Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM 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...

Models/ensembles still show the large scale flow evolving toward a
more typical summertime pattern as progressive flow becomes more
prominent across the northern tier U.S. and southern Canada with
time, while moderate mean troughing should persist near the West
Coast. Southwestern U.S. upper ridging should persist for most of
the period, while a separate Southeast/Gulf Coast upper ridge may
strengthen and merge with the Southwest ridge by next Tuesday.
There are continued uncertainties with specifics of initial energy
over the Plains as well as northern stream energy that may settle
near or offshore the East Coast. This pattern will likely bring the
greatest potential for anomalous heat to the northern-central
Plains. Areas of enhanced rainfall will be possible over parts of
the southern tier, along an ahead of a wavy northern tier front,
and with monsoonal convection over parts of the West.

...Guidance/Predictability Assessment...

As has been the case recently, the most prominent guidance
differences appear over the eastern half of the lower 48 into the
western Atlantic. 12Z/18Z runs diverge with respect to the eventual
strength and path of initial upper level energy over the Plains,
with some influence from timing differences for upper ridging that
cross the Midwest on its way into the East. These differences then
affect the longitude of a separate shortwave that could settle
along or offshore the East Coast. 24 hours ago the dynamical and
machine learning (ML) models were evenly distributed between the
ECMWF scenario (weak Plains energy that does not lift northeastward
due to a slower Midwest Ridge, plus an upper trough settling near
the East Coast) and the GFS (stronger Plains energy that ejects
northeastward plus farther eastward shift of features downstream).
Now 12Z MLs are 4-to-1 in favor of some variation of the GFS/UKMET
(CMC a somewhat weak compromise for the Plains energy, the 00Z
UKMET perhaps too strong), and the new 00Z ECMWF has indeed come in
with a trend toward what has become the dynamical/ML majority.
Along the West Coast, the majority of guidance maintains mean
troughing but with some differences in strength that have low
predictability. There are sporadic model runs that have been
showing a little more split to the flow. Guidance comparisons for
the 12Z/18Z cycles led to an operational model composite early,
trending toward a model/mean mix later, and incorporating a little
less 12Z ECMWF input than usual due to its less favorable
comparisons.

...Weather/Hazards Highlights...

Guidance continues to show the potential for episodes of locally
heavy rainfall within a corridor of enhanced moisture from
southern/eastern Texas to the southern Mid-Atlantic/Southeast
coast late this week. Thus the Days 4-5 Excessive Rainfall Outlooks
covering the Friday through Saturday night period depict a broad
Marginal Risk area across the southern tier. Within the Day 4
Marginal Risk, the region encompassing eastern North Carolina and
northeastern South Carolina has shown continuity in the guidance
for relatively greater heavy rain potential as an eastern U.S. cold
front approaches from the north, while wet ground conditions
prevail as well. Thus a Slight Risk area has been introduced for
this region. Some enhanced activity may continue along the
Southeast coast into Day 5 with upgrade potential if guidance
signals persist in future runs. The western Gulf Coast looks to be
another area of interest for potentially organized heavy rainfall
on Day 5, but signals are currently too diffuse for a Slight Risk.
Finally, it is worth noting that diverging guidance for flow
details aloft over the East by Day 5 leads to below average
confidence for the northern periphery of meaningful rainfall. For
now the Marginal Risk area leans to the conservative side
(representing the most common overlap) with some northward
expansion possible depending on trends of future runs.

Monsoonal moisture will continue to support episodes of diurnally
favored convection over the Four Corners region into the Great
Basin, with some shifting of coverage over the course of the period
based on pattern evolution. Instances of flash flooding will be
possible, especially near steep terrain and burn scars. The Days
4-5 EROs both depict Marginal Risk areas, with Day 5 representing
a trimming of the westward extent of potentially heavy convection
as the best moisture/instability shift eastward somewhat. Embedded
Slight Risk upgrades will be possible in future cycles, with
specifics depending on how guidance clusters relative to each other
and with sensitive burn scar areas/regions with wettest ground
conditions.

By Saturday-Saturday night the guidance suggests that a wavy front
moving into the northern Plains may start to produce areas of
heavier rainfall given potential interaction of shortwave energy
and deep moisture being carried along in southwesterly flow aloft.
The Day 5 ERO introduces a Marginal Risk centered over the eastern
half of North Dakota into northern Minnesota, as ground conditions
are still on the wet side over this region.

Beyond early Sunday, northern tier convection should gradually
continue eastward while areas of rainfall may expand across the
east-central U.S. depending on the details of initial Plains energy
aloft that may lift northeastward. Diurnally favored convection
should continue across parts of the southern tier while western
U.S. monsoonal activity should become more confined to southern
areas.

Expect the northern and central Plains to see the most persistent
anomalous heat from late this week into next week, with best
potential for some highs 10-15F above normal to shift from the
Dakotas on Friday into the central Plains by Sunday-Tuesday. These
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


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