Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
253 AM EDT Sun Apr 21 2024

Valid 12Z Wed Apr 24 2024 - 12Z Sun Apr 28 2024

***Showers and thunderstorms with heavy rainfall increase in
 coverage across the Central U.S. by the end of the week***

...General Overview...

The forecast period begins on Wednesday with an amplifying upper
trough over the Northeast U.S., and this will propel a cold front
across much of the Eastern U.S. for the middle of the week. Surface
high pressure builds in across the Great Lakes and then across the
East Coast region to close out the week. Meanwhile, a synoptic
scale trough develops across the Western U.S. by Friday and into
next weekend with a couple of shortwaves ejecting out across the
Plains, and supporting surface cyclogenesis east of the Rockies.
This will likely result in multiple rounds of showers and storms
for the Central U.S., with some heavy rain and severe weather
likely by the end of the week.

...Guidance/Predictability Assessment...

The first forecast challenge is centered on the approach of two
shortwaves in the northern stream, potentially phasing and closing
off in the Northeast by the Wednesday-Thursday time period. The
GFS and its ensemble mean continue to support a stronger/more
phased system that features a deep closed low over northern New
York by 00Z Thursday. The 00Z non-NCEP guidance has trended more
amplified compared to the 12Z runs, but have the core of the closed
low farther north than the GFS.

The other area of concern is across the West with the next round
of shortwave energy moving into the Desert Southwest and
eventually ejecting out across the Plains by Friday. Recent runs of
the GFS have been faster and somewhat stronger compared to the
non-NCEP guidance. Then the next system behind that amplifies
further across the Western U.S. over the weekend, with relatively
good agreement among the ECMWF and GFS, although recent CMC runs
have varied more with the depth and timing of the trough. The
fronts/pressures forecast was primarily derived from a general
deterministic model blend early on, with slightly more weighting to
the ECMWF and CMC through Thursday, and then gradually increasing
proportions of the ensemble means to about 50% by Sunday.

...Weather/Hazards Highlights...

The upper trough and cold front passing through the Northeast U.S.
Wednesday will be accompanied by widespread light to moderate rain,
and high elevation snow for portions of New Hampshire and Maine.
The GFS is the heaviest solution for QPF given its stronger upper
level system. Farther west, warm air advection across Texas and
Oklahoma will bring a return to scattered showers and storms across
this region, with some of the model guidance suggesting the
potential for some local one inch rainfall totals. However, the
guidance is not locked in on the location of these scattered QPF
maxima, and therefore it seems prudent to hold off on any potential
risk areas for the Day 4 outlook on Wednesday until a better model
signal develops for placement. More widespread showers and storms
are then expected to develop across portions of Kansas, Nebraska,
Iowa, and Missouri going into Thursday (Day 5) as moisture and
elevated instability increase with surface low pressure system
development across the Central Plains. There is enough support in
recent model runs to support a Marginal Risk area where multiple
thunderstorm complexes are likely to develop. The potential also
exists for some strong to severe storms ahead of the dryline in the
Southern Plains and extending northeast to Arkansas to close out
the work week, so this will continue to be monitored and the Storm
Prediction Center has additional information pertaining to this.

Much of the West should have above normal temperatures on
Wednesday, before returning to near or slightly below normal
levels for much of the West Coast and Intermountain West to close
out the week, as the upper level trough encompasses the region.
Warmer temperatures and increasing humidity are forecast for the
central/southern Plains by next Friday and beyond as the upper
ridge axis builds back, and southerly flow from the western Gulf
advects moisture northward ahead of the developing surface low.
This spreads northeastward to include the Ohio Valley and the
Appalachians, with more widespread above normal readings for
overnight lows (+10 to +20 degrees) with conditions more typical
of May or early June by next weekend. Highs could get over 100
degrees for portions of southern Texas by Saturday.

Hamrick


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