411
FXUS02 KWNH 030649
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 AM EDT Fri May 3 2024

Valid 12Z Mon May 06 2024 - 12Z Fri May 10 2024


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


...Overview...

An upper low that is strong for this time of year is forecast to
move through the West by early next week and track/meander into
the Plains and Midwest as the week progresses. 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 may 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 into later next week,
still also under some influence from the lead upper low/trough.

...Guidance/Predictability Assessment...

Model and ensemble solutions overall seem reasonably clustered for
Monday-Wednesday and generally in step with National Blend of
Models (NBM) guidance. This bolsters forecast confidence in a
composite GFS/ECMWF/UKMET/Canadian model blend to provide maximum
system detail as consistent with feature predictability. Opted to
switch preference to a composite of best clustered GFS/ECMWF and
GEFS/ECMWF ensemble means along with the NBM for later next week
to temper detail some, acting to mitigate growing but managebale
forecast spread. WPC product continuity is decently maintained in
this manner. Latest 00 UTC guidance has mainly stayed in line.

...Weather/Hazards Highlights...

As a main/closed upper low tracks out of the West as addiitonal
energies feed back through the West Coast, widespread precipitation
will include some enhanced snow potential given cooled
temperatures. The higher elevations of the Sierra Nevada, north
into the Cascades, and east into the Intermountain West/Rockies
will see May snow. Troughs/low aloft and frontal boundaries could
also produce some high winds across the Great Basin to Rockies to
High Plains. Precipitation is forecast to move steadily eastward
through early next week, reaching the northern Plains to Midwest
Monday and Tuesday. 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. 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 in the Day
4/Monday and into Day 5/Tuesday 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. The Storm Prediction Center also has portions of the
Plains delineated with potential for severe weather on Monday.
Convective rainfall is forecast to shift east into the Great Lakes
and Ohio Valley Tuesday into Wednesday with system progression and
intercecting return flow. A Day5 Marginal Risk ERO was introduced.

Upstream, aforementioned Pacific upper trough energy will work
inland over an unsettled West and over the south-central U.S. into
later next week, riding under the base of the lingering main upper
low/trough. These will interact with wavy and stalling fronts and
combine with pooling moisture and instability to fuel a favorable
pattern for rain and thunderstorms across the south-central
states/Mid-South and onward to the East where downstream energy
track may support moderate late next week coastal low development.

Much cooler than average maximum temperatures will slowly moderate
while spreading next week from the West to the northern Plains
with slow ejection of a closed upper low. 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







$$