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FXUS02 KWBC 211853
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
253 PM EDT Sat Sep 21 2024

Valid 12Z Tue Sep 24 2024 - 12Z Sat Sep 28 2024

...Eyes to the Caribbean for next week...


...Guidance/Predictability Assessment...

There continues to be a lot of uncertainty surrounding the upper
level pattern over the CONUS mid to late next week, which has huge
implications for the eventual track of a possible tropical system
through the Gulf of Mexico. Latest guidance runs seem to be
trending towards more split flow over the CONUS, as energy or an
upper low splits from a leading trough racing through the
Midwest/Northeast next Wednesday-Friday and meanders over the
Central Plains-Middle Mississippi Valley. This may help to pull any
sort of tropical development in the Gulf northward a little faster
than previous thoughts and latest guidance from NHC reflects this.
Very little confidence in any of the specifics associated with the
overall medium range flow pattern over the CONUS and, of course,
Gulf of Mexico tropical activity as models continue to flip-flop
and will probably continue to do so. Also a lot of uncertainty late
period as a cut off low tries to form over or off the Northeast
Coast, and timing inconsistencies as another shortwave moves into
the West.

Through the 00z/06z guidance, the GFS was the biggest outlier and
was not used in today`s updated guidance blend. The new 12z run
(available after forecast generation time) is more in line with
consensus. The WPC forecast for today leaned more heavily on the
ECMWF and Canadian, with some smaller contributions from the UKMET
and the means early period. By the second half of the period,
transitioned to as much as 70 percent ensemble means in the blend
to help smooth out unresolvable details at this point.


...Weather/Hazards Highlights...

Energy digging southward over the Rockies/High Plains in still
uncertain separating flow may offer a period favorable for enhanced
rainfall over the southern High Plains under upper diffluent flow
and with frontal proximity. A WPC Marginal Risk area remains in
place on the WPC Day 4/Tuesday Excessive Rainfall Outlook (ERO).

Meanwhile, upper trough energy and wavy frontal systems will be
kicked slowly into the Midwest/East early next week along with an
associated threat of showers/thunderstorms with some local runoff
issues given motion. WPC (ERO) Marginal Risk areas are depicted to
shift eastward Day4/Tuesday and Day/5 Wednesday across the Upper
Ohio Valley and northern Mid-Atlantic. Upper trough reinforcement
may support additional rainfall potential to monitor over the East-
Central U.S. mid-later next week in amplified but uncertain flow.
The eastward progression of this remains contingent on far upstream
development of a progressive upper trough from the northeast
Pacific early next week.

Meanwhile NHC and WPC will continue to monitor possible tropical
development in the Caribbean that may slowly lift northward into
the Gulf of Mexico to threaten the Gulf Coast late next week. The
ultimate position and southward breath of the upper trough over the
lower 48 will determine extent and speed of northward lifting.
Given some of the latest guidance trends, there is potential that
heavy rainfall may begin impacting parts of the central to eastern
Gulf Coast by late next week. The timing and exact location of
impacts is still very uncertain at this time.

Out West, most areas should stay dry. The Washington coastal
ranges/Cascades will be an exception with growing daily enhanced
rain potential with Pacific system energy and moisture approaches.


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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