672
FXUS02 KWBC 181859
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM 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 beginning 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...

Overall pattern over the CONUS at the start of the forecast period
features a lingering trough over the East Coast, a northern stream
trough over the Upper Midwest, a southern stream ridge over the
south-central U.S., and a southern stream upper-low over the Four
Corners region. The 00Z/06Z GFS and 00Z ECMWF, echoing prior
forecasts, show the streams remaining separate as both systems lift
to the northeast, with the Four Corners system entering the Plains
and bringing a potential threat of heavy rainfall to the central
Plains. The 00Z CMC continues to differ from the other guidance,
showing the northern and southern stream troughs phasing much
quicker compared to the other guidance.

Differences amongst the remaining guidance increase in the mid- to
late period as a secondary shortwave incoming from the west
amplifies into the Southwest, with the energy eventually
potentially becoming cutoff by Tuesday. This has become a much more
notable trend in the ECMWF guidance as compared to the GFS, with a
hint in the 00Z ECens mean. The EC AI model suite does tend to
favor this more than not, though with the lack of consensus and
support from the means it is difficult to resolve at this
timeframe. The overall pattern otherwise for the CONUS is fairly
agreeable on a larger-scale, with the central U.S. upper
low/trough lifting northeast in much of the guidance as the streams
phase, with large- scale ridging over the western U.S. similarly
shifting eastward in its wake as upstream energy reaches the
Pacific Northwest. The central U.S. trough in the latest 06Z GFS
does not lift into the Midwest as quickly as the ECMWF and the
ensemble means while also showing a closed high over the Southwest
embedded within the broader longwave ridging compared to the weak
cutoff low in the other solutions.

One last but important feature of interest is the potential for
tropical development in the western Caribbean mid-next week. There
remains a large amount of uncertainty in the development of a
system during this time frame, with the 00Z GFS/CMC and some GEFS
and ECens ensemble members faster in both developing a system and
bringing it northward into the Gulf, while the 00Z ECMWF does not.
Currently anticipate that an area of low pressure may move into the
Gulf of Mexico mid-next week, but there remains a large amount of
uncertainty and the NHC and WPC will continue to monitor. It is
noted that the lingering disagreement on the evolution of the
central U.S. trough will likely impact how fast this potential
system lifts northward.

The updated WPC forecast begins with a blend of the 00Z ECMWF, a
mix of the 00Z/06Z GFS, and 00Z UKMET. Similar to the prior
forecast, the 00Z CMC is not included given how quickly the
solution diverges over the central U.S. as the two streams phase.
The differences in the guidance as to if/where there is cutoff
energy over the Southwest, or a closed high as the 06Z GFS shows,
is harder to resolve given the overlap, so favored a large
contribution from the means (and did not include the 06Z GFS) for
the end of the period.


...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
stretching from Iowa west through the central Plains and into
eastern Colorado. The heavy rain is forecast to shift slightly
northeastward on Sunday, prompting a Marginal Risk from
northeastern Colorado east through southern Nebraska, northeastern
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 given the amounts currently
depicted in the deterministic guidance. Antecedent conditions are
generally dry, but heavy rain rates could pose flash flooding
issues regardless. Moderate to heavy rain is expected to 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. Moderate to heavy rainfall may reach portions of the
central to eastern Gulf Coast by mid-next week and beyond, but
there remains a lot of uncertainty on the exact location and timing
tied to the evolution of the potential tropical system.

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 as a ridge reminds in
place between the central U.S. and East Coast upper troughing. 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.


Putnam/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






$$