Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Tue Apr 23 2024
Valid 12Z Fri Apr 26 2024 - 12Z Tue Apr 30 2024
...Severe thunderstorms and heavy rain possible for the Plains and
Mississippi Valley late this week through this weekend...
...Overview...
The upper level pattern through this weekend will feature mean
troughing over the west-central U.S. while a ridge builds over the
East Coast. This pattern will result in varying degrees of
unsettled weather over the western/central U.S. and fairly benign
weather over the East. Of particular focus, a couple of potent
shortwaves/embedded upper lows will travel through the base of the
mean trough and eject northeastward across the Plains/Upper
Midwest, triggering rounds of active and potentially hazardous
weather across portions of the Plains and Mississippi Valley. The
pattern will progress gradually early next week, with a
northeastern Pacific upper low drifting toward British Columbia
(with a broad trough to its south) while the trough associated with
the second Plains system continues into the East and the East Coast
upper ridge finally moves offshore.
...Guidance/Predictability Assessment...
With guidance showing reasonable agreement for the leading central
U.S. system, primary discrepancies involve the second system to
emerge from the West into the Plains and then the Northeast Pacific
upper low and surrounding flow. For the second Plains storm, the
past couple ECMWF runs have been on the fast side of the spread
while the UKMET has been slowest. With 00Z ECMWF-initialized
machine learning models (MLs) also spread out for timing,
preference remains with an intermediate timing that is closest to
the GFS/CMC. The new 12Z ECMWF has adjusted slower to this favored
solution. Meanwhile there is a fair amount of spread among
dynamical models/ensembles and the MLs for the path and timing of
the Northeast Pacific upper low along with its surrounding flow.
The general theme is for moderately slow progression (but
potentially some progression of leading energy), and away from the
CMC/CMC mean that are quicker to bring additional energy behind
this primary feature.
Based on latest guidance comparisons among 00Z/06Z solutions, the
updated forecast started with a composite of operational models
early-mid period and then transitioned to a mix of models and 06Z
GEFS/00Z ECens means along with phasing out the CMC by next Tuesday
as it compared less favorably to other guidance over the Northwest.
...Weather/Hazards Highlights...
Late this week, an upper level shortwave will move northeast
across the Central U.S. and will be accompanied by a surface low
pressure system. The surface low will move across the Central
Plains and Upper Midwest on Friday, lifting a warm front north
across the South-Central U.S. and pushing a cold front across the
Central/Southern Plains and Middle/Upper Mississippi Valley. This
system should produce showers and thunderstorms over portions of
the Plains and Mississippi Valley, and the ingredients of
instability and moisture will be in place for locally heavy
rainfall totals. A broad Marginal Risk area remains in place for
portions of the Plains and Middle/Upper Mississippi Valley in the
Day 4 (Friday- Friday night) ERO. The southern tail of the trailing
cold front may stall across the Southern Plains while the next
system emerges from the West to develop another surface low over
the Southern/Central Plains. The combination of this front's
stalling, some signal for locally heavy rainfall, and wet
antecedent conditions from either recent rain (eastern Texas) or
rain forecast before Day 4 (Oklahoma/Arkansas) would appear to
support a relatively greater flash flood threat from far
northeastern Texas into Arkansas. Thus the Day 4 ERO plans to
introduce a Slight Risk area over this region. Meanwhile the
forecast pattern should focus another round of heavy rainfall
across the Southern Plains and Middle Mississippi Valley. Within
the Marginal Risk area being maintained for these regions in the
Day 5 (Saturday-Saturday night) ERO, this afternoon's update
proposes an upgrade to Slight Risk from north-central Oklahoma into
northwestern Missouri. There is a reasonable signal from the
guidance and general pattern evolution for heavy rainfall
potential, albeit with some lingering uncertainty for specifics.
The proposed Slight Risk incorporates the GFS/UKMET/CMC cluster
that is slower than the 00Z ECMWF for the second Plains system, and
is a rough middle ground among the ensemble guidance.
In addition to heavy rainfall, severe thunderstorms will be
possible ahead of the dryline in the warm sector of this system,
and the Storm Prediction Center has highlighted portions of the
Plains and Mississippi Valley in the Severe Weather Outlook for
Friday through Sunday. Isolated to scattered severe thunderstorms
will be possible and may bring a threat of damaging winds, large
hail, and tornadoes. On the western side of the dryline, strong
winds and low humidity will create and increased wildfire risk for
portions of New Mexico, west Texas, the Texas and Oklahoma
panhandles, and far southeastern Colorado.
As the second Plains System lifts northeastward and the upper
pattern shifts gradually eastward, the trailing cold front and its
associated rainfall will push eastward as well. By next Tuesday the
best focus for rain should be from the Northeast to the Lower
Mississippi Valley. Totals should generally be lower than during
Friday-Sunday but some locally heavy activity will still be
possible.
From the West into parts of the High Plains, mean upper troughing
will allow precipitation chances to persist across much of the
region through the end of this week into the weekend, but drier
conditions will likely develop over the Southwest by Sunday.
Precipitation will fall as rain at lower elevations and as snow at
higher elevations. In association with the system emerging from the
West, guidance is suggesting potential for anomalous moisture and
low level upslope flow to produce a localized area of heavy
precipitation (mostly rain) over the central High Plains. While
this scenario is not unanimous, there seems to be enough of a
cluster advertising significant totals to merit a Marginal Risk
area centered over northeastern Colorado in the Day 5 ERO. The
system approaching from the Northeast Pacific early next week may
produce some light to moderate rain/mountain snow over the
Northwest.
Temperatures will trend above normal across most of the eastern
half of the country late this week into the weekend as a warm
front lifts northward and the anchoring system's cold front stalls
ahead of the next system to emerge into the Plains by Sunday. High
temperatures will likely reach the 70s and 80s over most areas,
with 90s and potentially over 100 degrees possible for some
locations in south Texas by Saturday. The warmth will be confined
more to locations east of the Mississippi River by early next week
as the second Plains system pushes its cold front eastward.
Meanwhile cool highs of 5-15F below normal should progress across
the West and into the northern Plains Friday-Sunday, followed by a
warmer trend that may lift highs to 10-15F above normal over the
Four Corners region and parts of the Plains by next Tuesday.
Rausch/Dolan
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
