057
FXUS02 KWNH 220702
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Thu May 22 2025

Valid 12Z Sun May 25 2025 - 12Z Thu May 29 2025


...Heavy rain and flash flooding concerns continue in the Ozarks
and vicinity Sunday...


...Overview...

A pattern supporting heavy rain and thunderstorms will be in place
as the period begins Sunday, as an upper trough atop the Rockies
Sunday moves east with ample moisture and instability streaming
into a frontal system in the central U.S. ahead of it. Flash
flooding is possible in the Ozarks and vicinity, continuing from
the short range period. This trough tracking east will combine with
another trough stretching from the East westward across the Great
Lakes, causing the trough to dig atop the east-central U.S. and
finally press the front and the resultant rainfall southeast across
the southern tier into the Appalachians and eventually the Eastern
Seaboard. The rounds of troughing aloft will cause cooler than
normal temperatures in parts of the central and eastern U.S., while
upper ridging building in the Intermountain West should allow for
warmer than average temperatures there.


...Guidance/Predictability Assessment...

Model guidance continues to be agreeable on the broad scale (with
typical detail differences) at the start of the medium range period
Sunday, indicating an upper trough extending westward from an
Atlantic Canada upper low across the northern tier and another
trough atop the Four Corners toward the northern High Plains. These
troughs are forecast to combine and suppress the initial
subtropical ridge over the southern tier. At the surface there is
general consensus that this will push the main front south and
east, but there are some differences as a surface low emerges as
dominant in response to the trough. The low should track eastward
into the Lower Ohio Valley or so early Tuesday, but the 18Z GFS was
particularly fast to take the low (and spread QPF to) the Mid-
Atlantic by then. But there is reasonable agreement that the
combined trough will slowly move east Tuesday-Thursday over the
eastern half of the U.S., with newer 00Z models indicating an
embedded upper low in the Great Lakes vicinity around midweek.

Farther west, models are agreeable that an upper ridge will
dominate across the Interior West through much of next week. Broad
troughing will set up in the eastern Pacific with initial and then
reloading northeast Pacific upper lows. This will set up an omega
block pattern with the Pacific trough, West ridge, and East trough.
But the more uncertain piece is with potential for energy to break
off and form southern stream troughing or an upper low or two
moving into the West Coast. Dynamical and AI models all show a lot
of spread with this potential. With the 12/18Z model cycle, the CMC
was particularly aggressive with a stronger upper low atop
California early Tuesday compared to other guidance. Then
midlatitude Pacific energy may also reach California around
Thursday, though the 12/18Z GFS and 12Z ECMWF were offshore with
this feature. The 00Z model cycle fortunately seems to be showing
more convergence in solutions, as the 00Z CMC weakened somewhat
while the EC/GFS were a bit stronger Tuesday 12Z and thus align
well. Additionally, the next trough/possible low is trending a bit
farther east and reaching California by Thursday.

The WPC forecast was able to use a general model blend early in
the period, with increasing proportions of the ensemble means to a
little more than half Days 6-7 given increasing spread in the
individual models.


...Weather/Hazards Highlights...

A wavy west-east oriented front will be in place over the south-
central Plains and Mississippi Valley on Sunday. Moisture
(precipitable water) values likely over the 90th percentile and
ample instability will stream into the front, and the right
entrance region of the jet provides dynamical support. This will
lead to additional heavy rain, perhaps in multiple rounds, likely
centered in the Ozarks and vicinity on Sunday. A Slight Risk is in
place for the Day 4/Sunday ERO from the south-central Plains into
parts of the Lower and Middle Mississippi Valley. Over the Ozarks,
this is considered a higher end Slight Risk that may become a
Moderate Risk eventually, depending on the exact overlap between
Sunday`s rain and heavy rain during the short range period. Due to
models converging better on timing and placement of convection, was
able to reduce the size of the broader Marginal a bit on the
southern and eastern sides, compared to the previous couple of
issuances.

By Memorial Day, the trough aloft should finally press
the front gradually south toward the southern tier and east toward
the Appalachians. Moisture and instability in place could still
lead to heavy rain rates across the south-central U.S. into the
Tennessee Valley and Lower Ohio Valley, but at least there should
be relatively less potential for training storms as the front
moves. For the Day 5/Monday ERO, show a Marginal Risk delineated
across these areas as a starting point. Embedded Slights may be
needed in future issuances but the details still need to be refined
with time. On Tuesday, the eventual dominant low on the frontal
system should lift northeast and bring localized heavy rain to the
Eastern Seaboard, especially the Mid-Atlantic, while the trailing
front still lingers and causes showers and storms across the
southern tier Tuesday-Wednesday.

Farther north, the initial trough should produce some modest
precipitation in the central Rockies (even some snow in the highest
peaks) and the northern/central Plains early next week. The
frontal boundary curling back into the southern/central Rockies and
High Plains could help produce scattered convection there next
week as well. Meanwhile, rounds of fronts coming through the
Northwest could allow for light to moderate precipitation.
Summertime scattered storms are also likely across the Florida
Peninsula.

One more day of hot weather is likely for Texas across the
Southeast on Sunday. Temperatures well into the 90s to around 100
will support areas of Major HeatRisk in southern parts of Texas and
Louisiana. Meanwhile, cooler than normal temperatures by 5 to 15
degrees will take hold over the north-central to eastern U.S. into
early next week underneath upper troughing. The trough and front
pressing south will allow for temperatures to cool in the southern
tier Monday and Tuesday. The cooler temperatures should gradually
moderate closer to normal as next week progresses. But in the West,
a building upper ridge will lead to warmer than average
temperatures by 10 to 15 degrees for several days over the Great
Basin and northern/central Rockies.


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


$$