041
FXUS02 KWNH 290701
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Mon Apr 29 2024

Valid 12Z Thu May 02 2024 - 12Z Mon May 06 2024

...Widespread rain and thunderstorms are likely in the central U.S.
through at least Thursday...


...Overview...

As the medium range period begins Thursday, mean troughing
amplifying atop the Rockies will provide support for widespread
convection as moisture and instability spread into the central U.S.
and cause heavy rain and flash flooding. Lifting upper troughing
moving east by Friday should push rain chances into the east-
central and eastern U.S., though some may remain over the south-
central U.S. this weekend near a lingering frontal boundary.
Additionally, rounds of precipitation including some higher
elevation snow are possible at times across the Northwest into the
northern/central Rockies, but confidence in the details is still
lower than desired given ample spread in the model guidance.


...Guidance/Predictability Assessment...

Troughing across the West and ridging across the East will be the
main theme early in the medium range period, but even Thursday-
Friday there are some model differences within this overall
pattern. The core of the upper trough/low should be located in the
northern Plains/south-central Canada, but models show discrepancies
on the southward extent of the trough as it pivots eastward. GFS
runs (particularly the 12Z) and the 12Z UKMET dug the trough deeper
than the 12Z ECMWF/CMC. Now the newer 00Z GFS is not as deep and
is more agreeable with the EC/CMC, but the 00Z UKMET stayed deep
and seems to be more clearly an outlier. Meanwhile a smaller
shortwave/compact upper low upstream shows some differences
Thursday too as it comes into the Northwest, which may play a
factor in the evolution of the main trough downstream. This smaller
feature`s clustering in position seems better in the incoming 00Z
model guidance. The ridging in the East seems pretty agreeable in
the guidance.

By late week into the weekend, attention turns to an upper low
dropping southeast from the northeastern Pacific toward the West
Coast. Recent models are now somewhat more agreeable in showing a
midsize and closed upper low, rather than a huge upper low or just
troughing/energy, which were both possibilities shown by the models
a day ago. However, the position still varies greatly among the
deterministic models and ensemble members and to some extent the
AI/machine learning (ML) models. By early Saturday recent GFS runs
have been on the southwestern side of the spread to varying degrees
depending on the run, which then leads to the upper low farther
south into California as it translates east early next week. The
deterministic runs are generally deeper than the individual
ensemble members, limiting confidence in the operational runs.
Meanwhile the 12Z and now 00Z ECMWF do not dig the low as far
south, taking it across the Northwest over the weekend and quickly
into the northern Plains. The 12Z ML models were generally in
between these southwest and northeast extremes with the feature.
The 12Z CMC seemed like a more reasonable proxy for the low
position as it was in between the two, somewhat like the ML models.
There are also questions into early next week whether or not the
feature stays separate or gets combined with yet more energy and
troughing coming in from the Pacific. In all, the confidence for
this feature is low and changes to the forecast are likely.

The WPC forecast was based on a blend favoring the deterministic
models early in the period, but quickly ramped up the proportion of
ensemble means given low confidence in the deterministics. A split
between the EC ensemble mean (which was more like its
deterministic run in taking troughing east across the northern
tier) and the GEFS ensemble mean (with troughing pulled more
offshore) seemed to provide a reasonable middle ground.


...Weather/Hazards Highlights...

On Thursday, ingredients for widespread rain and thunderstorms
potentially causing flash flooding will be in place across central
parts of the country. Lowering heights as upper troughing digs in
the Rockies will provide lift and combine with ample moisture and
instability. On Thursday model guidance already shows heavy rain
totals of 3-5 inches in some areas and high rain rates are likely
as well. A large Slight Risk remains in place for the Day
4/Thursday ERO for eastern parts of the Plains into much of the
Mississippi Valley. Within this risk area, there may be somewhat
greater potential around northeast Texas to eastern Oklahoma and
western Arkansas--and this general area will likely be sensitive to
additional rain due to wet antecedent conditions. There is
potential for embedded upgrades within the Slight Risk depending on
details of prior convection and how guidance consolidates for
amounts/location of heaviest rainfall. The main change to the
previous Slight Risk was to expand it northward into Wisconsin.
There, morning rainfall is expected to be widespread and serve to
wet the ground before a more unstable airmass comes in as a warm
front lifts through the area for thunderstorms with possibly higher
rain rates during the evening. By Friday, the cold front
progressing east in the northern two-thirds of the country or so
will transition rain chances more into the Great Lakes and
Ohio/Tennessee Valleys, and toward the Appalachians to Eastern
Seaboard on Saturday. However, the frontal boundary looks to linger
longer or possibly lift across the the southern Plains and produce
additional rounds of rain and storms through the weekend. A
Marginal Risk is in place Friday across portions of the lower
Mississippi Valley where there may be some heavier rainfall focus.

Meanwhile, areas from the Northwest into the northern and central
Rockies may see rounds of precipitation including snow in higher
elevations through the latter part of the week into the weekend. On
Thursday, the cooling associated with the upper trough could bring
some early May snow into the northern and central Rockies. There
is some chance that snow may make it into the Front Range of
Colorado on Thursday. There is still quite a bit of model spread
into late week and the weekend, which affects the precipitation
forecast. Currently the best guidance cluster shows another round
of precipitation in the northern Rockies Friday with a compact
upper system crossing the area, while an upstream Pacific system
renews precipitation of varying intensity across the Northwest from
late Friday or Saturday through Sunday. The eastern and southern
extent of the precipitation is uncertain and will be refined with
time.

The eastern third of the country will be warmer than average by
10-15 degrees through late week, with 80s reaching the Ohio Valley
and Mid-Atlantic. Meanwhile cooler than average temperatures are
forecast across the Rockies and parts of the Plains. These cooler
temperatures should push east behind a cold front and help moderate
temperatures a bit in the East, though could remain a few degrees
above normal. Temperatures across the Rockies to Plains are
forecast to warm up into early next week due to upper ridging ahead
of the Pacific trough. The West Coast may become cooler than
normal this weekend, but this will depend on how far east/onshore
the Pacific trough reaches.


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



$$