Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 PM EDT Sun Sep 15 2024

Valid 12Z Wed Sep 18 2024 - 12Z Sun Sep 22 2024

...A coastal storm could spread heavy rain into the Mid-Atlantic
states while locally heavy rain could impact northwestern Montana
on Wednesday into early Thursday...


...Overview...

Deep upper troughing will be in place across the western U.S.
through this week to promote cooler weather with rain and high-
elevation snow. Rain on the backside of a deepening low could
impact northwestern Montana on Wednesday into early Thursday. There
appears to be a good chance for bands of heavy rain and strong
thunderstorms across the northern and central Plains ahead of a
slowing front late this week into the weekend with warmer than
average temperatures. Meanwhile in the East, a broad upper low
initially accompanied with tropical moisture from a coastal storm
will be slow to exit the East Coast with marginal heavy rain
potential mainly across the northern Mid-Atlantic Wednesday into
Thursday.


...Guidance/Predictability Assessment...

The most uncertain portion of the medium-range forecasts continues
to be along the East Coast where the track and forward speed of a
coastal storm remain in question into the short-range, even down
to the current time with the analysis. The GFS has been the fastest
guidance, taking the system north then northwestward into South
Carolina and pretty much dissipates well inland by the time
medium-range begins on Wednesday. The ECMWF has flipped its
eastern track since a couple of days ago, now tracking the system
slowly westward for a day or so before swinging it onshore into
North Carolina on Tuesday. Present satellite loop indicates that a
northward motion has begun after initially being stationary. The
WPC medium-range forecasts are based on a consenus of the guidance
with a general northward track bringing the system well inland by Day 3.


...Weather/Hazards Highlights...

The surface low that may be tropical or subtropical per NHC/WPC
coordination remains more uncertain than desired, but for now is
most likely to be located in Virginia or vicinity by Wednesday,
spreading rain into the Mid-Atlantic and Appalachians. For
Wednesday/Day 4, a Marginal Risk area remains in place across much
of the Mid-Atlantic region and stretches into southern parts of the
Northeast given a slightly faster trend in the rain spreading
north and east. Above normal moisture looks to linger into
Thursday/Day 5, prompting a continued Marginal Risk in the Mid-
Atlantic especially after a wet previous couple of days. Showers
are likely to persist into late week along parts of the Eastern
Seaboard as upper troughing remains overhead. The Florida Peninsula
could also see some focused convection due to a frontal boundary
in a moist environment. Currently this thunderstorm activity seems
to be sub-Marginal Risk for flash flooding, but if guidance is
agreeable in trending upward with amounts in future cycles, future
Marginals cannot be ruled out, especially for the typically
sensitive urban corridor of Southeast Florida.

The mean trough aloft over the West will provide ample
lift/forcing to support rounds of precipitation there this week.
Heavy rain may linger across parts of Montana on Day 4/Wednesday
for a continued Marginal Risk, as the region could see heavy rain
rates for multiple hours near the track of a leading upper low
ejecting through the northern Rockies, with potential for some
instability to be in place. The secondary upper low should then
spread precipitation from the West Coast states midweek into the
Great Basin and Rockies. The higher elevations can expect snow with
both of these systems given the depth of the upper low. Farther
east, scattered rain and thunderstorms are likely in the Plains for
multiple days this week near and ahead of frontal systems.
Rainfall amounts may increase in the north- central U.S. by later
week, as the front trailing from the initial Montana into Canada
system stalls for a period of time while awaiting some combination
of northern stream and western U.S. dynamics (with possible surface
low development).

Underneath the upper trough in the West, much cooler than normal
temperatures are expected. High temperatures will be 15-25F below
normal across much of the West, gradually moderating closer to
normal across the West Coast states but with some minus 10-15F
anomalies persisting in the Interior West and reaching the northern
High Plains over the weekend. Some areas may reach the freezing
mark in the higher elevations of the Great Basin/Rockies.
Meanwhile, the central U.S. should see warmer than normal highs by
10-15F and lows by 15-20F, including highs in the 90s in the
southern half of the Plains, through the latter half of the week.
The scope of above normal temperatures will decrease into the
weekend and mainly focus across the Upper Great Lakes to the Ohio
Valley. The Northeast can expect warmer than average temperatures
until a cold frontal passage late week, while temperatures south of
there are forecast to be near to below normal.


Kong/Tate


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