Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
234 PM EDT Tue Sep 17 2024

Valid 12Z Fri Sep 20 2024 - 12Z Tue Sep 24 2024

...Heavy rain with possible flooding is forecast for the north-
central Plains to Upper Midwest this weekend...


...Overview...

Upper troughing over the West late week will pivot eastward and
provide support for likely heavy rain across portions of the
central U.S. that looks to peak over the weekend. The trough
pushing east along with frontal boundaries should gradually cool
the central U.S. closer to normal after a warm period through late
week. 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. A stalling front may focus rain over South
Florida at times for perhaps nonzero flooding concerns over the
urban corridor.


...Guidance/Predictability Assessment...

Model guidance shows good agreement with the overall pattern as
the period starts Friday, indicating upper troughing over the West
with a possible embedded upper low over California/Nevada, ridging
over much of the central U.S. though limited in its northward
extent due to northern stream energy, and East Coast troughing.
Models continue to exhibit notable uncertainty in the
evolution/progress of the Western low and how, if at all, it
interacts with the northern stream shortwave. The biggest outlier
continues to be the CMC which shows more phasing of these two
features to create one upper low in the north-central U.S. on
Sunday. The rest of the guidance shows less phasing and two
distinct features/shortwaves. The last couple of runs of the GFS
have been faster though with the overall Western trough/embedded
features compared to the ECMWF and UKMET, and was not incorporated
as much into the blend. The next shortwave into the Northwest late
period also shows timing and amplitude inconsistencies as it pushes
east towards the Central U.S. next Tuesday.

Some guidance, especially recent Canadian and GFS model runs,
offer increased tropical potential up from the Caribbean into early
next week. Uncertain position and digging of upper troughing by
then to the Southeast would affect any lower latitude interactions,
so WPC and NHC will continue to monitor.

The WPC forecast leaned more heavily on the ECMWF early in the
period, with lesser contributions from the GFS and UKMET. The CMC
was not included at all given its early period differences across
the West. With overall increasing spread mid to late period,
increased the ensemble means into the blend to 70 percent of the
blend by Day 7. Overall, maintained good continuity with the
previous WPC forecast, but did make some notable changes to frontal
structure across the Central U.S. this weekend.


...Weather/Hazards Highlights...

Low pressure/frontal systems over the central U.S. will help
produce scattered rain and thunderstorms on Friday. There is low
confidence in any area of focus for heavy rain, so continued to
leave the Day 4/Friday ERO blank pending better model agreement.
However, by Saturday, dynamical support for thunderstorms with
heavy rain will increase as the Southwest upper low approaches.
Models therefore show a considerable uptick in rainfall amounts on
the cusp of the instability gradient, with 3-5 inches locally. For
the Day 5/Saturday ERO, continue to depict a broad Marginal Risk
centered over Nebraska/Kansas and vicinity. There may be a need for
higher risk levels within the Marginal in the future if models
align more agreeably on an heavy rain axis. Moderate to heavy rain
is expected gradually shift into the Midwest Sunday-Monday.

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

In the East, troughing remaining overhead and weak Atlantic
fronts/surface lows could promote showers over parts of the Eastern
Seaboard late this week into early next week. These showers look
to remain light, but persistent onshore wind flow may create a
coastal flooding hazard along the Mid-Atlantic coast before an
offshore low moves away. The Florida Peninsula could also see some
focused convection due to a frontal boundary in a moist
environment. This front stalling into the weekend and beyond could
increase rain totals, which may create flooding problems especially
for the typically sensitive urban corridor of Southeast Florida.

Initial troughing aloft in the West will lead to continued below
normal temperatures through late week, though moderating compared
to the short range to only 5-10 below normal. The West Coast,
especially California, looks to warm to a bit above normal by early
next week pending the upper level pattern uncertainties. Cooler
than average temperatures will migrate into the north- central
Plains under the trough early next week. Warmer than normal
temperatures, including 90s to near 100F in the southern Plains,
will last into late week and then gradually moderate. The scope of
above normal temperatures will decrease into the weekend and mainly
focus across the Upper Great Lakes to the Ohio Valley. The Eastern
Seaboard looks to be around normal on average through the period,
with parts of the Mid-Atlantic cooler than usual for highs.


Santorelli/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