117
FXUS02 KWBC 021858
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM 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 may approach by early next week but with uncertainty in
timing/evolution. The forecast pattern may support increasing
coverage and intensity of showers/storms over the central or south-
central Rockies and Plains late week into the weekend.


...Guidance/Predictability Assessment...

Within the general western ridge/eastern trough mean pattern that
most guidance loosely shows in some fashion, there is continued
wide spread for details in the shape of ridge along with
depth/broadness of the upper trough (plus anchoring upper low
track). There is also a lot of spread and variability for eastern
Pacific upper troughing that may being pushing into the ridge by
next weekend, to go along with divergence for the northward extent
of an upper low initially tracking toward Baja California. If
anything, the forecast for much of Canada into the lower 48 seems
to have become more complicated over the past couple days.

In general the most recent runs of the machine-learning guidance
(ECMWF-initialized and GFS GraphCast) have been tilting a little
south or southeast versus most dynamical solutions with the path of
the primary upper low expected to cross parts of the Great
Lakes/Northeast or southeastern Canada. Isolated runs have been as
far south as the northern Mid-Atlantic, which the new 12Z GFS also
briefly wobbles over around early next Sunday. Among guidance
available for forecast preparation, the 06Z GFS/00Z CMC were
closest to the ML models in principle. While the ensemble means
track the upper low a little farther north, they still like a
fairly amplified upper trough over the eastern U.S., in contrast to
some ECMWF/UKMET runs that become much more shallow with eastern
troughing. The new 12Z ECMWF shows a different wrinkle, showing an
amplified trough into Friday followed by dominance of upstream
west-northwesterly flow that leads to a more shallow East Coast
trough thereafter. Ongoing wild cards in the forecast include
ridging over Hudson Bay/northern Ontario (a stronger ridge pushing
the upper low farther south) and the ultimate evolution of a
developing west-central Canada upper low which guidance seems to
be more defined with than in previous days but ultimately with wide
divergence for what becomes of it after Friday.

Meanwhile, GFS trends toward slower eastern Pacific trough
progression into Friday-Saturday have led to better guidance
agreement near the West Coast into early Saturday but then guidance
as a whole diverges for specifics. Aside from typical detail
differences, the ML models are remarkably agreeable in showing and
upper trough finally reaching the West Coast by early Sunday. In
contrast, the ECMWF and sporadic GFS runs (00Z and 12Z) have been
dropping an upper low southward well offshore with minimal
troughing yet to approach the Pacific Northwest. The GEFS/ECens
means also keep the overall trough axis west of 130W as of early
Sunday.

With the UKMET continuing to look excessively flat with northern
tier flow, the updated forecast based on 00Z/06Z solutions started
with a blend of the 06Z GFS and 00Z ECMWF/CMC early in the period
followed by a transition toward a model/ensemble mean (06Z GEFS/00Z
ECens) mix with the operational component increasingly skewed
toward the 06Z GFS/00Z CMC relative to the 00Z ECMWF. This approach
seemed to provide the best compromise among dynamical and ML ideas,
with the overall pattern also maintaining fairly good continuity.
Frontal details become more uncertain/variable later in the period,
depending on low-predictability shortwave impulses.


...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 in the Day 4/Wednesday Excessive Rainfall Outlook 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. Most guidance is still not too
enthusiastic about heavy rainfall potential, but available
moisture/instability ahead of a fairly strong upper low may be
enough to produce some fairly intense rain rates. Thus the Day
5/Thursday ERO depicts a Marginal Risk area for the interior
Northeast and northern Mid-Atlantic near the best forcing and
lower flash flood guidance. Uncertainties persist regarding the
precise evolution and track of the approaching upper low/trough
which will affect the speed of the surface 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 and next weekend given the
persistence of upper troughing.

Farther west, some return flow of moisture (including perhaps
Pacific moisture advecting around the upper 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. There is
currently not a sufficiently strong signal for existence/intensity
of this activity to warrant a risk area in the Day 5 ERO. 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 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.


Rausch/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





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