174
FXUS02 KWBC 201857
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 PM EDT Fri Sep 20 2024

Valid 12Z Mon Sep 23 2024 - 12Z Fri Sep 27 2024


...Guidance/Predictability Assessment...

As early as Day 4, there is increasing uncertainty in the overall
upper level flow pattern over the CONUS. 00z runs of the UKMET and
CMC, along with the 06z GFS, had trended towards less separation of
energy from a shortwave diving south out of the Northern Rockies
early in the week. ECMWF and some ensemble members (and now the new
12z CMC) however continue to show energy breaking off moving back
towards the southwest as the parent trough continues east into the
Midwest. After this though, there is some notable agreement that
additional energy will break off into a cutoff low over the Midwest
or the Northeast late period, but uncertainty in the timing of
this. Regardless, steering flow and evolution of possible tropical
system development into the Gulf of Mexico late next week is
entirely dependent on the overall flow pattern over the CONUS. The
guidance continues to show good agreement on the development of
some sort of system in the southern or western Gulf, but where it
tracks and what impacts it has on the Gulf Coast region of the US
is highly uncertain.

The WPC blend today leaned closely towards the GFS, the ensemble
means, and WPC continuity amidst growing uncertainty in the
guidance. This was also consistent with NHC preferences for Gulf of
Mexico tropical activity.


...Weather/Hazards Highlights...

If energy over the West splits southward over the Rockies, then
southwestward toward the Southwest through early-mid next week.
This would provide a period favorable for enhanced rainfall over
the southern High Plains and southern Rockies Tuesday into
Wednesday in upper diffluent flow and with frontal proximity. A WPC
Marginal Risk area continues in the Day 5/Tuesday ERO period.

Meanwhile farther north, upper trough energy and wavy frontal
systems will be kicked from the Plains to the Midwest/East early
next week along with an associated threat of showers/thunderstorms
with some local runoff issues. WPC Excessive Rainfall Outlook (ERO)
Marginal Risk areas are depicted to shift eastward Day4/Monday and
Day/5 Tuesday across the Great Lakes/Ohio Valley to the northern
Mid-Atlantic. Upper trough reinforcement may support additional
heavy rainfall potential to monitor over the East mid-later next
week in amplifying flow. This occurs as far upstream development of
a potent and slowly progressive upper trough from the northeast
Pacific early next week provokes downstream upper ridge
amplification and warming, with an eastward shift from the NOAM
West Coast to mid-continent next week.

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

Farther west, most areas should stay dry under mean ridging. The
Washington coastal ranges/Cascades could be an exception and offer
growing potential over time with subsequent ample Pacific energy
approach. Then in South Florida, showers and storms may focus near
a frontal boundary lingering in a moist environment. These do not
look to be too heavy next week until around late week, when there
may be some surge of tropical moisture northward ahead of any
potential tropical development.


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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