175
FXUS02 KWBC 031900
PMDEPD

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

...Possible heavy rain across the interior South later next week...


...Overview...

Models and ensemble means continue to indicate a synoptic pattern
that swings upper-level troughs/lows onshore across the western
U.S. that would favor cyclogenesis over the mid-section of the
country followed by gradual lifting of the troughs toward New
England. The beginning of the medium-range period (Monday morning)
will likely feature one such cyclogenesis over the central to
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 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/Canadian model blend to provide maximum system
detail as consistent with feature predictability. Opted to edge
toward the ECMWF and EC mean by Days 6 and 7 in favor of the more
southern placement of the heavier rainfall across the interior
South for next Thursday and Friday. WPC product continuity is
decently maintained in this manner. The WPC forecast package was
based on 40% blend of the 00Z ECMWF/EC mean, 40% from the 06Z
GFS/GEFS, and 20% from the 00Z Canadian/GEM mean, increasing to 55%
from the EC solutions by Day 7.

...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
Monday 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 Monday
and Tuesday with some timing differences among models (faster with
the ECMWF). 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 gradual 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 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 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 5 as the fronts push farther toward the east and
northeast.

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
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.

Kong/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









$$