Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Sun Jun 2 2024

Valid 12Z Wed Jun 05 2024 - 12Z Sun Jun 09 2024

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


...Overview...

Broadly, a western upper ridge and eastern upper trough pattern
will set up later this week into next weekend. As the medium range
period begins Wednesday, expect an upper low in south-central
Canada along with a surface low pressure/frontal system to spread
rain and thunderstorms to the east-central U.S. 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 will expand
from Texas to affect much of the western U.S. midweek and beyond,
producing potentially hazardous heat. An eastern Pacific upper
trough or two may approach by early next week but with uncertainty
in timing.


...Guidance/Predictability Assessment...

Model guidance remains in good agreement that an upper low in
south-central Canada will produce a trough axis extending into the
Midwest midweek. However, by Thursday-Friday, the model diagnostics
process becomes much more challenging as the 12Z cycle of
dynamical and AI/machine learning (ML) models diverge with the
track of the upper low. Dynamical models from the 12/18Z cycle
generally trended toward the low shifting/expanding eastward (not
much southward) into late week, along with the AIFS ML model. Thus
the associated troughing was not as deep into the Southeast as the
previous 00Z model cycle showed. However, all other available AI/ML
models that are EC based, as well as the GFS Graphcast, dove
energy southeastward and formed an upper low in the east-central
U.S. by Friday. This divergence of solutions does not inspire
confidence in the forecast especially from Friday onward, despite
the supposed deterministic model agreement. The ensemble means were
a bit deeper with the troughing in the East compared to the
deterministic models. It seemed prudent to lean toward these as a
bit of a nod toward most ML models, and also to be closer to
continuity. Thus for the WPC forecast, had to heavily favor the 18Z
GEFS and 12Z EC ensemble means in the forecast, reaching half the
model blend by Thursday and comprised the whole model blend over
the weekend. Fortunately the 00Z GFS and CMC at least have trended
farther south with the upper low center and the southern extent of
its trough by late week. The 00Z ECMWF is less so, possibly in part
due to differences in splitting energy into an upper low farther
west in central Canada that was unlike other guidance.

This ensemble mean-heavy blend also worked well for the pattern in
the West. The upper ridge should be well established across Texas
to the Four Corners states by Thursday, but then there are some
differences in timing and placement of a southern stream upper low
expected to be west of Baja California Wednesday-Thursday sneaking
north or northeast for late week into the weekend. Meanwhile, a
trough in the northeast Pacific continues to show spread in timing
of its approach but also with the potential to break off an upper
low diving south in the eastern Pacific. The variations in the
deterministic and ML models yield low confidence so the ensemble
means seemed like a good intermediate solution.


...Weather/Hazards Highlights...

On Wednesday, a cold front anchored by a southern Canada surface
low will be pushing through the east-central U.S., helping to focus
rain and storms along and ahead of it. A broad Marginal Risk is in
place for Day 4/Wednesday from the Lower Mississippi Valley into
the Tennessee and Ohio Valleys toward the Great Lakes region, with
a moist and unstable environment in all these areas for scattered
convection that could produce heavy rainfall rates. Embedded Slight
Risk(s) in future cycles cannot be ruled out, but the reasonably
fast movement of the storms would be a limiting factor for flash
flood potential. Some storms are possible into the Eastern Seaboard
on Wednesday but without as much dynamical support as farther
west. But by Thursday, the eastward push of the front and upper
trough will shift the best rain chances into the Eastern Seaboard.
Rainfall totals trended lower than the previous forecast, but still
could be heavy enough to cause flooding concerns that could
warrant a Marginal Risk in the Day 5/Thursday ERO for the interior
Northeast and northern Mid-Atlantic near the best forcing and lower
flash flood guidance. Uncertainties in the shape and axis of the
supporting upper trough will affect the speed of the front and in
turn intensity/duration of heaviest rainfall. Even behind the cold
front, there may be diurnally enhanced episodes of rainfall in the
northeastern U.S. into late week given the persistence of upper
troughing.

Farther west, some return flow of moisture (including perhaps
Pacific moisture advecting around the ridge) is possible across the
southern/central Rockies and Plains, with one or more surface
boundaries helping to focus showers and thunderstorms. Scattered
showers and storms may begin Thursday but increase in coverage and
magnitude Friday into next weekend. Some scattered rainfall will
also be possible late in the week over parts of the Interior
West/northern Rockies, with details depending on the position of
the upper ridge axis and how much moisture/shortwave energy may
lift northward to the west of this axis.

South Texas westward into the Rio Grande Valley will remain hot
especially through Wednesday, where the combination of hot
temperatures into the 90s and 100s as well as humidity will keep
heat indices in the upper 100s to 110+. Some slight moderation of
the heat indices is forecast later in the week in Texas. But the
next big concern will be increasing heat across the West as upper
ridging builds in. Highs broadly will run 10-20 degrees above
average, with even warmer anomalies of 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. The higher
anomalies look to drift into the Northwest for the weekend with
some minor moderation of the temperatures in the Southwest.
Meanwhile, most areas east of the Rockies (generally aside from
Texas and Florida) should trend to slightly below normal behind the
cold front.


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