773
FXUS02 KWBC 191858
PMDEPD

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

Models are pretty consistent with a split flow pattern over the
lower 48 at the beginning of the period, eventually phasing into a
deep mid-upper-level trough over eastern North America toward the
middle-to-end of next week. Meanwhile, an upper ridge develops over
the West and propagates into the Central U.S. by mid-week. The
main area of uncertainty pertains to a potential tropical system
that is forecast to propagate into the Gulf of Mexico sometime
during the middle-to- latter half of next week. The operational GFS
has trended slower and farther south with next week`s
Caribbean/GoM tropical system.

A mixture of the 00z ECMWF/UKMET/CMC and 06z GFS were used in the
days 4 and 5 blends with the EC and GFS favored. The 00z ECE and
06z GEFS were introduced to the blend on day 5 with the operational
Canadian removed due to a seemingly overamplification of a mid-
level vort over the eastern Pacific and its more bullish/progressive
solution with respect to the tropical disturbance in the
Caribbean/Gulf of Mexico. The Canadian ensemble is introduced to
the blend on day 6 and continued through day 7 to mitigate for
potential over-suppression of the tropical by the EC (especially)
and GFS suites.


...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.


Kebede/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




$$