Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Wed Jul 24 2024

Valid 12Z Sat Jul 27 2024 - 12Z Wed Jul 31 2024

...Hazardous heat to shift into the northern-central Plains...
...Showers/storms with locally heavy rainfall possible from the
western Gulf Coast into the Southeast and northward into the Upper
Midwest...


...Overview...

Guidance shows a somewhat complex evolution over the eastern half
of the country and western Atlantic during the weekend into the
start of the week, while progressive northern tier flow will carry
energy ejecting out of a persistent West Coast mean trough. By next
Wednesday the merger of a retrograding and strengthening
Southeast/Gulf coast upper ridge with a separate ridge lingering
over the southern Rockies should ultimately yield a strong southern
Rockies/Plains ridge with a hint of troughing over the Great
Lakes/Northeast. This evolution will support an expanding area of
hot weather across the northern-central Plains and into the Great
Lakes/Northeast. Locations from the South through the east-central
U.S. into the Upper Midwest may see one or more days of locally
heavy showers/storms, while monsoonal convection over the West will
likely be more confined to southern locations after Saturday.

...Guidance/Predictability Assessment...

Dynamical and machine learning (ML) guidance is still showing a
fair degree of spread for specifics of the upper low/shortwave
energy ejecting northeastward from the Plains along with separate
East Coast energy that may form a weak upper low along or offshore
the coast. For the ejecting Plains energy, the UKMET has tended to
be on the strong side and the CMC on the weaker side. Farther east,
the average of 12Z ECMWF-initialized ML models would suggest an
upper low would most likely form/track over a position
south/southeast of New England by early next week and then lift
northward as ridging to the east becomes more pronounced. This
feature would then eject northeastward with the approach of
upstream flow (faster than the 12Z ECMWF, and the new 00Z run has
trended faster). Specifics of southern tier ridging and the
progressive flow on its northern periphery have low predictability
at extended time frames. Also, the details of Pacific Northwest
mean trough become increasingly ambiguous by the early-middle part
of next week, with various ideas for individual shortwaves that
may feed into/eject from the feature. 12Z/18Z guidance comparisons
led to starting the updated forecast with an operational model
composite early in the period (with more GFS/ECMWF weight relative
to the CMC/UKMET) followed by a trend toward 60 percent total
weight of the ensemble means by next Wednesday.

...Weather/Hazards Highlights...

For the Days 4-5 Excessive Rainfall Outlooks valid this weekend,
one area of focus will be over the northern Plains/Upper Midwest
where shortwave energy and a wavy surface front will interact with
enhanced moisture, with ground conditions still on the wet side.
Although guidance signals for QPF have become a bit more diffuse
during Saturday versus continuity, moisture anomalies/instability
still seem to support the ongoing Marginal Risk area (Level 1 of
4). The GFS/ECMWF and their ensembles suggest greater heavy
rainfall potential by Sunday but remaining guidance differs in some
respects, so for now the plan is to include the region as part of a
larger scale Marginal Risk area with the understanding of upgrade
potential to a Slight Risk pending improved clustering.

Farther south, the weekend will start with a broad area of locally
heavy rainfall potential with anomalous moisture extending from
the western Gulf Coast to the southeastern coast (the latter with a
front sinking southward) and extending north into the Middle
Mississippi Valley/Lower Ohio Valley (along/ahead of an upper
low/trough ejecting northeast from the central Plains). The Day 4
ERO depicts a broad Marginal Risk area across these areas with one
or more embedded Slight Risk areas possibly arising if short term
guidance refines areas of best focus. By Day 5 the corridor of best
moisture/instability should extend from the Southeast into the
Midwest, possibly getting close to connecting with the northern
tier moisture. Lingering moisture over southern Texas will support
a Marginal Risk area there as well.

Shortwaves and moderately above-climatology moisture will support a
Marginal Risk area in the Day 4/Saturday ERO over and near the
southern two-thirds of the Rockies. Much of the West will see a
push of drier air by Day 5 Sunday, confining the Marginal Risk area
to southeastern Arizona/southwestern New Mexico.

Expect the evolving pattern to support an expanding area of hot
weather over portions of the Plains and extending into the Great
Lakes and Northeast. The most persistent and extreme high
temperature anomalies should be over the central Plains where some
locations may see multiple days with highs 10-15F above normal. The
Northeast could see some highs reach 10F or so above normal after
Sunday. The experimental HeatRisk likewise reflects an expanding
area of Moderate to Major (Level 2-3 of 4) risks of heat-related
impacts during 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 low 100s over the central Plains and upper 80s to around 90
farther northeastward. Climate Prediction Center forecasts indicate
some variation of this pattern may persist well beyond next
Wednesday. Much of the West will likely see near to slightly below
normal highs through the period. The Southwest could be a little
warmer on Saturday, and then again by midweek as southern Rockies
upper ridging rebuilds. Lingering rainfall over the southern tier
should support near to below normal highs, especially over
southern/eastern Texas especially during the weekend.

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