925
FXUS02 KWBC 310710
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
310 AM EDT Fri May 31 2024

Valid 12Z Mon Jun 03 2024 - 12Z Fri Jun 07 2024

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


...Overview...

The early part of next week will feature progressive shortwaves
and frontal systems in the northern two-thirds or so of the lower
48, leading to precipitation chances in the Northwest on Monday and
possibly multiple rounds of convection in the central and eastern
U.S. through midweek. Meanwhile very hot and humid conditions
should continue across southern Texas under the northern periphery
of an upper ridge anchored over Mexico, to the south of the storm
track. By the latter half of the week, a more amplified and
stagnating pattern develops, as upper ridging expands from Texas
into the Interior West (expanding the coverage of potentially
hazardous heat), while an upper trough axis develops downstream in
the East.


...Guidance/Predictability Assessment...

Model guidance is reasonably agreeable as the forecast period
begins Monday, with a shortwave over the north-central U.S. that
lifts northeast by Tuesday while additional energy approaches the
Northwest. This latter feature will be a concern for model
diagnostics and for sensible weather though, showing more model
spread. Deterministic models all develop a closed upper low with
this system by Tuesday, but differ considerably with its
positioning/how progressive it is, especially by midweek. The 12Z
ECMWF is the slowest/farthest southwest of the deterministic
guidance, with GFS runs faster and the CMC in between. The ECMWF`s
slow track seemed unlikely considering that the EC-based AI/machine
learning models were all faster/farther northeast than the
operational ECMWF. Thus preferred the 18Z GFS/12Z CMC upper low
track. The 12Z GFS seemed agreeable through Wednesday-Thursday but
then dove energy southeast to form a closed low atop the southern
Appalachians Friday, which was an outlier. With the incoming 00Z
guidance, the models are generally faster than their previous
runs (though the 00Z CMC seems a little rogue with diving an upper
low toward the Mid-South Friday). The new 00Z ECMWF now looks more
like the older 18Z GFS, while the 00Z GFS is even farther
east/faster than its previous runs. So choosing the faster models
may have been good for this forecast with only the 12/18Z guidance
cycle available at the time, but overall there is still uncertainty
with the timing yet to be resolved.

Models are agreeable in indicating an upper ridge building into
the Southwest for midweek and beyond. However, there are
uncertainties with southern stream troughing breaking off in the
east Pacific west of Baja California, and with the northeastern
extent of the ridge in the central U.S. dependent on where the
east-central U.S. trough ends up setting up. By Friday models go
out of phase with the flow pattern nearing the Northwest, with GFS
runs showing troughing while EC/CMC runs have ridging.

Considering these factors, the WPC forecast was based on a blend
of deterministic guidance early on, reducing the proportion
particularly of the 12Z ECMWF and eventually the 12Z GFS in favor
of the GEFS and EC ensemble means, with the means reaching 60
percent of the blend by Day 7.


...Weather/Hazards Highlights...

An upper-level wave and atmospheric river coming into the
Northwest will bring heavy rainfall continuing into Monday.
Marginal Risks are in place for flooding concerns with the heavy
rain. Snow levels may be lowering some by Monday in the Cascades
and northern Rockies as the shortwave moves overhead, but there is
still some concern for rain at elevations that still have a
snowpack, possibly increasing the flooding threat. Precipitation
should then exit Tuesday-Wednesday, with dry conditions across the
West as ridging builds.

An initial low pressure/frontal system in the central U.S. and the
next one coming in from the West will lead to rounds of showers
and thunderstorms broadly across central and eastern portions of
the country. The focus for potentially heavier rain is rather
uncertain for Monday as instability abounds in widespread areas,
but there still seems to be a broad model signal for
Missouri/Arkansas and surrounding states to maintain a Marginal
Risk there for the Day 4 ERO. By Day 5/Tuesday, the focus for
convection is a little more clear as instability pools ahead of the
low pressure system tracking near the U.S./Canadian border and its
cold front extending south. A Marginal Risk is in place across
northern and central parts of the Mississippi Valley Tuesday. As
the cold front tracks east, rain should shift toward the Ohio
Valley and Appalachians by Wednesday-Thursday. Additional weak
upper-level energies from the Southern Plains to the Southeast will
also keep up the potential for additional storms.

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 early in the week 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 be running
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 reaching over 110F in the Desert
Southwest. Per the Climate Prediction Center, this hot pattern
could continue at least into next weekend.


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


$$