Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EDT Sat Jul 20 2024

Valid 12Z Tue Jul 23 2024 - 12Z Sat Jul 27 2024

***Heat wave continues across the West, and showers/storms with
 heavy rainfall expected from the Four Corners to portions of the
 East***


...General Overview...

Expect the amplitude of next week's upper pattern to become
somewhat less pronounced toward the end of next week. A strong
ridge extending from the Southwest U.S. into west-central Canada
(producing hazardous heat from the West into the northern High
Plains) should begin to weaken and get pushed east/southeast ahead
of a Pacific upper low tracking into western Canada and trailing
trough that settles near the West Coast. Monsoon conditions will
promote daily episodes of showers/storms over the Four Corners
states and vicinity under and near upper ridging over that part of
the country. Meanwhile, one or more wavy fronts will be on the
leading side of Great Lakes into southern Plains mean troughing
aloft, leading to multiple days of rain/thunderstorms with areas of
heavy rainfall from the southern Plains into the Mid-Atlantic and
parts of New England. The Great Lakes and Northeast should
eventually trend drier late week as the northern part of the trough
moves eastward. Consensus still shows the Atlantic upper ridge
building into the Southeast for a time, peaking in strength around
Wednesday-Thursday.

...Guidance/Predictability Assessment...

The most prominent forecast issue in terms of synoptic-scale
features that have some degree of predictability in the extended
range is with the ultimate progression of Great Lakes troughing
late in the week after Canadian energy feeds into it by Wednesday.
By Friday-Saturday the spread becomes fairly dramatic between the
slower ECMWF/ECMWF mean and fast GFS runs (though with the 12Z GFS
not as fast as the newer 18Z/00Z versions). ECMWF-initialized
machine learning (ML) models continue to favor an intermediate to
moderately progressive solution, not as fast as latest GFS runs but
suggesting low probability of the slow ECMWF scenario. The new 00Z
ECMWF has indeed nudged somewhat faster than prior runs. There are
some amplitude differences as well, but without prominent
clustering one way or the other.

The other forecast consideration is that 12Z/18Z GFS runs strayed
to the fast side of the spread for upper low energy crossing
southern Canada, while ML models at least support a farther
north/northeast upper low than advertised by the 12Z ECMWF by next
Saturday. The ensemble means and an average of the models fit this
general idea.

The updated forecast started with a composite of 12Z/18Z
operational models early and then gradually increasing
incorporation of latest GEFS/CMCens/ECens means late, reaching 50
percent total by next Saturday. By the latter half of the period
the blend kept ECMWF/ECens weight a little lower than would
normally be the case to account for preferences with the Great
Lakes/Northeast upper trough.

...Weather/Hazards Highlights...

Expect dangerous heat over the West to extend into next week with
high temperatures reaching the 90s and 100s and warm overnight lows
providing only limited relief. There should be a fairly broad area
of highs 5-15F above normal through midweek or so from California
into the far northern Rockies. Gradual cooling will likely move in
from the Pacific Northwest (where highs may be slightly below
normal by late week) as an upper low tracking into British
Columbia and trough to its south/southwest helps to lower heights
aloft. Meanwhile the most anomalous heat should shift into
Montana/northern High Plains during the mid-late week period, in
particular Wednesday-Thursday when Heat Risk shows the most
prominent signal for major impacts. The prolonged period of dry
heat over the West could also result in enhanced wildfire danger.
In contrast, the persistent upper trough/weakness over the central
U.S. will promote below normal highs from the central-southern
Plains into the Ohio/Tennessee Valley and southern Appalachians
around midweek, eventually becoming more confined to Texas and
vicinity by next Saturday.

Monsoonal moisture will linger over the Four Corners region and
promote daily episodes of showers/thunderstorms through the period.
Isolated to scattered instances of flash flooding will be
possible, especially near steep terrain and burn scars. The Days
4-5 Excessive Rainfall Outlooks covering Tuesday-Wednesday maintain
Marginal Risk areas, with potential for embedded Slight Risk
upgrades in future cycles depending on how guidance clusters
relative to each other and sensitive burn scar areas/regions with
wettest ground conditions.

The upper trough extending from the Great Lakes into southern
Plains along with a couple leading wavy fronts will support a
fairly broad corridor of locally heavy shower and thunderstorm
potential from the southern Plains northeastward next week. At the
moment, the best guidance signals in terms of coherence and
rainfall magnitude (with ample moisture and instability) exist
over southern/southeastern Texas during the Tuesday-Wednesday
period covered by the Days 4-5 Excessive Rainfall Outlooks. As a
result, both days of these outlooks have introduced a Slight Risk
area and reflect a modest south/southeast drift of heaviest
activity with time. Surrounding Marginal Risk extends northeastward
through the Mid-Atlantic and parts of the Northeast both days, with
some embedded Slight Risk areas possibly emerging as guidance
refines the most likely regions for heavy rainfall potential in the
shorter term. Meanwhile a Marginal Risk area over the Upper Midwest
into Great Lakes Tuesday-Wednesday accompanies an area of rainfall
generated by an upper shortwave and surface wave/frontal system.
Recent guidance runs have tempered the higher end of the envelope
for rain rates but the dynamics/surface features still suggest
potential for some locally heavy rainfall. By Thursday-Saturday,
the northeastern quadrant of the lower 48 should trend drier as
upper troughing moves eastward to some degree while showers/storms
persist over the southern Plains and Southeast.

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