Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Sat Jun 1 2024

Valid 12Z Tue Jun 04 2024 - 12Z Sat Jun 08 2024

...Excessive heat likely to continue over southern Texas, while
heat builds in the West for the latter half of next week...


...Overview...

As the medium range period begins Tuesday, expect an upper-level
shortwave to track across the north-central U.S. along with a
surface low pressure/frontal system, spreading rain and
thunderstorms to much of the Mississippi Valley. This shortwave
trough will meet a somewhat blocky pattern farther east and
slow/deepen over the east-central U.S. late week into the weekend,
with rain chances gradually shifting toward the eastern U.S. along
and ahead of the cold front. Meanwhile an upper ridge atop Mexico
and southern Texas Tuesday will expand to affect much of the
western U.S. midweek and beyond, producing potentially hazardous
heat. Then an upper trough may approach the West Coast late week
but with uncertain timing.


...Guidance/Predictability Assessment...

Recent model guidance is more agreeable than it was a day ago
regarding the timing of the shortwave tracking near the
U.S./Canadian border Tuesday-Wednesday. This feature is likely to
form a closed upper low by Thursday near to northwest of the Upper
Great Lakes. Deterministic and AI models are more variable with the
placement of the upper low and the southern extent of the
resultant trough. The newer 00Z GFS/CMC end up reaching farther
south with the trough than the previous 12/18Z cycle, but the 00Z
ECMWF elongates the upper low west-east strangely by next Saturday.
Thus the details are still in question by late week/weekend with
this feature.

The relatively better agreement with the north-central to eastern
U.S. trough also yields better model consensus regarding the
western U.S. ridge, but uncertainties remain on the ridge's
periphery. The evolution of the eastern trough/low will affect the
ridge's extent north, with the greatest differences of ridging vs.
troughing arising in the north-central U.S. by Friday-Saturday.
Southern stream energy and possible closed upper low west of Mexico
through much of the period could play a role in the ridge's shape
on the southwest side. Then, there is model spread with timing of
troughing in the eastern Pacific approaching the Northwest late
week. The ECMWF is showing ridging strongly holding on in the
Northwest with the Pacific trough well west all the way through
Saturday, but EC-based AI/ML models indicate the trough farther
east. However, the GFS runs are well on the eastern side of the
envelope. A middle ground more like the ensemble means seemed best
for timing of that approaching troughing.

The WPC forecast utilized a multi-model deterministic blend early
in the forecast period given good agreement in the large scale
pattern at least. As the period progressed, transitioned to a blend
incorporating the GEFS/EC ensemble means to half Day 6 and more
Day 7, given the increasing spread and uncertainty.


...Weather/Hazards Highlights...

A vigorous shortwave and frontal system (anchored by southern
Canada low pressure) reaching the Dakotas into the Upper Midwest on
Tuesday will provide forcing and a focus for rain and
thunderstorms as instability abounds ahead of it. Smaller scale
shortwaves farther south should also track eastward and promote
scattered convection. A Marginal Risk is in place for the Day
4/Tuesday ERO for much of the Mississippi Valley and expanded a bit
into the Mid-South based on recent model trends. Some locations
within this area will be sensitive due to rainfall from prior days.
The best focus for widespread moderate to heavy rain is likely
across the Upper Midwest, but uncertainty in amounts and placement
as well as the effects of the rainfall precludes a Slight Risk
issuance at this time. Into Wednesday, convection is forecast to
shift east ahead of the cold front into the Great Lakes region and
Ohio and Tennessee Valleys, and a Marginal Risk is delineated for
those areas. For both days, the forward motion of the storms may be
a limiting factor for flooding to become too concerning. Steady
progression of the front will take rain chances into the East for
Thursday-Friday, with possible focus of heavier amounts from the
northern Mid-Atlantic into the interior Northeast where the best
forcing is.

Some lingering precipitation is forecast in the Northwest on
Tuesday before drying out midweek with the amplifying ridge aloft.
Gusty winds are possible in the northern Rockies behind the strong
southern Canada/northern tier U.S. surface low on Tuesday. Into
late week, some return flow of moisture is possible across the
southern/central Rockies and Plains, but with considerable
uncertainty in coverage and amounts of rain. The same can be said
of possible precipitation affecting the Northwest late week.

Next week will remain hot across South Texas westward through the
Rio Grande Valley, where the combination of hot temperatures into
the 90s and 100s as well as humidity will keep heat indices in the
upper 100s, locally 110+. Temperatures nearing or exceeding 100F
may stretch across the southern High Plains through Tuesday but
should moderate somewhat by midweek behind a frontal passage. Some
above normal temperatures are possible across the northern tier at
times. But the next big concern will be increasing heat across the
West as upper ridging builds in. Highs broadly will run 10-15
degrees above average, with even warmer anomalies of 15-20+ degrees
for the central California valleys and portions of the Great
Basin. Temperatures should reach over 100F across the central
California valleys, and near/over 110F in the Desert Southwest.
Scattered daily record high minimum and maximum temperatures are
possible. Per the Climate Prediction Center, this hot pattern could
continue into early next week.


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