190
FXUS02 KWBC 041831
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
231 PM 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...

Latest 00z and 06z guidance continues to show good agreement for
the overall synoptic pattern, though plenty of uncertainty still
in the details which has impactful implications on QPF and surface
features. A general compromise of the deterministic solutions
offered a good starting point for the Tuesday-Thursday/Day 3-5
forecast. After this, there remain significant differences between
model guidance and run to run continuity on how far south and east
the lingering northern Plains energy gets into the Great
Lakes/Northeast. Also uncertainty out west with the evolution of
energy into the Great Basin and a blocky upper ridge/high to the
north. The WPC blend trended towards more ensemble mean influence
late period to mitigate these concerns. This is consistent both
with previous WPC continuity as well as the ECMWF-initialized AI/ML
models.

...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 into Day 5/Wednesday for this
area given the potential for protracted heavy rain, but likely
without widespread significant flooding impacts. 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 remains 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 remains in the
vicinity of the Mid-South Day 5/Wednesday as energy interacts with
a wavy and stalling front and combines with pooling moisture and
instability. The Storm Prediction Center also highlights a broad
area of the Midwest to Mississippi Valley/parts of the Southern
Plains for severe weather potential Tuesday and Wednesday. Beyond
Wednesday, a favorable pattern will continue 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, likely
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 Tuesday-Thursday
before moderation by Friday and Saturday.

Santorelli/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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