Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 PM EDT Fri Jul 26 2024

Valid 12Z Mon Jul 29 2024 - 12Z Fri Aug 02 2024

...Hazardous heat to shift into the central Plains early on...
...Heat builds across the northern tier of the country late next
week...
...Showers/storms with locally heavy rainfall possible over
parts of the eastern half of the lower 48...


...Overview...
Guidance continues to show the pattern evolving toward a
strengthening southern Rockies/Plains upper ridge by the middle to
late part of next week as a retrograding Gulf Coast ridge merges
with the lingering ridge over the southern Rockies. This should
help set up what should be a prolonged period of hot weather over
many areas with the most anomalous heat expected over the Central
Plains. Shortwave energy moving through the Midwest with eventual
mean troughing that develops over the eastern U.S., will support a
broad area of showers/thunderstorms drifting across the eastern
half of the country. Shortwaves feeding into an initial mean
trough aloft over the Pacific Northwest coast should bring some
rainfall to the region early next week. This system should eject
eastward mid-late week as larger scale troughing takes shape over
the northeastern Pacific. Despite the overall pattern, heat
increases with time across the Lower 48, with the largest positive
anomalies temperature-wise migrating towards the US/Canadian
border


...Guidance/Predictability Assessment...
The guidance showed reasonable agreement, with the 00z ECMWF on the
strong side of the guidance with an occluded low that it seems to
want to develop a subtropical cyclone out of early on. There are
some differences with a trough moving by the Great Lakes, with the
00z ECMWF a little out of sync with the rest of the guidance. 

The pressures/fronts, winds, and 500 hPa heights used a blend of
the 06z GFS, 00z Canadian, 00z UKMET, and 00z ECMWF for the first
half of the period which helps deals with details around New
England effectively. After this, increased contributions from the
00z NAEFS and 00z ECMWF ensemble means was used, maximizing at 40%
of the blend by late next week. The QPF was generally a slightly
amped up version of the 13z NBM, except across the Northwest where
some 12z GFS, 12z Canadian, and 12z UKMET were used per
coordination with PDT/the Pendleton OR forecast office. Other
grids were based more strongly on the 13z NBM. Overall, this
maintained reasonable agreement with the previous WPC forecast.


...Weather/Hazards Highlights...
By Monday, there will be a broad corridor of above-average
precipitable water values extending from the Upper Midwest into the
Southeast, gradually shifting eastward with time on the backside
of a weakening deep layer cyclone. The Days 4 and 5 Excessive
Rainfall Outlooks covering Monday- Tuesday night show broad
marginal risks across portions of this region. There was enough
agreement in the guidance for small slight risks across portions of
the southern- central Appalachians where flash flood guidance is
typically lower, but moreso lately due to recent rains and
additional rainfall expected on Sunday, prior to the medium range
period. Much of the Eastern third of the country should remain
generally unsettled next week as a upper trough is slow to budge
out of the region while it generally weakens. Elsewhere, above
normal moisture associated with a shortwave should bring some
modest rainfall to portions of the Pacific Northwest on Monday,
with much lighter/more scattered rainfall as the shortwave shifts
inland. Monsoonal convection over the Southwest should stay
confined to southern areas through midweek, though it may
eventually expand a little northward depending on the shape of the
southern Rockies/Plains ridge and upper level energy rotating
around its western periphery.

The forecast pattern evolution will support an expanding area of
warmer than average temperatures across the Lower 48, possibly into
the Ohio Valley/Mid-Atlantic by next Thursday. The most persistent
and extreme high temperature anomalies should be over the Central
Plains early on where some locations should see multiple days with
highs 10-15F or so above normal and high temperatures broach 105F.
An incoming front will cause some easing of the heat late next
week. The experimental HeatRisk likewise reflects an expanding
area of Moderate to Major risks of heat- related impacts from the
weekend through next Wednesday, and even some pockets in the
Extreme category over the central U.S. Forecast temperature
anomalies would yield highs in the upper 90s towards 110F in
isolated spots over the central Plains and upper 80s to 90 or so
farther northeastward. Climate Prediction Center forecasts indicate
some variation of this pattern may persist into next weekend.

Much of the West will likely see near to slightly below normal
highs through the first half of next week before the strengthening
ridge pushes temperatures above normal by next Thursday, and more
significantly so next Friday with lower elevations seeing 100F+
high temperatures. The length of the northern tier of the country
could see some highs reach 10F or so above normal during the first
couple days of August/late next week, with the strongest anomalies
expected in eastern WA and the ID Stovepipe. Lingering rainfall
over the southern tier should support near to below normal highs,
especially over southern/eastern Texas at the start of the week.

Roth/Santorelli


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