609
FXUS02 KWNH 020702
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Thu May 2 2024

Valid 12Z Sun May 05 2024 - 12Z Thu May 09 2024


...Heavy rain will continue in the south-central U.S. into
Sunday...


...Overview...

An upper low that is strong for this time of year is forecast to
move through the West early next week and track east 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 then shifts into the northern Plains
to Midwest. Farther south, a subtropical jet coming into the south-
central U.S. and the western side of a front stalling will lead to
rain and thunderstorms in the southern half of the Plains and
Mississippi Valley into Sunday, potentially in some areas that have
received ample rainfall recently and/or will during the short
range period. Then mean upper ridging will lead to warming
temperatures in much of the central and eastern U.S. next week.


...Guidance/Predictability Assessment...

Recent model guidance, especially the newer 00Z cycle, is in
reasonably good agreement with an upper low coming through the
Great Basin as the period begins Sunday that shifts east Monday and
then pivots northward and slows over the northern Plains. A strong
surface low (that models show could break low pressure records for
May) will accompany it. Model spread was well within typical
ranges through Tuesday, though the details of exact strength and
track and QPF waffle. A model blend favoring the deterministic GFS,
ECMWF, and CMC with small proportions of the GEFS and EC ensemble
means worked well for the first few days of the forecast.

By Wednesday and Thursday, model spread increases as the upper low
gets trapped between ridging to its east and an upper high to its
northwest in a blockier pattern. In general the 12/18Z model cycle
took the surface low a bit farther north while the older and newer
00Z runs had it more suppressed. Reinforcing energy may dive in
from western Canada into the Northwest and complicate matters. CMC
runs generally have this energy weaker but entering the U.S.
sooner, allowing the troughing to progress eastward, while GFS runs
are stronger but delayed with the energy and leave it in the West.
The CMC initially appeared to be an outlier but many of the EC-
based AI/machine learning models were similar to it. The 12Z ECMWF
was more like the GFS but the newer 00Z run is more similar to the
CMC and AI models. So there is certainly some remaining spread with
this. The WPC forecast shifted toward favoring more GEFS and EC
ensemble means late in the period given the increasing model
spread.


...Weather/Hazards Highlights...

As the upper low tracks into and through the West, widespread
precipitation is likely. The higher elevations of the Sierra
Nevada,
north into the Cascades, and east into the Intermountain
West/Rockies will see some May snow. The trough/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. Thus a Marginal Risk is in place in the Day 5/Monday ERO for
this area given the potential for unusually heavy rain but without
widespread significant flooding impacts. The Marginal Risk extends
southeast into the Upper/Middle 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. Rainfall is forecast to shift east into the Great
Lakes and Ohio Valley Tuesday-Wednesday.

Farther south, the subtropical jet looks to be active coming into
the south-central U.S. Sunday, focusing convection in a moist and
unstable environment. Many areas in the south-central U.S. will
have quite saturated antecedent conditions because of recent and
upcoming short range events, so this will be a factor for flooding
as well. Though there is still some variability, models seem to be
converging on a position for heavier rainfall in eastern Oklahoma
and vicinity, so the ERO for Day 4/Sunday will embed a Slight Risk
within the broader south-central U.S. Marginal for that area.
Fortunately into the workweek, rain chances should lessen for those
waterlogged areas. The Eastern Seaboard can also expect some rain
on Sunday, with perhaps a nonzero chance of locally heavy rain.

Much cooler than average temperatures by around 15-25 degrees for
highs will be present in the West on Sunday underneath the upper
low. The anomalously cool temperatures should moderate somewhat as
the week progresses, but spread into the northern High Plains
after a warm day on Sunday. But farther southeast, mean upper
ridging will cause warm temperatures to spread across the south-
central and east-central U.S. into the Eastern Seaboard. 90s are
forecast to become more widespread across the southern tier, with
temperatures over 100F (and even higher heat indices) into
southwest Texas and possibility for the first excessive heat days
of the year.


Tate


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














$$