431
FXUS02 KWNH 270659
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Fri Sep 27 2024

Valid 12Z Mon Sep 30 2024 - 12Z Fri Oct 04 2024

...Overview...

Incoming Pacific flow, with leading energy reaching the northern
Plains by early next week, will finally dislodge the persistent
eastern U.S. upper trough/low responsible for multiple days of
rainfall enhanced by Helene`s moisture. The new pattern will favor
progressive mean flow across the northern tier, pushing along a
couple frontal systems, and southern tier ridging (though with
possible breaks in places). The Mid-Atlantic/Northeast will
gradually trend drier over the course of next week while Florida
and possibly portions of the Gulf Coast could see some rainfall at
times. Fronts associated with northeastern Pacific systems could
bring light rain to the northern Pacific Northwest. Expect the most
notable temperature anomalies next week to be on the warm side of
normal, over the southern half of the West on a consistent basis
and more briefly over parts of the central U.S.

...Guidance/Predictability Assessment...

Dynamical and machine learning (ML) models/ensembles agree fairly
well for next Monday-Tuesday, as a progressive Canadian/northern
tier U.S. shortwave leads to an opening of the upper low over the
East. The front ahead of the upper shortwave will cross the central
U.S. and weak Mid-Atlantic wave should evolve along a front
lingering near the East Coast. A storm tracking toward the Alaska
Panhandle should bring another front into the Northwest by Tuesday.

Guidance steadily diverges in various respects by the latter half
of next week. One issue is how the leading trough energy evolves
once it reaches the Northeast and vicinity. In very different ways,
the 12Z/00Z CMC runs are on the aggressive side of the spread
though the 12Z CMC`s closed low does have some similarity to one of
the 12Z ML models. There are hints from some other solutions for
weaker energy to pass through the Northeast. Specifics of this
energy will affect rainfall details across the region.

Farther west, differences arise with energy entering western North
America by Wednesday and then upstream Pacific flow. By next
Friday the most common theme exists among most ML models/AIFS mean
and dynamical means, reflecting an eastern Pacific upper trough
(supporting a well-defined surface system), western North American
ridge, and downstream east-central trough that should be fairly
shallow over the lower 48. ECMWF runs compare best to this idea.
12Z/18Z GFS runs also fit this pattern over the Pacific/western
North America but the 18Z GFS strayed out of phase downstream.
Meanwhile the 00Z GFS has strayed faster with the Pacific trough
and the 00Z CMC splits Pacific energy well westward.

Late in the period, guidance also differs regarding signals for
tropical development in the northwestern Caribbean/Gulf of Mexico,
with GFS/CMC runs showing greater potential than other guidance.
Remaining solutions that have some indication of low pressure over
the region suggest it may be weaker and over the southwestern Gulf
by next Friday.

Guidance comparisons led to updating the forecast with an
operational model composite early in the period, followed by a
50-70 percent 18Z GEFS/12Z ECens weight while phasing out GFS/CMC
input and maintaining some 12Z ECMWF. This yielded fairly good
continuity from previous forecast.

...Weather/Hazards Highlights...

The lingering upper low over the East and accompanying moisture
from Helene will continue to produce some showers and thunderstorms
over portions of the central Appalachians and Mid-Atlantic into
Monday, though much lighter in intensity compared to
ongoing/short-term rainfall. The Marginal Risk area depicted on the
Day 4 Excessive Rainfall Outlook is unchanged from the prior Day 5
outlook based on support from latest guidance. Heavy rainfall
during this period should not be as widespread but wet conditions
from prior days of rainfall may be an aggravating factor in terms
of the flooding potential. By Day 5/Tuesday the guidance develops a
more spread for finer details at the surface and aloft, leading to
a more diffuse signal for where any pockets of enhanced rainfall
may exist. Therefore the Day 5 ERO depicts no risk areas at this
time. Beyond Tuesday, some rain may continue northeastward from
the Appalachians/Mid-Atlantic but with low confidence in the
coverage and amounts.

Elsewhere, Florida and eventually parts of the Gulf Coast farther
to the west may see some rainfall at times. Northern parts of the
Pacific Northwest may see a couple episodes of light rain, one with
a front reaching the region on Tuesday and another around Friday.

Expect temperatures to remain quite warm to hot across the
southern half or so of the West next week, especially Monday-
Wednesday when plus 10-15F anomalies will be common for highs with
numerous daily records possible. More transient anomalous warmth
will be likely over parts of the central U.S. ahead of cold fronts,
including areas from the Upper Midwest southwestward on Monday and
then the north-central to south-central Plains on Wednesday (and
lingering to some degree into Thursday over southern areas). The
Northwest into northern High Plains will tend to be near to below
normal. The East will tend to see highs vary somewhat but average
near normal, while clouds/rain will keep highs above normal during
the first half of the week.

Rausch

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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