Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Mon Sep 16 2024

Valid 12Z Thu Sep 19 2024 - 12Z Mon Sep 23 2024

...Overview...

Mean upper troughing across the West into later this week will
promote cooler weather with rain and high elevation snow. Rounds of
rain and thunderstorms are likely in the central U.S. in the
vicinity of frontal systems ahead of this troughing, which are
likely to become more widespread Friday and especially Saturday
into Sunday as the trough's dynamics move east. Meanwhile in the
East, upper troughing will linger for late week into the weekend,
which along with weak surface frontal systems over the western
Atlantic may lead to showers across parts of the Eastern Seaboard.

...Guidance/Predictability Assessment...

Overall, the WPC medium range product suite was primarily derived
from a composite blend of best clustered guidance from the
GFS/ECMWF/Canadian and GEFS/ECMWF ensemble means along with the
National Blend of Models and WPC continuity. This blended solution
tends to mitigate lingering timing/ejection variances in guidance
that are primarily associated with system energies progressing into
the West and the effect on kicking systems downstream. Forecast
spread has modestly decreased in this flow over recent days.

Some guidance, especially recent Canadian model runs, offer
increased tropical potential up from the Caribbean into early next
week, but with limited broader guidance signal support as per NHC.
Uncertain position and digging of upper troughing by then to the
Southeast would potentially effect any lower latitude interactions.

...Weather/Hazards Highlights...

An upper low to start the period over the West will provide
lift/forcing to produce precipitation across parts of California
into the Great Basin on Thursday and shifting into the Rockies
Friday. The higher elevations can expect snow given the depth of
the upper low. Farther east, scattered rain and thunderstorms are
likely in the Plains to Upper Midwest for multiple days later this
week into the weekend near and ahead of frontal systems, and as the
mean trough moves eastward. Model guidance shows more spread with
the placement and amounts of heavy rain that may occur, making
confidence in the details less certain. There is continued
consideration of drawing a Marginal Risk into the ERO for Day
5/Friday around the central Plains to parts of the Midwest, but
with some recent model runs of the UKMET and ECMWF trending down in
rainfall amounts and the overall spread, held off at this
juncture. It appears that Saturday into Sunday may be the wettest
days with the best dynamical support.

Upstream, the questions with the pattern aloft create uncertainty
with the QPF in the Northwest into the weekend, but there is a
growing signal in support to focus a deeper influx of moisture and
system energy into the region in about a week to monitor.

In the East, remnant (partially tropical) moisture will promote
showers persisting into late week and the weekend given troughing
remaining overhead and weak Atlantic fronts/surface lows. At this
point, recent models are overall modest/sporatic with rainfall
amounts for the Carolinas/Mid-Atlantic for Day 4/Thursday, so no
Marginal Risk areas are in place there. However, the flash flooding
risk may be nonzero especially for areas that have wet antecedent
conditions by then. It remains in question how far north into the
Northeast the showers reach. The Florida Peninsula could also see
some focused convection due to a frontal boundary in a moist
environment. This front stalling into the weekend could increase
rain totals, which may create flooding problems especially for the
typically sensitive urban corridor of Southeast Florida.

Underneath the upper trough in the West, much cooler than normal
temperatures are expected. High temperatures will be 15-20F 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 part 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.

Tate/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