578
FXUS02 KWBC 081856
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 PM 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) should
reach the eastern Great Lakes or vicinity by the start of the medium
range period early 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...

As of Thursday the guidance shows some lingering detail differences
for post-tropical Beryl with solutions waffling with respect to
exact upper level details and resulting strength/track of the
surface reflection. The 06Z GFS/00Z ECMWF strayed a bit on the
weaker/eastern side of the spread for Beryl`s surface low but other
models and a relative majority of ECMWF-initialized machine
learning (ML) models would recommend a stronger/westward solution
(but not as slow as the 12Z CMC). A composite of latest models with
minor adjustment provided a reasonable starting point reflecting
the official NHC track. Guidance continues to show steady weakening
of the Great Lakes/Midwest upper trough into 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 specifics late
in the week. By Sunday-Monday, consensus still depicts shallow
cyclonic flow expanding into the Northeast while ridging persists
over the South (but weakening a bit by Monday). 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, with the combination of these
features beginning to pull monsoonal moisture farther northward
over parts of the West during the weekend into early next week. To
the north of the upper ridge, guidance clusters well for a leading
trough/upper low reaching western Canada late this week and then
likely opening up and progressing eastward with time. Then the
relative majority of dynamical and ML guidance suggests that
another shortwave will reach western Canada by Saturday and
possibly amplify gradually as it reaches a position between Hudson
Bay and the northern Plains/Upper Midwest by next Monday. At that
time corresponding surface low pressure would be over or near
Ontario with the trailing front pushing farther south into the
northern tier relative to a leading front that may stay near the
Canadian border. The 06Z GFS strayed out of phase from Alaska and
vicinity eastward by mid-late period, favoring exclusion of its
forecast at that time. The 00Z and new 12Z GFS runs compare better
to remaining guidance.

Above considerations based on 00Z/06Z guidance led to a forecast
update consisting of a blend of operational guidance (splitting GFS
input among the 00Z/06Z runs) early in the period and then
incorporating some 06Z GEFS/00Z ECens means while shifting GFS
input to the 00Z run exclusively.


...Weather/Hazards Highlights...

Several ingredients will come together for heavy rain and flash
flooding potential along 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
surface low will be tracking northeastward as well, and ample
moisture (precipitable water values over the 90th-95th percentile)
will be in place. A stalled front, possibly beginning to lift north
some as Beryl`s low approaches, may help to focus rainfall as well.
The Day 4 ERO (Thursday) update accounts somewhat more for
consensus regarding anomalous moisture, some instability, and
average of the past couple ECMWF runs plus ensemble signals in
expanding the central New England and vicinity Slight Risk area
eastward into portions of Maine. Most non-ECMWF models are less
enthusiastic about potential for significant QPF, so confidence is
still somewhat below average.

Farther south, guidance continues to suggest a favorable setup for
significant rainfall near/inland from the Mid-Atlantic coast with
the combination of a stalling surface front and a band of enhanced
deep moisture that stays in place during Thursday-Friday. Based on
a composite of guidance signals and WFO coordination, the Day 4 ERO
proposes maintenance of the existing Marginal Risk area extending
south into the Carolinas (with initially dry ground conditions
being a mitigating factor) while the Day 5 ERO will introduce a
Slight Risk area from southern New Jersey into eastern North
Carolina given increased sensitivity due to rainfall expected in
the Day 4 period. This proposed Slight Risk area represents the
best overlap of guidance but there will be heavy rainfall potential
within the surrounding Marginal Risk area as well. Some showers
and storms may linger into the weekend for portions of the East,
but rain amounts do not look as focused or heavy as the surface
front/moisture band become more diffuse. 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, guidance agrees upon increasing/northward spread of
moisture over the Southwest/Four Corners states this weekend and
early next week so expect more convective rains across the region
during that time frame.

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.


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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