299
FXUS02 KWBC 281858
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Tue May 28 2024

Valid 12Z Fri May 31 2024 - 12Z Tue Jun 04 2024


...General Overview...

Guidance shows an East Coast upper trough as of Friday moving into
the western Atlantic/Canadian Maritimes thereafter (with possible
embedded upper low), while cool Canadian surface high pressure
provides the East with pleasant weather for the end of this week.
Expect a warming trend over the East by early next week with above
normal temperatures also gaining coverage over other parts of the
lower 48 as well. Meanwhile, a rather unsettled weather pattern
should be in place across the Central U.S. with multiple shortwave
disturbances aloft and surface waves/fronts fueling rounds of
showers and thunderstorms late this week into early next week.
Expect the Pacific Northwest to trend wetter over the course of
the period as a strong jet underneath a Northeast Pacific mean low
brings in moisture and a series of fronts. Most of the Southwest
and the Northeast should remain mostly dry, and the upper ridge
anchored over Mexico may gradually increase its northward extent
and thus maintain hot and humid conditions over southern Texas.

...Guidance/Predictability Assessment...

Early in the period there is decent agreement in principle for a
Plains/Mississippi Valley shortwave and separate northern tier
flow to the south of a central Canada upper low, but scale of the
initial Plains shortwave reduces predictability for specifics and
associated surface reflection. Meanwhile consensus shows a Pacific
shortwave moving into the West during the weekend. By early next
week solutions diverge regarding the relative strength of this
shortwave and a potential upstream feature, as well as strength of
upper ridging that may reach the northeast. Among recent guidance,
the GFS/ECMWF and their means provided a relative majority
depicting continued eastward progression of this western shortwave
into east-central North America by next Tuesday. These solutions as
well as 00Z ECMWF-initialized machine learning (ML) models suggest
that latest CMC runs may be too strong with their Northeast upper
ridging late in the period. As for the potential upstream Pacific
shortwave arriving into the Northwest by early next week, dynamical
models and MLs are split over the relative strength with some run-
to-run variability, favoring an intermediate solution at this time.

The updated forecast based on 00Z/06Z guidance started with an
operational model composite for about the first half of the
period, with greater weight on the 06Z GFS/00Z ECMWF relative to
the 00Z UKMET/CMC. The remainder of the blend incorporated some of
the 06Z GEFS/00Z ECens means to account for increasing guidance
spread for details at the more distant time frames.

...Weather/Hazards Highlights...

Numerous showers and thunderstorms are likely to remain ongoing
across much of the central and southern Plains going into Friday
and the weekend. Multiple weak disturbances aloft, with the best-
defined one likely drifting from the Plains into Mississippi Valley
Friday into early Saturday, will interact with a warm and humid
air mass and one or more surface waves/fronts. A broad Marginal
Risk area continues for the Day 4 Excessive Rainfall Outlook period
Friday-Friday night from parts of Texas and Lower Mississippi
Valley into the Midwest. There is an embedded Slight Risk area
centered over the ArkLaTex region, where MCS activity is most
likely to develop based on the latest deterministic guidance. In
addition, wet ground conditions from recently observed rain or
what falls in the short term may contribute to the flash flooding
threat. There may be some abatement in the coverage of convection
on Saturday across the Central U.S., but still enough to merit a
few Marginal Risk areas where the best combination of upper level
forcing and moisture convergence will likely exist. Of these, one
over Texas and another over and south of the Lower Ohio Valley have
relatively better confidence based on latest guidance. Some strong
to severe thunderstorms are also likely each day. The potential
exists for scattered QPF maxima of 2-4 inches for the Friday to
Saturday time period for some of these areas that get repeated
rounds of convection.

After early Sunday, an advancing northern tier frontal system may
increase convective emphasis over the Midwest for a time before
extending farther southward. Meanwhile the Pacific Northwest
should trend wetter by the weekend and early next week, perhaps
with some totals on the heavy side for the time of year. Low-
predictability shortwave details will determine the magnitude and
duration of rainfall on a day-to-day basis.

Cool high pressure will bring below-normal temperatures, especially
for morning lows, to areas from the Lower Great Lakes/Ohio Valley
into Mid-Atlantic Friday into Saturday. Flow around the high as it
settles over the western Atlantic will support near to above
average readings thereafter, with some locations in the Northeast
possibly reaching at least 10F above normal early next week.
Meanwhile above normal temperatures should spread across a majority
of the West late this week into the weekend. The warmth will likely
expand to include the High Plains into Upper Midwest from late
weekend into the next work week. Best potential for one or more
days of highs at least 10F above normal will be between California
and the High Plains. It will remain very hot and humid for much of
southern Texas through the period under the northern periphery of
persistent upper ridging anchored over Mexico. The wetter pattern
forecast over the Pacific Northwest may bring highs down to near or
below normal values during the weekend and early next week.

Rausch/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



















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