339
FXUS02 KWBC 061940
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
340 PM EDT Mon May 6 2024

Valid 12Z Thu May 09 2024 - 12Z Mon May 13 2024


...Gulf Coast States to Appalachians Heavy Rainfall Threat into
Thursday/Friday...


...Overview...

Positively tilted upper troughing atop much of the lower 48 to
start the period will push rain and thunderstorms through the South
and East along and ahead of a cold front Thursday-Friday. By the
weekend into early next week, this trough should consolidate and
linger atop the eastern U.S. with split flow across the West.


...Guidance/Predictability Assessment...

The early medium range period shows reasonably good agreement with
a train of energy embedded within the positively tilted trough,
with some energy focused in the central Great Basin lingering into
late week while some tracks from the Midwest into the Northeast.
With the latter, associated surface lows are in reasonable
agreement but with typical spread. Thus the early period was based
on a blend of the 06Z GFS and 00Z ECMWF and CMC, which maintained
good continuity with the previous forecast.

By early Saturday or so additional energy looks to dive into the
north-central U.S., with the 00Z GFS an outlier in how much and how
far west it fed in. Meanwhile, the 00Z CMC took southern stream
energy east while other models had it lingering in the Southwest
through Saturday. The newer 12Z CMC looks to be in better
agreement. But then, the ECMWF holds this energy farther southwest
longer than other guidance--and longer/southwest than the EC-based
AI/machine learning models. With many of the deterministic models
showing increasing spread, quickly increased the proportion of
ensemble means in the model blend by days 5-7 to minimize the
individual differences.


...Weather/Hazards Highlights...

Main lows and associated fronts to focus rainfall seem set to work
from the Great Lakes/Ohio Valley through the Northeast/Mid-
Atlantic later this week with complex system/energy interactions
forging offshore low developments downstream. Meanwhile, a
wavy/trailing frontal system and impulses aloft will gradually
work across the South to intercept return flow with deeper
instability and moisture. Accordingly, a WPC Marginal Risk
Excessive Rainfall (ERO) area is in place from eastern Texas across
the Lower Mississippi Valley through the central Gulf Coast states
to the Appalachians for Day 4/Thursday. The Marginal for this
forecast cycle will expand farther north into portions of the Lower
Great Lakes region. Despite lower instability than farther south,
anomalous moisture will be in place there while the surface low and
a coupled jet (right entrance/left exit region) aloft could lead
to flooding concerns in a somewhat sensitive region. An embedded
Slight Risk is in place from portions of
Louisiana/Mississippi/Alabama (expanded eastward from the previous
forecast) because ample instability and the west-east orientation
of the front and thus the convection will provide an environment
favorable for training storms with very heavy rain rates. By Day
5/Friday, the front will continue to press eastward and areas from
southern Alabama and Georgia into northern Florida could see some
thunderstorms with flooding concerns, so a Marginal Risk remains in
place there with a trend slightly southward from the previous
issuance. Downstream energy may lead to additional but uncertain
coastal lows with wrapback precipitation in the northeastern
quadrant of the country. Additional rainfall potential back to the
south-central U.S. is highly speculative in guidance at this time
for into next week.

Expect cooler than normal temperatures for parts of the West into
Thursday under upper troughing. Meanwhile, mean upper ridging will
favor lingering warmth from the far South through the Mid-Atlantic
to threaten record seasonal values Thursday. 90s are still forecast
across the far southern tier, with potential for south Texas
excessive heat with temperatures over 100F and higher heat indices.
Temperatures broadly cool later week onward across the South and
East, with warming across broad portions of the West in more benign
weather pattern underneath a building upper ridge.


Tate/Schichtel


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

























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