165
FXUS02 KWBC 040816
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
416 AM EDT Tue Jun 4 2024

Valid 12Z Fri Jun 07 2024 - 12Z Tue Jun 11 2024

...Major HeatRisk expected for many areas in the West late week
with record high temperatures...

...Overview...

Broadly, a western upper ridge and eastern upper trough pattern
will set up late this week, though weak troughing may approach the
West Coast early next week. The ridge will produce potentially
hazardous heat across much of the West through the end of the week.
The forecast pattern should support increasing coverage and
intensity of showers/storms over the central or south-central
Rockies and Plains as systems ride the ridge through much of the
forecast period. Meanwhile, periods of light to modest rain are
likely in the northeastern U.S. underneath the slow-moving
trough/low.


...Guidance/Predictability Assessment...

While model guidance is generally agreeable in showing the western
ridge/eastern trough pattern in some fashion, differences in the
details will impact sensible weather across the Lower 48. Most
models have fortunately converged somewhat regarding the placement
of the Great Lakes upper low and the depth of the trough extending
south through Friday-Saturday, though the 12Z CMC seemed too fast
to move it eastward. Into Sunday and beyond though, the
low/trough`s movement eastward (or lack thereof) becomes
questionable. One concern with the evolution of this upper low is
the possible interaction with another upper low to its northwest
over Canada. Overall the 12Z guidance indicated this low over
Manitoba or so on Sunday and moving southeastward, serving to kick
the Great Lakes low east early next week. The 18Z GFS was generally
alone in keeping this low well northwest of the other guidance
where the two lows do not interact much. So leaned away from the
18Z GFS...but the newer 00Z model solutions came in more like the
18Z GFS, with the Canada upper low farther removed northwest and
the Great Lakes upper low stalling and keeping the trough deeper
across the East early next week. So there will likely need to be
some changes to the next forecast to show this trend.

In the West, ridging through late week is agreeable in the model
guidance. Models show a southern stream upper low near/west of Baja
California opening up as it approaches the Southwest this weekend,
with some expected detail differences but will at least help
decrease the heat. Much farther north, upper low development in the
northeast Pacific Friday-Saturday will extend troughing south that
approaches the West Coast early next week. The upper pattern over
the Gulf of Alaska is complicated as two upper lows exhibit a
Fujiwara evolution, but models generally show weakening troughing
approaching the Northwest by Sunday. This trough should track east
Monday-Tuesday but with the details dependent on the pattern
downstream.

The WPC forecast used a blend of deterministic guidance favoring
the ECMWF and GFS early in the forecast, lessening the
incorporation of the deterministic models (particularly the 18Z
GFS, possibly incorrectly) as the period progressed in favor of the
GEFS and EC ensemble means. The means reached 60 percent of the
blend by Days 6-7. Additional changes to the forecast are expected,
particularly given how the trends in the Great Lakes upper
low/trough will impact fronts and temperatures and QPF.


...Weather/Hazards Highlights...

Some return flow of moisture (including Pacific moisture advecting
around the upper ridge) is likely across the southern/central
Rockies and Plains, with one or more surface boundaries helping to
focus multiple rounds of showers and thunderstorms. Placement of
this convection is uncertain, as reflected by the relatively low
QPF, but potential for much higher amounts exists. A Marginal Risk
is delineated on the Day 4/Friday ERO across Kansas and Oklahoma
and clipping surrounding states. By Day 5/Saturday, moisture
transport is forecast to increase even more and the instability and
overall setup will likely cause heavy rain rates. Recent model
guidance is notably agreeable at least for now in a heavy rainfall
axis of 3-5 inches near the Oklahoma/Kansas/Missouri/Arkansas
intersection. Given these factors, have a Slight Risk delineated
for that area on Saturday within a broader Marginal covering south-
central parts of the Plains and Mississippi Valley. Though the
exact placement may have to shift around in future cycles, the
ingredients will certainly be sufficient for flash flooding
concerns. Additional rounds of rain are likely in the Plains and
south-central parts of the Mississippi Valley early next week.
Rain chances should also gradually increase across the southern and
central Rockies late week through the weekend and remain into
early next week. The threat for heavy rainfall to cause flash
flooding there seems sub-Marginal at this point given recent drier
than average conditions. Scattered light rainfall is possible late
in the week over parts of the Interior West/northern Rockies,
likely increasing in coverage and amounts early next week as upper
troughing approaches.

Rounds of rain and possible thunderstorms are expected across the
Lower Great Lakes to the Northeast underneath the upper trough. The
heaviest rain amounts during the medium range period are forecast
on Friday, but since the initial front will be coming through and
amounts look to decrease compared to the short range period, do not
have any Marginal Risk in the ERO for Day 4/Friday. However, there
is a non-zero risk of flooding concerns especially if heavy rain
falls in similar areas as the previous days.

The western ridge will promote notably/record hot temperatures
across the West especially for the latter part of this week. Highs
broadly will run 10-20 degrees above average and locally 20+
degrees above average in the Great Basin. High temperatures above
110F are forecast for the Desert Southwest (including Las Vegas),
with 90s and 100s stretching into the central California valleys to
the Great Basin to much of Texas. Daily record high minimum and
maximum temperatures are possible. Through Friday, the experimental
HeatRisk product shows a large area of Major (level 3 of 4) and
some Extreme (level 4 of 4) outline. The higher temperature
anomalies look to shift into the Northwest for the weekend with
some minor moderation of the temperatures and HeatRisk in the
Southwest as the weakening upper low approaches from northwestern
Mexico. Meanwhile, most areas east of the Rockies should trend to
slightly below normal behind the cold front. Exceptions to this
trend will be Texas (especially southern areas), and Florida where
highs several degrees above normal may lead to some daily records.


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




$$