402
FXUS02 KWNH 190706
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
305 AM EDT Thu Sep 19 2024

Valid 12Z Sun Sep 22 2024 - 12Z Thu Sep 26 2024

...Heavy rain with possible flooding is forecast for the central
Plains to Midwest and Ohio Valley into early next week...


...Overview...

Northern and southern stream upper troughing over the central U.S.
early next week are forecast to combine and gradually shift east
into the Great Lakes to Mississippi Valley as the week progresses.
This pattern aloft and surface frontal systems will promote showers
and thunderstorms, including potentially heavy rain across the
central Plains, Midwest, Great Lakes region, and Ohio Valley.
Farther south, 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 is slowly coming into better agreement with the
pattern over the lower 48 early next week. Namely, a small southern
stream upper low over the central High Plains Sunday is forecast
to move northeast and open up Monday-Tuesday. After multiple runs
of the CMC showing phasing of the northern and southern streams by
Sunday unlike the GFS/ECMWF consensus, the newer 00Z CMC finally
showed this separate upper low/trough like the GFS/ECMWF have been
showing for several runs.

Models overall then indicate energy digging farther west atop the
Rockies/High Plains Monday, but show increasing spread with what
becomes of this energy Tuesday-Thursday. ECMWF runs have been most
aggressive with showing this energy cutting off in the southwestern
or south-central CONUS, while other models like the 12/18Z GFS
indicate weak energy but keep most in the northern stream. EC-based
AI models vary; the 12Z AIFS for example did not show the low
cutting off but the 18Z AIFS does. So confidence is low for what
becomes of this energy and the resulting pattern.

Another important feature of interest is the potential for
tropical development in the western Caribbean by mid-next week.
While many models and ensemble members suggest that a tropical
system is likely to develop, the timing of it lifting into the Gulf
and the track are quite uncertain. Generally, CMC ensemble members
show a TC lifting north most quickly into the eastern Gulf. EC
members are slower and many end up on the west side of the Yucatan.
GFS members are mostly more like the CMC with the track, but with
some (including some recent operational runs) farther west. The
disagreements in the pattern across the CONUS will certainly affect
the track of any tropical low that may develop. Tend not to favor
the fastest solutions from the CMC suite, but other than that it is
difficult to choose any preferred track given the spread at this
time.

The WPC forecast used a blend favoring the 18Z GFS and 12Z ECMWF
early in the period, with lesser proportions of the UKMET and GEFS
and EC means. As the period progressed, increased the proportions
of the ensemble means to over half Days 6-7 given ample
uncertainty.


...Weather/Hazards Highlights...

The southern stream upper low over the central High Plains early
Sunday and pivoting northeast will provide ample dynamical support
for thunderstorms with heavy rain on the cusp of the instability
gradient. On Sunday, models generally show support for 2 to locally
4 inches of rain across parts of the central Plains into the Mid-
Mississippi Valley. Continue to depict a broad Marginal Risk for
those areas, and there may be a need for higher risk levels within
the Marginal in the future if models align more agreeably on a
heavy rain axis. Antecedent conditions are generally dry, but heavy
rain rates of 1 to 2 inches per hour could pose flash flooding
issues regardless. Chances for moderate to heavy rain should shift
east into the Great Lakes and Ohio Valley on Monday as the
shortwave moves east. Rainfall totals look to decrease as the
shortwave weakens, but with some upper level support and
instability, a Marginal Risk still seems warranted for Day
5/Monday, though perhaps lower end than on Day 4/Sunday. Rain is
forecast to focus along the eventually stalling front in the east-
central U.S. Tuesday-Thursday. Rain chances also are likely to get
renewed in the south-central U.S. by Tuesday with another front and
energy aloft.

Farther west, most areas should stay dry under mean ridging. The
Washington coastal ranges/Cascades could be an exception at times.
Then in South Florida, showers and storms may focus near a frontal
boundary lingering in a moist environment. These do not look to be
too heavy next week until around midweek, when there may be some
surge of tropical moisture northward ahead of any potential
tropical development farther south in the Caribbean that may slowly
make its way north into the Gulf during next week. There is still
considerable uncertainty in the track and timing of any possible
tropical system, so continue to check updated forecasts.

Cooler than normal temperatures particularly in terms of highs
will peak across the central Plains on Sunday, with highs generally
only reaching the low 60s underneath the upper trough. Meanwhile
farther east, above normal temperatures by 5-15 degrees are
forecast for the south-central Mississippi Valley northeast to the
Ohio Valley and Great Lakes region ahead of a front. The above
average temperatures will decrease in scope into Monday, focusing
mainly over the Ohio Valley and then gradually moderating there
too. The Eastern Seaboard could see below average highs into early
next week. On the other hand, building ridging in the West will
allow for some warmer than average temperatures by 5-15 degrees,
bringing temperatures above 100F to the Desert Southwest once
again.



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


$$