194
FXUS02 KWNH 230615
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
214 AM EDT Thu May 23 2024

Valid 12Z Sun May 26 2024 - 12Z Thu May 30 2024


...Heavy Rainfall/Flooding Threats from parts of the Midwest to
the Northeast Sunday-Monday..

...South Texas to the Central Gulf Coast and South Florida Heat
Threat into early next week...


...Overview...

An upper trough (which may be a combination of different energy
streams) will traverse the CONUS through the medium range period.
This will serve as the main driver for heavy rainfall potential
early in the period from the Midwest/Mid-South into the East. After
Monday, the overall pattern should turn more amplified as an upper
ridge builds over the interior West allowing the downstream trough
to deepen and slow as it reaches the East early next week.
Upstream, a deep upper low anchored off the West Coast should
eventually send energy into the Northwest and northern tier around
the middle of next week. Meanwhile, ridging over Mexico will help
maintain a hazardous heat threat from south Texas to the Gulf Coast
and southern Florida through about Monday before a cold front
sweeps through the South.


...Guidance/Predictability Assessment...

Models and ensembles continue to show reasonably good agreement on
the overall large scale upper level pattern through much of the
medium range period, but there are some small-mid scale system
differences and run to run variances that remain more uncertain.
Early in the period, a northern stream shortwave into the northern
Plains and a leading southern stream shortwave should interact with
a compact closed low and amplified troughing over the Ohio Valley
by Monday. After Monday, there is a lot of uncertainty still on the
evolution and speed of this trough which is dependent on
additional closed low energy dropping in from south-central Canada.
Eventually, this should create renewed troughing over the East
that could be slow to move out of the region.

Shortwave energy from a closed low just off the British Columbia
coast should enter the Northwest around the middle of next week,
but there is also a lot of uncertainty on the speed and amplitude
of this feature. Through the 12z/18z May 22 model runs, the ECMWF
was the most amplified with this, but the new 00z GFS did come in
to match. But given the late period timing, a lot can still change.


The WPC forecast tonight was based on a blend of the GFS, ECMWF,
and CMC through Tuesday when model agreement was still decent.
After this, trended more towards the GFS with the ensemble means to
help mitigate the smaller scale differences while still
maintaining some system definition. This also maintained good
continuity with the previous WPC forecast for Days 3-6.


...Weather/Hazards Highlights...

A surface low and associated fronts will be the focus for strong
to severe thunderstorms and heavy rainfall/flooding potential
Sunday and Monday from parts of the Midwest to the East. A marginal
risk of excessive rainfall remains in place across much of the
Ohio Valley and into the Tennessee Valley on Sunday/Day 4, shifting
into the Northeast on Monday/Day 5. Especially Sunday, the
guidance continues to show elevated QPF totals, but still with a
lot of spread in exactly where. It is possible that a the guidance
signal becomes more defined, an embedded slight risk may be needed
somewhere.

The heaviest rainfall should exit the East Coast after Monday with
the cold front, but renewed troughing across the region will allow
for showers across the Great Lakes and into the
Appalachians/Northeast beyond Monday. Elsewhere, rain chances
should increase from the Northwest to northern Plains while
additional showers expand across the Southern Plains as energy
ejects out from the Southwest late period.

Expect hazardous heat to continue into the medium range period
from South Texas through the Central Gulf Coast and South Florida
with daytime highs as much as 10-15 degrees above normal with max
heat index values possibly reaching 115 degrees, especially for
South Texas. Highs near 100 degrees could stretch farther north
across the southern High Plains on Sunday as well. The combination
of dangerous heat indices and lack of overnight recovery/cooling
will allow for a major to extreme heat risk across South Texas,
with widespread major heat risk also extending into the Central
Gulf Coast region as well as southern Florida. Temperatures should
trend closer to normal after Monday across much of the South
following a strong cold front passage, but southern Florida should
remain warm into mid-next week. The forecast pattern will favor
below average highs over the Northwest to northern Plains for
Sunday still but above normal temperatures will build again across
the West early next week underneath of amplified upper ridging that
shifts into the Central U.S. Above average temperatures along the
eastern seaboard will moderate back to normal next week with some
chance for below normal highs around mid-next week.

Santorelli


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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