869
FXUS02 KWBC 121855
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
255 PM EDT Sun May 12 2024

Valid 12Z Wed May 15 2024 - 12Z Sun May 19 2024

...Another southern tier heavy rainfall threat mid-late week...


...Overview...

A system reaching the Mid-Atlantic by early Wednesday and a cold
front just to its north/west will support a wet midweek over
portions of the East before the surface low continues eastward into
the Atlantic. Farther west, shortwave energy emerging from the
Rockies should support a wavy frontal system pushing eastward from
the Plains, spreading another broad area of rainfall over the
central/eastern U.S. with highest totals still expected to be
across the southern tier. Guidance is still having difficulty in
resolving some important details though. Potential stalling of the
system`s trailing front near the Gulf Coast could support at least
a localized heavy rainfall threat into the weekend.
Models/ensembles still differ for specifics of flow rounding
eastern Pacific ridging from Wednesday onward with effects
translating eastward with time, but at least the prior extremely
broad solution envelope has narrowed somewhat.

...Guidance/Predictability Assessment...

There was good enough agreement the first half of the period
(Wednesday-Friday) to warrant a forecast blend between the latest
GFS, ECMWF, CMC models. The UKMET was a little quick/strong with
western Canada energy into the northern tier as early as Thursday,
and was not used in the blend today. After Friday, there is a lot
of uncertainty with timing and amplitude of shortwave energy over
the Upper Midwest-Great Lakes and possible phasing with the
southern stream shortwave lifting towards the east-northeast.
Another shortwave should drop into the Northwest next weekend as an
upper low well off the California coast gets drawn eastward.
Models struggle with the details of how these two systems may
interact and also with more weak energy into the Southern Plains
downstream. Given this, opted to trend more towards the ensemble
means to mitigate the differences. This maintains good continuity
with the previous WPC forecast as well.


...Weather/Hazards Highlights...

The primary focus for heavy rainfall potential still looks to be
in the southern half of the Plains by the Wednesday-Wednesday night
time frame covered by the Day 4 Excessive Rainfall Outlook.
Reflecting some developing spread for approaching dynamics aloft
(from the Rockies, as well as a separate Southwest shortwave that
may have some interaction with northern stream flow),
model/ensemble guidance continues to differ for convective
specifics over this region. Although guidance doesn`t show
increasing instability and moisture in this region, models are
showing increasing uncertainty in where heaviest rainfall sets up,
and backing away from the previous general theme of a central Texas
focus. Given this, and after collaboration with the affected WFOs,
have opted to remove the previous slight risk area from the
Wednesday ERO given the downtrend in QPF. There is still somewhat
of a developing signal from some dynamical and machine learning
output toward better convective emphasis a little farther north--
over an area where short term forecast rainfall could lead to
wetter soils heading into Day 4 than currently observed. Will hold
off on any slight risk upgrades to gain better confidence in any
potential adjustments. Meanwhile continuity looks good for the
lingering Marginal Risk over northern Florida as the front reaching
the area stalls, accompanied by instability and above normal
moisture.

For the Day 5 ERO (Thursday-Thursday night), aforementioned spread
for details aloft continue to influence convective details from
the Plains into the Gulf Coast states. The outlook maintains a
Slight Risk area extending from southeastern Texas into southwest
Mississippi, where guidance shows a pronounced surge of moisture
into this region with adequate instability as well. An eventual
need for an upgrade to a moderate risk, especially across very
sensitive regions of southeast Texas into central Louisiana, is
possible but more dependent on additional rainfall in the short
term period. A front farther north may help to focus less extreme
but still locally heavy rainfall, which is accounted for by a
Marginal Risk area that extends into parts of the south- central
Plains and Midwest.

Expect the moisture shield to continue across the eastern U.S.
late in the week, with heaviest rainfall most likely extending
across the Southeast. Depending on specifics of upper flow, the
front trailing from the surface system associated with this episode
could stall near the Gulf Coast and provide a focus for additional
rainfall during the weekend.

An eastern Pacific upper ridge extending into the West should
support above normal temperatures over most of the West Coast
states and parts of the Great Basin during Wednesday-Thursday, with
some areas seeing plus 10-15F anomalies. Thereafter, expect the
warm readings to persist over California/Nevada with eventual
expansion into the southern Rockies while the Northwest trends
closer to normal (possibly below normal over Montana). Most of
Florida may see several days with highs up to 5-10F above normal,
in a warm pattern to the south of a couple fronts expected to stall
upon reaching the far northern part of the state. Combined with
high dew points, these temperatures may bring heat indices as high
as 105-110 across southeast/southern Florida, signaling the first
hazardous heat days of the summer. Otherwise, areas to the east of
the Rockies will tend to be within a few degrees of normal for
highs, aside from below normal over the southern Plains on Thursday
and possibly an axis of modestly above normal readings dropping
southeast from the northern Plains Friday onward. Most of the
eastern U.S. should see somewhat above normal lows through the
period.

Santorelli/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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