747
FXUS02 KWNH 100658
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Fri May 10 2024

Valid 12Z Mon May 13 2024 - 12Z Fri May 17 2024

...Excessive Rainfall/Flooding Threat for the Gulf Coast and
Southeast...


...Overview...

A system progressing across the southern half of the central-
eastern U.S. during the first half of the week will bring the
threat of heavy rainfall to areas near the Gulf Coast. This system
and a northern stream front will combine to spread varying
intensity of rain across areas farther north. Another system
currently expected to reach the Plains/east-central U.S. after
midweek may produce another broad area of rainfall with highest
totals again possible over parts of the southern tier. Meanwhile
guidance diverges significantly for flow from the eastern Pacific
into western North America by mid-late week, lowering confidence in
forecast details over that region and to some extent downstream at
that time frame.

...Guidance/Predictability Assessment...

An operational model composite provided a reasonable starting point
for the early portion of the forecast. 12Z ECMWF-initialized
machine learning (ML) models offer the potential for the system
crossing the eastern U.S. to become a little faster than the
current average, and the mean of new 00Z runs thus far does appear
to be a tad faster.

Behind this system, a shortwave reaching the Northwest by Monday
should ultimately press southeastward and support a frontal system
that would most likely reach the Plains/east-central U.S. by
Thursday-Friday. Some details will become increasingly dependent on
very uncertain flow details upstream. Already by Wednesday the full
range of ensemble members becomes very diverse over the Pacific
Northwest and vicinity (ranging between a ridge or trough) with
this spread broadening further into late week. This spread
originates from differences in how central Pacific trough energy
may separate around Tuesday. Beyond this spread, operational model
runs and even ensemble means have been waffling between
ridge/trough solutions. While the 12Z MLs varied for details as
well, they at least supported lower heights aloft than latest
GFS/GEFS/ICON runs which represented the strong ridge cluster. At
the same time, latest consensus suggests that upper ridging may
hold on over the Northwest a little longer than depicted by the 12Z
CMCens mean. Preference tilted somewhat more toward the 12Z
ECMWF/ECMWF mean relative to the 12Z GFS/GEFS mean (which were less
extreme with upper ridge strength than in the 18Z cycle), along
with modest inclusion of the 12Z CMC.

...Weather/Hazards Highlights...

The system progressing eastward from the southern Plains during the
first half of the week will be one generator of potentially heavy
rainfall. The best signal currently exists along and near the Gulf
Coast Monday-Tuesday, in association with a warm front lifting
north through the region and followed by the system`s trailing cold
front. The new Day 4 Excessive Rainfall Outlook maintains
continuity with a Slight Risk area extending from southeastern
Texas through southern Alabama and a portion of the Florida
Panhandle while the Day 5 ERO introduces a Slight Risk area over
southern Georgia and northern Florida. Over these two days, this
region shows the best combination of instability and anomalous
moisture along with synoptic features to enhance focus. A Marginal
Risk area extends farther north into parts of the Middle
Mississippi Valley/Ohio Valley into southern Mid-Atlantic over the
two-day period, corresponding to a favorable combination of
instability, guidance signals for scattered heavy QPF maxima, and
currently wet ground conditions. There are two other areas worth
monitoring for Day 5, one over the central Plains/Midwest where a
front may stall, and another over the Northeast where a northern
stream front may enhanced rainfall. At this time there is too much
spread among the guidance to merit any risk area but that could
change if solutions gravitated toward the heavier side of the
envelope.

From early Wednesday onward, first expect some lingering rainfall
near the East Coast as the leading system departs. Then areas from
the southern Plains eastward may see yet another episode of heavy
rain as a front pushes into and through the Plains/Mississippi
Valley and the first system`s trailing front lifts back north as a
warm front. Rain of varying intensity will also be possible farther
north across parts of the central/eastern U.S.

Uncertainty with the flow aloft over the northern half of the West
by mid-late week leads to a potential range between dry conditions
with above normal temperatures or some precipitation spreading
through the Pacific Northwest/northern Rockies with temperatures
closer to normal.

Farther south, California and Nevada should be more consistently
above normal next week with a scattering of plus 10-15F anomalies.
The northern Rockies/High Plains should also be on the warm side
early in the week. The clouds and heavy rainfall potential over
the Gulf Coast/Southeast early in the week should initially keep
highs below normal over that region. Otherwise, areas to the east
of the Rockies will tend to be within a few degrees of normal for
highs while the eastern U.S. should see somewhat above normal lows
Tuesday-Friday.

Rausch


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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