Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Mon Sep 9 2024

Valid 12Z Thu Sep 12 2024 - 12Z Mon Sep 16 2024


***Heavy rainfall and potential flooding for the lower Mississippi
 River Valley on Thursday as the tropical storm moves inland***


...General Overview...

The main thing that will make weather headlines for the end of
this week will be the tropical storm progressing inland across
Louisiana, Arkansas, and Mississippi. The result will be heavy
rainfall that could produce flooding and gusty winds, and the rain
from this system will likely lift northward towards the Ohio
Valley as it weakens further by the weekend. Elsewhere, a pattern
change will be arriving to the West Coast and Intermountain West as
an upper trough amplifies over the region and heralds the arrival
of much cooler temperatures and increasing rainfall chances,
compared to recent conditions. Meanwhile, warm temperatures are
expected across much of the north-central U.S. to the Great Lakes
region with an upper level ridge building in.

...Guidance/Predictability Assessment...

The 00Z model guidance suite features good overall synoptic scale
agreement across the nation on Thursday and continuing into
Friday, especially with the development and evolution of the
trough across the West. For the tropical low moving inland across
the Deep South, the 12Z CMC was a southwestern outlier compared to
the other guidance and the ensemble means, and did not line up with
the NHC track. The new 00Z run is much better and closer to the NHC
track, even though it is slightly to the west of the
UKMET/GFS/ECMWF guidance. By the end of the forecast period Monday,
the GFS is several hundred miles to the east of the CMC/ECMWF
guidance with the next trough that builds in across the West Coast.

For the WPC overnight forecast, a blend of the deterministic
GFS/UKMET/ECMWF was used as a baseline through the end of the
week, and less of the 12Z CMC owing to its southwestern solution
with the tropical system. However, the CMC better matched the
consensus once the tropical system dissipated, so it was brought
back into the blend once the UKMET was at the end of its run.
Ensemble means were added gradually to make up about 50% of the
blend by Monday, which helped to smooth out increasing mesoscale
differences.

...Weather/Hazards Highlights...

The core of the heaviest rainfall associated with the weakening
tropical storm is forecast to be across eastern Arkansas and into
western Tennessee and northern Mississippi by Thursday, and the QPF
from the ECMWF/GFS best matched the NHC track and is overall more
progressive compared to earlier forecasts. Therefore, the Slight
Risk area on Day 4/Thursday has been nudged northward some, and
also extends eastward over western Alabama where the inflow band
may reside and result in locally higher rainfall totals as well.
Given the recent dry conditions and remaining uncertainty in the
swath of heaviest QPF, there is no need for any Moderate Risk areas
for the Day 4 time period at this time. For the Day 5/Friday time
period, the overall QPF decreases as the low lifts farther north
and steadily weakens across the Ohio Valley region, where a
Marginal Risk is planned. This Marginal Risk also includes the
southern Appalachians where moist upslope flow could produce even
higher rainfall totals.

Otherwise, the trough arriving over the West will result in
numerous showers across the Northern Rockies. The heaviest QPF
from this will likely be over northwestern Montana, with snow for
the highest mountain ridges. Closed upper low passage and
increased ensemble enhanced QPF probs seems to also support a
Marginal Risk ERO area for Day 4/Thursday. Showers and some storms
then reach the central/northern Plains by Friday, while the Great
Lakes and the Northeast U.S. remains mostly dry.

Temperatures will still feel more like August across much of the
north-central U.S. through the end of the week and into the weekend for
both highs and lows, with potential +20 degree anomalies for lows
across North Dakota and northern Minnesota by Friday morning.
Meanwhile across the West Coast region and the Intermountain West,
a return to refreshingly cooler temperatures is expected by midweek
in response to the building upper trough, and feeling more like
October. Widespread highs in the 60s can be expected for the lower
elevations behind the cold front. It will likely be cooler than
average across portions of the Mid-South for daytime highs to close
out the week owing to increased clouds and rainfall.

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