382
FXUS02 KWNH 010701
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Wed May 1 2024

Valid 12Z Sat May 04 2024 - 12Z Wed May 08 2024


...Renewed heavy rain threat in the south-central U.S. this
weekend...


...Overview...

An upper low that is strong for this time of year is forecast to
move into the West this weekend and track east into the Plains as
next week progresses. This will support cooler than normal
temperatures, gusty winds, and precipitation including higher
elevation snow in the West. Ahead of this, 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 this weekend, potentially in
some areas that have received ample rainfall recently and/or will
during the short range period. Rain and storms are also possible
farther east. Then mean upper ridging downstream will lead to
warming temperatures in much of the central and eastern U.S. into
next week.


...Guidance/Predictability Assessment...

The first model diagnostics concern during the medium range period
is with the upper low that drops through the eastern Pacific and
enters the West this weekend. Though models have been gradually
converging over time toward a middle ground, there are still some
remaining differences in the position and depth of the upper low.
For multiple days now, GFS runs have showed the farthest southwest
and deepest low/trough diving south and then east, while ECMWF runs
have been shallow and generally fast. This continued in the 12/18Z
cycle though with less dramatic differences than in earlier
cycles. Given the convergence toward a middle solution and
AI/machine learning models generally in the same ballpark, a blend
of the 12Z GFS (which was less extreme than the 18Z GFS) combined
with the 12Z ECMWF/CMC and some GEFS and EC ensemble means seemed
to create the preferred intermediate solution. Fortunately the
incoming 00Z model suite seems to finally be in better alignment
with the timing and depth of the low at least through the weekend.

The upper low is forecast to pivot east/potentially northeast into
the north-central U.S. during the workweek, along with a
reasonably deep surface low. Models continue to show spread with
those features with the main outlier the 12Z CMC which was farther
east. Additionally, upstream, yet another round of energy and
troughing may dive through the Pacific Monday-Wednesday. The 12Z
CMC was also quite aggressive with this energy. Overall favored a
blend heavy on the GEFS and EC ensemble means along with some
lingering components of the GFS and ECMWF late in the forecast
period.


...Weather/Hazards Highlights...

As the upper low tracks into and through the West, widespread
precipitation is likely. Moisture streaming perpendicular into the
foothills of the Sierra Nevada could cause a heavy rain and
flooding threat there, and a Marginal Risk remains in place for Day
4/Saturday. Meanwhile the higher elevations of the Sierra, north
into the Cascades, and east into the Intermountain West/Rockies
will see some May snow. Precipitation is forecast to move steadily
eastward through the weekend into early next week, reaching the
northern Plains to Midwest Monday and Tuesday. The trough/low aloft
and frontal boundaries could also produce some high winds across
the Great Basin to Rockies.

Meanwhile this weekend, the subtropical jet looks to be active
coming into the south-central U.S., 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. On Saturday and Sunday, Marginal Risks are delineated in
the ERO across southern/central portions of the Plains and
Mississippi Valley. Will also maintain an embedded Slight Risk on
Day 4/Saturday for portions of southwest Oklahoma and into portions
of northwest Texas where potential exists for an MCS with heavier
rainfall rates, though the position varies somewhat between models.
Farther east on Saturday, an axis of instability is forecast
across the southern/central Appalachians and into the Piedmont
between the synoptic cold front and backdoor front in the East as a
shortwave aloft moves through. These ingredients could produce
heavy rainfall and a Marginal Risk is in place there for Day 4 as
well. Showers and thunderstorms should continue for parts of the
eastern U.S. on Sunday.

For the weekend, portions of the east-central U.S. are forecast to
see above average temperatures, but the Northeast and Mid-Atlantic
could be cooler than average at least for highs due to a backdoor
front. Temperatures this weekend should warm in the Intermountain
West to northern Plains ahead of the West upper low, while cooler
than normal temperatures by 10-20 degrees for highs will impact the
West. As the upper low/trough shifts east, this should cool down
temperatures toward the north-central U.S. while staying below
average in the Northwest. On the other hand, 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 into southwest Texas.


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










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