Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
216 PM EDT Tue Jul 23 2024

Valid 12Z Fri Jul 26 2024 - 12Z Tue Jul 30 2024

...Hazardous heat to shift into the northern Plains...
...Showers/storms with locally heavy rainfall expected over the
Four Corners and from the western Gulf Coast into the southern
Mid-Atlantic/Southeast...


...Overview...

In the big picture, the latest forecast guidance continues to show
the evolution of the large scale pattern moving toward a typical
summertime pattern. This will feature main weather system tracks
across the northern tier of the U.S. and southern Canada. Mean
troughing is expected to persist over the West Coast while a
building/merging ridge over the Southeast into the Southern Plains
will hold strong. This type of weather pattern will bring anomalous
heat to the northern Plains while excessive rainfall is possible
along the Gulf Coast into the Southeast as well as the monsoonal
moisture areas over parts of the West.

...Guidance/Predictability Assessment...

Today's model guidance showed relatively good agreement through the
period, with the best clustering for the western U.S. troughing
that is expected to persist for much of the period. Meanwhile,
while the models, including the ensembles and Machine Learning
(ML) guidance, show a building ridge over the southern/southeast
U.S. by Day 5-7, the exact placement/strength and amplitude of it
is still a bit uncertain and models do show some variability with.
The models are slow/sluggish to dissipate/evolve the bagginess and
weak vort over the Central U.S. early on in the period as well as
a lingering trough off the East Coast. By early next week though,
there is reasonably good agreement that the ridge over the Southern
U.S. reasserts itself and perhaps centers over the S. Plains areas
while the northern tier experiences a fast parade of storm
systems, likely to keep some level of troughing from the northern
Plains into the western/northern Great Lakes. Forecast confidence
began the period above average to average but by Day 7 settled on
mostly average given the typical summertime pattern and
variability. The WPC model blend preference included mostly
ECMWF/GFS/UKMET/CMC at the beginning (Day 3) but incorporated a
fair amount of ensemble means by Day 5 through Day 7.


...Weather/Hazards Highlights...

The threat of excessive rainfall will continue over the course of
the period, with the Day 4 Excessive Rainfall Outlook featuring a
Slight Risk over portions of the coastal and Piedmont regions of
the Carolinas, where the combination of additional heavy rainfall
over wet soil/ground conditions could result in scattered instances
of flash flooding. Elsewhere, a deep moisture plume will exist
across portions of Texas Gulf Coast through the Southeast U.S.
Additionally, a continuation of monsoonal moisture will bring a
localized flash flood threat to much of the Four Corners region
into portions of the Intermountain West. Across portions of
southern Arizona into New Mexico, forecast rainfall amounts and
the amount of moisture/instability may necessitate a future upgrade
to Slight Risk, but confidence on exact placement of heavy rainfall
and potential rain rates wasn't high enough to introduce at this
time. On Day 5 (Saturday 12Z to Sunday 12Z), broad Marginal Risks
(Level 1 of 4) exist for portions of Four Corners/Rockies, much of
the Southeast and Mid Mississippi Valley and across the far
northern Plains where a passing weather system may lead to robust
convection and locally heavy rainfall.

Temperatures will generally be above normal (5-10F) across much of
the northern tier, especially the northern Plains where the
experimental HeatRisk is showing a large area of Moderate to Major
areas (Level 2-3 of 4) Friday through Sunday. The expected
temperature anomalies would yield highs in the upper 90s to low
100s. Some of this heat should spread eastward across the Great
Lakes/Northeast by Sunday- Tuesday, with highs 5-10F above normal
(upper 80s to around 90). Much of the West will likely see near to
slightly below normal highs through the period, with the exception
of the Southwest which should see some lingering above normal
readings through Friday and possibly Saturday. The wet pattern
across the southern tier late this week into the weekend will keep
most areas below normal for highs, especially southern/eastern
Texas. The rest of the southern tier should trend closer to normal
by early next week while southern Texas may remain a bit on the
cool side.


Rausch/Taylor


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