312
FXUS02 KWBC 080704
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
304 AM EDT Mon Jul 8 2024

Valid 12Z Thu Jul 11 2024 - 12Z Mon Jul 15 2024


...Heavy rain and flooding concerns for the Eastern Seaboard linger
into late week...

...Dangerous heat wave in the West persists through late week...


...Overview...

What is left of Beryl (namely a post-tropical/frontal low) is
forecast to push across the Lower Great Lakes region as the medium
range period begins Thursday. A stalling front, energy and moisture
from Beryl, and a favorable upper pattern with Great Lakes
troughing and a retrograding shortwave south of western Atlantic
ridging will promote a wet pattern for the Eastern Seaboard for
late week with heavy rain and flooding concerns. Meanwhile, a
persistent upper ridge over the West will continue to support a
dangerous and likely record-setting heat wave across much of the
region. This ridge looks to combine with the westward expanding
Atlantic ridge, while progressive shortwaves move across Canada and
the U.S. northern tier.


...Guidance/Predictability Assessment...

At the start of the period Thursday, models agree that troughing
will be in place over the Great Lakes, in part from Beryl`s
remaining energy, with some spread that affects Beryl`s (frontal)
surface low. The 18Z GFS was among the faster/farther northeast
guidance from the 12/18Z model cycle, but the 00Z model cycle had
the GFS a bit farther west while the ECMWF was east. A model blend
favoring the 12Z ECMWF and both the 12Z and 18Z GFS runs provided a
compromise position. There is general model agreement that this
trough should steadily weaken through early Saturday as strong
Atlantic ridging (anchored by a 600+ dm high near 60W longitude by
Friday-Saturday) builds into parts of the East. There is still a
signal for some weak energy to approach the East Coast from the
Atlantic on the western periphery of the core of strongest ridging,
with some potential influence on rainfall late in the week. By
Sunday-Monday, consensus suggests that shallow cyclonic flow may
expand into the Northeast while ridging persists over the South.
Low predictability for specific details mid-late period would
continue to favor a composite/mean approach to represent the larger
scale pattern for this part of the forecast.

Dynamical/ML guidance continues to agree fairly well for the
eastward drift of the western U.S. ridge from California and Nevada
into the Four Corners/Central Rockies region over the course of
the period. This drift should allow for weak troughing to develop
near the West Coast by the weekend. There is still some spread for
shortwaves rounding the top of the ridge, so there is low to medium
confidence in the specifics across the northern tier of the U.S.,
but there is some clustering for a frontal system to meander and
then finally press southward into the Upper Midwest by next Monday.

The WPC forecast thus used a deterministic blend favoring the 12Z
ECMWF and 12/18Z GFS early in the forecast period, including the
GEFS and EC ensemble means by Day 5 and increasing to around half
means by Day 7 to minimize individual model differences within a
generally agreeable large scale pattern.


...Weather/Hazards Highlights...

Several ingredients will come together for heavy rain and flash
flooding potential in the Eastern Seaboard for late week. A
positively tilted trough with its axis over the Great Lakes will
provide lift in the right entrance region of the jet. Beryl`s low
will be tracking northeastward as well, and ample moisture
(precipitable water values over the 90th-95th percentile) will be
in place while a front stalls. Marginal Risks are in place across
the region for the Days 4-5/Thursday-Friday EROs, with an embedded
Slight added over parts of the Interior Northeast on Thursday
especially if the main rainfall axis related to Beryl lingers from
Day 3/Wednesday into Thursday morning. Within this broad risk area
there are various antecedent ground conditions, such as drought
over the Mid-Atlantic and wet conditions over parts of the
Northeast, so various levels of impacts are possible. One change to
the previous ERO for Thursday was to limit the Marginal to
northeastern portions of Florida where urban areas may increase
sensitivity to heavy rain in an otherwise swampy/high Flash Flood
Guidance area. Some showers and storms may linger into the weekend
for portions of the East, but rain amounts do not look as focused
or heavy. Shortwaves and frontal boundaries could also promote
rounds of rainfall across the Midwest to Great Lakes.

Farther west, some moisture may feed into the Four Corners states
in a monsoonal pattern. A Marginal Risk is in place on Thursday
mainly for New Mexico--especially considering sensitivities due to
burn scars and areas that have seen ample rainfall recently. Less
overall coverage of showers/storms is currently forecast on Friday
along with less focus in the most sensitive areas of New Mexico
(Sacramento Mountains), so do not have a Marginal delineated in the
Day 5/Friday ERO. The chance for flash flooding may be nonzero but
it seems to be less than 5 percent, the Marginal threshold.
However, expect increasing moisture to promote more convective
rains in the Four Corners states this weekend.

The West will continue to see a major to extreme and long-lived
heat wave through late this week. Highs exceeding 110F should be
widespread in the Desert Southwest, with typically hotter locations
exceeding 120F. Farther north, highs into the 100s are forecast
into the Intermountain West as temperatures soar 10-20F above
normal. Numerous daily records are likely to be broken during this
heat wave, with monthly/all-time records not out of the question.
The prolonged nature of the heat wave and record warm overnight
temperatures will increase heat stress. Some moderation of the
temperatures closer to normal (but still a bit above) is finally
forecast in the Desert Southwest over the weekend. By the medium
range time frame, temperatures will have moderated in the Pacific
Northwest with possible troughing aloft, though may still be a bit
above normal. Meanwhile the warmer than average temperatures will
be spreading farther east into the northern and central Plains late
this week/weekend, and into the Midwest and East this weekend into
early next week.


Tate/Rausch


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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