Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
230 PM 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...

A rather complex weather pattern is expected at the start of the
forecast period (Saturday July 27), featuring troughing over the
Northwest U.S. and Northeast U.S. but also bagginess in the flow
pattern over the Plains. Over time, the Southeast U.S. ridge is
expected to build/strengthen as it retrogrades toward the Southern
Plains by the middle of next week, leading to a rather large and
prolonged period of hot weather for many areas, with the most
anomalous heat for the Plains. Meanwhile, monsoonal convection over
the West will likely be more confined to southern locations after
Saturday.

...Guidance/Predictability Assessment...
The beginning of the period still shows a fair amount of model
uncertainty and lower predictability for several areas across the
CONUS. First, a weak mid level feature is forecast to stall/wobble
off the Northeast coast and potentially retrograde or slowly drift
northwestward toward the coast late in the weekend into early next
week. The 12Z GFS was the most robust with the retrograding idea
and also closes off the low at 500 mb compared to the other
guidance which is weaker/more open by 12Z Monday. Meanwhile, a
fairly progressive, quasi-zonal flow pattern is expected from the
northern ROckies into the northern Plains with the latest guidance
now trending toward the idea of a deepening trough evolving over
the Great Lakes by the end of the forecast period. Finally,
bagginess/weakness in the trough over the Plains early on is
expected to dissipate and be replaced by a building/strengthening
upper level ridge, initially over the Southeast U.S. but by next
week retrogrades toward the southern Plains. By Day 6-7, the
consensus among the guidance is for a large, sprawling ridge
becoming centered from the Four Corners into the southern Plains.


...Weather/Hazards Highlights...

For the Day 4 Excessive Rainfall Outlooks, the main focus areas
for heavy rainfall leading to flash flooding is across the Four
Corners and Rockies regions, tied to the deep monsoonal moisture
lifting through the area. Across the northern Plains, a passing
shortwave and weather system will bring a threat of progressive but
potentially deep convection with heavy rainfall. Finally, the
combination of a wavy frontal boundary, copious amounts of deep
moisture, and daytime heating will lead to scattered to numerous
showers and thunderstorms from the Texas Gulf Coast through much of
the Southeast U.S. and into the Mid Mississippi  Valley. For all
of these areas, WPC is highlighting a Marginal (Level 1 of 4)
Excessive Rainfall Outlook.

By Day 5, the threat for heavy rainfall will shift to the UPper
Midwest through Ohio Valley and into the Southeast U.S. while
confining to mainly the Texas Gulf Coast as well. A narrow plume of
deeper moisture helping to produce heavy rainfall from showers and
thunderstorms across southern Arizona and southwest New Mexico may
product localized flash flooding.

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/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