894
FXUS02 KWBC 040646
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
246 AM EDT Sat May 4 2024

Valid 12Z Tue May 07 2024 - 12Z Sat May 11 2024


...Northern Plains to Midwest Heavy Rain Threat early next week...

...Mid-South Excessive Rainfall Threat into next Midweek...


...Overview...

It remains the case that recent models and ensembles indicate a
synoptic pattern that swings upper-level troughs/lows onshore
across the western U.S. that lead to and subsequently favor
downstream upper focus and cyclogenesis over the northern mid-
section of the country followed by gradual lifting of the troughs
toward the Northeast. The beginning of the medium-range period
(Tuesday morning) will likely feature one such cyclogenesis over
the northern High Plains. This will support cooler than normal
temperatures, gusty winds, and precipitation including higher
elevation snow in the West early in the week. Rain that could be
locally heavy shifts into the northern Plains and downstream
through the Midwest. Meanwhile farther south, mean upper ridging
will lead to warming pre-frontal temperatures for much of the
central and eastern U.S. that could yield some record values.
Upstream, Pacific upper trough energy will work inland over an
unsettled West and to the south-central U.S.. This is likely to
interact with a wavy and stalling lead front and combine with
pooling moisture and instability to fuel rain and thunderstorms
over the south-central states/Mid-South and onward/eastward into
later next week, still also under some influence from the lead
upper low/trough.

...Guidance/Predictability Assessment...

18 UTC GFS and 12 UTC ECMWF/Canadian model solutions were well
clustered for Tuesday-Thursday and generally in step with ensembles
and National Blend of Models (NBM) guidance. This bolsters
forecast confidence in a composite model blend to provide maximum
system detail as consistent with a period with above normal
predictability. Gravitated guidance preference toward ensemble
means into later next week, and opted to edge toward the tad more
robust ECMWF ensemble mean. WPC product continuity is decently
maintained with this plan. Latest 00 UTC guidance remains in line.

...Weather/Hazards Highlights...

As a main/closed upper low tracks out of the West with additional
energies reaching the West Coast, widespread precipitation will
include some enhanced snow potential given cooled temperatures. The
higher elevations of the Great Basin into the Intermountain
West/Rockies will see May snow as the medium-range period begins
Tuesday morning. Vigorous troughing aloft in conjunction with a
potent cold front will lead to windy conditions and a threat of
high winds from much of the Rockies to High Plains as the
cyclogenesis should be well under way across the northern High
Plains into Tuesday. Precipitation is forecast to be ahead of the
system as it spreads across the northern Plains to Midwest Tuesday
into midweek. Rainfall on the west side of the strong surface low
could be heavy across eastern Montana into the Dakotas given strong
upper-level support for lift with a slowdown of the storm track.
However, this region is not particularly sensitive to flooding
concerns, and instability should be limited wrapping around the
low. A WPC Excessive Rainfall Outlook (ERO) Marginal Risk area
remains in place Day 4/Tuesday and now into Day 5/Wednesday for
this area given the potential for protracted heavy rain, but likely
without widespread significant flooding impacts. The Marginal Risk
extends southeast into the Upper/Mid-Mississippi Valley, where
more instability will be in place for higher rain rates, but storms
will likely be faster-moving early week. The Storm Prediction
Center also has portions of the Plains and Mississippi Valley
delineated with potential for severe weather early through midweek.
Convective rainfall is forecast to shift east into the Great Lakes
and Ohio Valley Tuesday into Wednesday with system progression and
intercepting return flow. The Marginal Risk ERO was maintained
farther east into the Great Lakes, Ohio Valley and down toward the
mid-Mississippi Valley for Day 4/5 as the fronts push farther
toward the east and northeast. A Slight Risk threat area has been
introduced for areas in vicinity of the Mid-South Day 5/Wednesday
as energy interacts with a wavy and stalling front and combines
with pooling moisture and instability. This will fuel a favorable
pattern for rain and thunderstorms across the south-central
states/Mid-South and broadly onward to the East where downstream
energy track may support moderate late next week coastal low
development, likley off New England.

Much cooler than average maximum temperatures will slowly moderate
while spreading next week from the West to the northern Plains
as the closed upper low ejects eastward. But farther southeast,
mean upper ridging will meanwhile spread quite warm pre-frontal
temperatures from the South through the Appalachians/Mid-Atlantic
to threaten record seasonal values. 90s are forecast to become more
widespread across the southern tier, with temperatures over 100F
(and even higher heat indices) into south Texas and the possibility
for the first excessive heat days of the year.

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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