Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
427 AM 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 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 could
lead to showers across parts of the Eastern Seaboard.


...Guidance/Predictability Assessment...

Model guidance is agreeable with the large scale pattern at the
beginning of the period Thursday, showing a closed upper low atop
California and vicinity and a positively tilted trough toward a
compact upper low in south-central Canada, with ridging from the
southern Plains to Midwest and troughing atop the East. There is
some spread with the evolution of troughing in the East, but the
most uncertain aspect of the forecast is with the West trough
moving eastward and how it is affected by the northern stream flow.
By late week, there is already spread with the timing of the
trough's eastward movement. The 12Z CMC was on the slower side of
the deterministic guidance, with the 12/18Z GFS faster and the 12Z
EC in between. However, in looking at the EC-based AI models, the
bulk of them were faster than the deterministic EC and more like
the GFS runs, at least in part due to the timing of northern stream
troughing. The ensemble means were also on the faster side. Thus
leaned more toward the GFS/AI models/ensemble means for the
forecast. However the 00Z models have generally come in slower than
their previous runs, including the GFS and the EC (EC still slower
than the GFS). So there is still considerable uncertainty with the
trough's timing and this causes sensible weather differences like
with the QPF, for low confidence in the details.

Then the flow upstream across the eastern Pacific into the
Northwest is also quite uncertain over the weekend into early next
week. 12/18Z deterministic models were really favoring ridging
moving into the Northwest Sunday into Monday, but meanwhile the
ensemble means were completely out of phase and indicating some
troughing (especially the CMC mean that was considered overdone
with the troughing). The AI models showed a lot of spread with the
pattern as did the individual ensemble members, not increasing
confidence. Leaning toward the ensemble means with flatter flow but
with some troughing seemed most prudent. The newer 00Z models are
generally at least flatter with the flow coming into the Northwest,
but expect the forecast to continue to change there, with
precipitation chances and frontal positions being affected.

The WPC forecast used a blend of mainly the deterministic guidance
favoring the 12/18Z GFS runs early on, with gradual decrease of
the deterministic models in favor of the ensemble means from the EC
and GFS as the period progressed, reaching half means by Day 6 and
more by Day 7 given the increasing spread.


...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 quite a bit of
spread with the placement and amounts of heavy rain that may occur,
making confidence in the details low. Considered drawing a
Marginal Risk into the ERO for Day 5/Friday around the central
Plains to parts of the Midwest, but with some recent models (00Z
UKMET and ECMWF) trending down in rainfall amounts and the overall
spread, held off at this point, but some ERO risk could certainly
be needed in future issuances. It appears that Saturday (Day 6) may
be the wettest day with the best dynamical support. Upstream, the
questions with the pattern aloft create considerable uncertainty
with the QPF in the Northwest into the weekend.

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 trending down with rainfall amounts for
the Carolinas/Mid-Atlantic for Day 4/Thursday, so decided to remove
the Marginal Risk that had been 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


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