898
FXUS02 KWBC 301911
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
311 PM EDT Sun Jun 30 2024

Valid 12Z Wed Jul 03 2024 - 12Z Sun Jul 07 2024

...Major heatwave expected for the lower elevations of California
and the Desert Southwest next week, and remaining hot and humid for
the south-central to southeastern states...

...Overview...

Multiple shortwave passages across the northern tier of the nation
will fuel scattered to numerous showers and storms for the middle
to end of the week, particularly across the Midwest and portions
of the Central Plains/Middle Mississippi Valley where the heaviest
rainfall is likely. Heat and humidity will continue to make
weather headlines across much of the southern tier states as a
strong ridge of high pressure aloft governs the overall weather
pattern, and heat also expands across much of California and the
lower elevations of Arizona and southern Nevada as the upper ridge
builds across the West by the end of the forecast period. There
will also be copious moisture in place across much of the Southern
Rockies through midweek that will fuel scattered storms, with a
brief break in the moist pattern before it likely returns this
weekend.


...Guidance/Predictability Assessment...

Recent model guidance remains in good agreement with the mid- to
large scale features with just some minor mesoscale differences
with the shortwaves crossing southern Canada through around Day
5/Friday. The greatest differences in the northern stream arise
with a shortwave diving from southwestern Canada into south-central
Canada and/or the northwestern U.S. over the weekend. The 00Z GFS
seemed most aggressive in pushing troughing from this shortwave
into the western U.S. compared to the consensus of other models.
The incoming 12Z model suite still shows some spread with this
feature.

Also of concern for model diagnostics is the track of Hurricane
Beryl as it is forecast to reach the Bay of Campeche or
southwestern Gulf of Mexico this weekend. Deterministic and AI/ML
models and ensemble members show spread in its position. The
largest clustering shows Beryl being steered into northern Mexico.
But there is some guidance that curves the system farther north
into the U.S., including the 00Z CMC that seemed to be a fast and
northeastern outlier. Beryl`s track will continue to be monitored
over the next several days, and see NHC`s website for official
forecasts.

The updated forecast was based on a blend of the deterministic
models favoring the 06Z GFS and 00Z ECMWF early in the period, with
inclusion and gradual increase of the GEFS and EC ensemble means
to about half by Day 7.


...Weather/Hazards Highlights...

There will likely be a few mesoscale convective complexes that
cross the Central Plains and Midwest on Wednesday in conjunction
with a quasi-stationary frontal boundary. Deep moisture advecting
northward from the western Gulf will intercept this boundary and
fuel the development of showers and strong thunderstorms, some of
which could train over the same general areas and lead to some
instances of flooding. With the potential for some 1-2 inch per
hour rainfall rates, a Slight Risk area remains valid going into
the Day 4 ERO Wednesday across eastern Kansas into Missouri and
western Illinois. Marginal Risk areas stretch east into the Ohio
Valley/Lower Great Lakes where scattered convection with
potentially heavy rain rates is likely to develop ahead of a cold
front. A couple of rounds of thunderstorms farther west in Kansas
and vicinity prompted including that region in a Marginal Risk area
as well, in the biggest change to the previously issued outlook.
By Day 5/Independence Day, the overall QPF signal in the guidance
is looking more diffuse with scattered higher QPF maxima, so a
broad Marginal Risk still seems to be the best way to go for now,
with potential Slight Risk areas in future updates as the details
become more evident. Farther north, additional mesoscale convective
systems are likely to form and track through north-central parts
of the country. A Marginal Risk is in place for parts of the
Dakotas on Day 4 and into the Upper Midwest Day 5. Good support for
lift should be present there with a shortwave, but with less
anomalous moisture as farther south. A cold front slowly pressing
east will eventually spread thunderstorms to the Eastern Seaboard
into the weekend, while more rounds of storms are possible in the
central U.S. as well.

Monsoonal flow into the Southwest states will likely continue the
threat for showers and storms going into Wednesday. There is a
Marginal Risk in place across parts of Arizona and New Mexico into
the central High Plains for the Day 4 ERO, with saturated soils in
many of these areas owing to heavier rainfall in the short range
forecast period. The good news is the deeper moisture should be
south of the Mexico border for the 4th of July, so no Day 5 ERO
risk is needed there. This break may be brief, however, as tropical
moisture may advect into the region again by the weekend.

It will continue to remain hot and humid across the Southern U.S.
for the middle of the week and into next weekend, with the hottest
conditions centered across the Southern Plains on Wednesday-
Thursday where triple digit high temperatures are likely. The heat
will abate somewhat there into late week and the weekend (but still
feeling summer-like), while the Southeast to Mid-Atlantic should
see temperatures and heat indices rising. It will also be getting
very hot for the inland lower elevations of California and into the
Arizona Deserts with widespread 105-115 degree readings likely,
and locally higher for the normally warmer locations. Above normal
temperatures by 10-20 degrees should stretch into Oregon and
Washington late week into next weekend as well.


Tate/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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