Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 PM EDT Thu Jul 25 2024

Valid 12Z Sun Jul 28 2024 - 12Z Thu Aug 01 2024

...Hazardous heat to shift into and northeast of the central
Plains...
...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. Despite the
ridge/trough pattern expected, mid-level height rises occur
nationwide, setting 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 ejecting from the
Plains/Mississippi Valley Sunday onward, and then mean troughing
that develops over far eastern U.S., will support a broad area of
showers/thunderstorms drifting across the eastern half of the
country. Energy feeding into an initial mean trough aloft over the
Pacific Northwest coast should bring some rainfall to the region
early next week. This energy should eject eastward mid-late week as
larger scale troughing takes shape over the northeastern Pacific.


...Guidance/Predictability Assessment...
The latest forecast update (pressures, winds, and fronts) based on
00Z/06Z guidance started with an operational model composite and
then trended toward even weight of the models/means by the end of
the period next Thursday. The remainder of the grids used the 13z
NBM as a starting point.

Guidance is still working out the finer details of the complex
evolution over the eastern U.S./western Atlantic into the first
half of next week, as shortwave energy ejects from the
Plains/Mississippi Valley and a forming weak upper low just off
the East Coast may track into or near New England. A general model
blend works well here.

Around the northern periphery of the building Rockies/Plains upper
ridge (which has some modest spread for details with low
predictability several days out in time), there is some spread but
also improving clustering regarding shortwave details by the latter
half of the period. Consensus agrees that Northwest heights should
rise after midweek as this shortwave departs and larger scale
troughing develops over the northeastern Pacific. This should cause
whatever lingering front existing across the Great Basin to weaken
out of existence.


...Weather/Hazards Highlights...
During the Sunday-Monday night period covered by the Days 4-5
Excessive Rainfall Outlooks, there will be a broad corridor of
above-climatology precipitable water values extending from the
Upper Midwest into the Southeast, with this moisture drifting
gradually eastward. On Sunday into Sunday night, the guidance
suggests two relatively greater areas of focus for heavy rainfall
potential. One will be over the Upper Midwest where shortwave
energy and a wavy surface front will interact with a moist and
unstable environment, with the other centered in or near the
eastern portions of Tennessee. Both regions generally lean toward
wet antecedent conditions. The southern area exhibits a bit more
overlap in the guidance, allowing for continuation of the Slight
Risk area, while preference is to await somewhat better agreement
before introducing a Slight Risk over the Upper Midwest where
guidance remains a bit more dispersive. Locally heavy rainfall and
some flash flooding issues will be possible anywhere within the
broad Marginal Risk area. This broad area of showers and
thunderstorms will drift eastward Monday/Monday night. An
increasing signal for heavy rainfall from southeast KY into
northwest GA led to the introduction of a day 5/Monday into Monday
night Slight Risk in that area. Elsewhere, lingering activity over
southern Texas merits a Marginal Risk on Sunday/Sunday night with
no change in continuity while monsoonal activity and an associated
Marginal Risk from Sunday into Monday night will be confined to
parts of Arizona/New Mexico.

Upper troughing aloft will tend to maintain an unsettled pattern
over the East after early Tuesday, with showers/storms possibly
becoming a little lighter and more scattered by Wednesday-
Thursday.  Monsoonal convection over the West 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. A front approaching/reaching the Pacific
Northwest early in the week may produce a period of rainfall,
though currently with a fair amount of spread in the guidance for
intensity.

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 where some locations should see multiple days
with highs 10-15F or so above normal. The Northeast could see some
highs reach 10F or so above normal early in the 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. by next week. Forecast temperature
anomalies would yield highs in the upper 90s to 100s 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 somewhat above normal by next Thursday. 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/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