into the
Atlantic.

The WPC medium-range forecast package was based on 40% from the 06Z
GFS/GEFS, 40% from the 00Z ECMWF/EC mean, and 20% from the CMC/CMC
mean. This blend provided enough system details while maintaining
maximum WPC product continuity. The 12 UTC ECMWF offers a faster
eastward progression of the shortwaves and mean trough over the
eastern U.S. together with a faster ejection of the Southwest U.S.
upper trough into the central/southern Plains early next week, but
not nearly as fast as the ECMWF-AIFS guidance.


...Weather/Hazards Highlights...

A main parent low and frontal system working toward the Northeast
Friday will help focus generally moderate rainfall, with activity
aided and prolonged Saturday as downstream energy transference
may lead to coastal low genesis, although a general model trend for
the mean trough to sag farther south will tend to eject the coastal
low offshore. Additional northern stream systems will periodically
offer bouts of mainly lighter rains from the north-central U.S. to
the Great Lakes/Northeast from Sunday to next Tuesday but with some
uncertainty in their timings.

Meanwhile to the south, a cooling and trailing/wavy front will
moderate lead warmth while working through the Southeast to the
Gulf of Mexico and northern Mexico. A WPC Day 4/Friday Excessive
Rainfall Outlook Marginal Risk area is maintained for parts of the
Southeast given possible training of thunderstorms ahead of a cold
front under lingering deep moisture/instability and favorable
upper jet/diffluence support. The slowing front and a reinforcing
front/upper impulses are expected to periodically focus
precipitation from the southern Rockies/Plains out through the Gulf
of Mexico and Florida over the weekend into next week. By early
next week, there appears increasing guidance support for moisture
to increase along the Gulf Coast ahead of the returning warm front.
Heavy rain could move farther inland later next Tuesday as the
front is forecast to edge northward.

Kong/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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