Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
310 AM EDT Wed Sep 18 2024

Valid 12Z Sat Sep 21 2024 - 12Z Wed Sep 25 2024

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


...Overview...

Upper troughing centered over the Four Corners states as the
medium range period begins Saturday will gradually push into the
central U.S. into next week while reloading. This trough and
frontal systems at the surface will lead to cooler temperatures
behind them, but provide support for likely heavy rain across
central portions of the U.S. particularly this weekend. In the
East, upper troughing will linger into the weekend, which along
with weak surface frontal systems over the western Atlantic may
lead to showers across parts of the Eastern Seaboard, along with
onshore flow for some possible coastal flooding. This trough looks
to push away early next week in favor of ridging. A stalling front
may focus rain over South Florida at times for perhaps nonzero
flooding concerns over the urban corridor. Meanwhile NHC and WPC
will continue to monitor possible tropical development in the
Caribbean that may slowly move northward next week.


...Guidance/Predictability Assessment...

Model guidance begins in somewhat good agreement with the upper
pattern at the start of the period Saturday, consisting of an upper
low over Arizona/Utah, low amplitude northern stream troughing
over the north-central U.S., ridging from the Southern Plains
northeast to the Midwest, and lingering troughing in the East. But
even over the weekend, some guidance quickly diverges particularly
in the west-central U.S. Some models phase the northern stream and
southern stream upper low quickly by Sunday. The 12Z CMC was
particularly egregious with this while the 00Z CMC is not as
dramatically different, but still phases the streams well before
the GFS/EC consensus. This leads to the CMC and UKMET having
considerable QPF differences, which was not favored. Then
additional energy digging on the west side of the trough as it
moves into the central and east-central U.S. is also uncertain.
Some guidance pulls it far south enough to create an upper low in
the southern stream somewhere across the West, but with varying
placement. Then fortunately the 00Z model guidance consensus is
better with the timing of the upstream pattern in showing ridging
across the Northwest early week with troughing coming in by next
Wednesday.

Model and ensemble guidance continues to show the potential for
tropical development in the Caribbean, with a tropical system being
pulled north next week. GFS and CMC deterministic runs and many of
their ensemble members are faster to pull the system north than
the EC deterministic and ensemble members. 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 began with a blend favoring the 12/18Z GFS and
12Z ECMWF, with small proportions of the GEFS and EC ensemble
means. Did not include the CMC and UKMET given their differences.
As the period progressed, gradually increased the proportion of
ensemble means to over half by Days 6-7 given the increasing
spread. Continue to make changes to the frontal structures in the
central U.S. as the pattern evolves.


...Weather/Hazards Highlights...

By Saturday, dynamical support for thunderstorms with heavy rain
will increase as the upper low approaches from the Four Corners.
Models therefore show a considerable uptick in rainfall amounts on
the cusp of the instability gradient, with 3-5 inches locally. For
the Day 4/Saturday ERO, continue to depict a broad Marginal Risk
centered over Nebraska/Kansas and vicinity. The heavy rain is
forecast to shift slightly northeastward on Sunday, prompting a
Marginal Risk for parts of Nebraska/Kansas and stretching into the
Mid/Upper Mississippi Valley for the Day 5 ERO. On both days, 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.
Antecendent conditions are generally dry, but heavy rain rates
could pose flash flooding issues regardless. Moderate to heavy rain
is expected gradually shift across the Midwest Monday and the
east-central U.S. Tuesday. While uncertain, rain chances may get
renewed by Tuesday in the south-central U.S. as well.

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 some signal for moderate to heavy precipitation in
the Washington coastal ranges/Cascades toward the northern Rockies
at times.

In the East, troughing remaining overhead and weak Atlantic
fronts/surface lows could promote showers over parts of the Eastern
Seaboard this weekend 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. This is
all ahead of potential tropical development farther south in the
Caribbean that may slowly make its way north into the middle of
next week.

Cooler than normal temperatures particularly in terms of highs
will be in place for the Rockies westward on Saturday and focusing
in the northern/central High Plains on Sunday underneath the upper
trough. Meanwhile farther east, above normal temperatures by 10-15
degrees are forecast for the south-central Plains northeast to the
Ohio Valley and Great Lakes region Saturday. The above average
temperatures will decrease in scope early next week, focusing
mainly over the Ohio Valley and then gradually moderating there
too. The Eastern Seaboard could see below average highs into early
next week. Temperatures may be near normal in most places by the
middle of next week, but pending upper level pattern uncertainties.


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