Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM EDT Tue Sep 10 2024

Valid 12Z Fri Sep 13 2024 - 12Z Tue Sep 17 2024

...Heavy rainfall associated with Francine continuing through
Friday across parts of the Mid-South and Southeast...

...General Overview...

The main thing that will make weather headlines for the end of
this week will be the eventual remnants of Francine progressing
inland across the Tennessee and southern Ohio River valleys. The
result will be moderate to heavy rainfall that could produce some
instances of flooding. Elsewhere, a trough exiting the Northern
Rockies Friday will likely be replaced by another large scale
trough across the West Coast by Monday, bringing another round of
cooler weather and increased rainfall chances. Meanwhile, a general
upper level ridge to the north of the remnant low from Francine
will be in place from the Midwest/Northern Plains to the Great
Lakes, keeping summer-like temperatures in place.

...Guidance/Predictability Assessment...

The 00Z model guidance suite features good overall synoptic scale
agreement across the nation on Friday and continuing into the
weekend. For the remnants of Francine moving inland across the
western Tennessee Valley, the CMC and UKMET remain slightly faster
than the official NHC track and so the preference for the QPF
forecast was towards the ECMWF and GFS. The low continues to weaken
further and become just a surface trough or weak reflection based
on the guidance by Sunday. Looking ahead to Monday, the GFS remains
quicker with the arrival of the next trough across the West Coast,
but 12z guidance today has all trended slightly faster (though
still not as fast as the GFS).


For the WPC forecast, a blend of the deterministic guidance,
weighted towards the ECMWF and GFS, was used through the weekend.
Ensemble means were added gradually to make up about 50% of the
blend by early next week, which helped to smooth out increasing
mesoscale differences. The ECMWF was weighted more than the GFS
late period given better consistency with the ensemble means.
Overall, good continuity was maintained with the previous WPC
forecast.

...Weather/Hazards Highlights...

The center of what most likely will be post-tropical cyclone
Francine by the start of the period should be reaching the
Tennessee Valley by Friday, and the majority of the rainfall should
be located to the east and southeast of the low pressure center.
There will likely be a remaining inflow band across central
Tennessee and extending into western Kentucky, where 1-2 inches of
rain can be expected with locally higher amounts. Orthogonal low
level flow across the southern Appalachians could also result in
some higher rainfall totals here as well, and both of these regions
are encompassed in a broad Marginal Risk area for Day 4/Friday.
Not much in the overall atmospheric set-up changes for Day
5/Saturday, except for a slightly weaker signal in the model
guidance and further weakening of the remnant low. However, the
Marginal Risk is still planned for many of those same areas in Day
4 given the grounds will likely be more saturated.

Elsewhere across the country in terms of rainfall, showers should
be decreasing in coverage across Montana and the northern High
Plains as the first western U.S. trough lifts out of the region in
time for the weekend. Showers and thunderstorms are likely this
weekend for parts of the Southwest with a increased moisture
connection ahead of a possible tropical disturbance lifting north
over the southern Baja Peninsula. Elsewhere across the West,
increasing showers and storms are likely by early next week as the
second trough builds in across the West Coast and then the
Intermountain West, and scattered showers/storms also for the
southern Plains as tropical moisture gets wrapped in ahead of the
Western trough.

Temperatures should moderate some across the northwestern U.S. by
the weekend as the first trough lifts out, with highs returning to
near seasonal averages. Another cool-down appears likely going into
early next week for the West Coast with readings 10 to perhaps 20
degrees below normal for portions of the interior West Coast states
by Tuesday. Meanwhile, warm conditions are forecast to continue
from the central/northern Plains to the Great Lakes and parts of
the Northeast where upper level ridging will tend to govern the
overall weather pattern.

Santorelli/Hamrick


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