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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1858Z Jul 10, 2024)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Wed Jul 10 2024

Valid 12Z Sat Jul 13 2024 - 12Z Wed Jul 17 2024

...Hot temperatures persist in the West and spread eastward, but
not quite as extreme as recent/short range heat...


A persistent upper ridge over the West will gradually drift east
and at times combine with ridging stretching westward from a strong
Atlantic upper high this weekend into early next week. This will
cause heat to spread across the central and eastern U.S., but as
the ridge weakens, temperatures should become slightly less extreme
during the period. The ridge shifting east will allow for some
monsoonal moisture to spread into the West. To the north of the
upper ridge, progressive shortwaves moving across Canada and the
U.S. northern tier will push possibly multiple surface fronts into
the northern states. Accompanying rounds of showers and
thunderstorms will be possible from the north-central U.S. to

...Guidance/Predictability Assessment...

The latest guidance continues to show overall agreement with the
large scale pattern, but plenty of uncertainty in the details,
especially with the lower predictability shortwaves progressing
through the northern stream. The main focus area is with a
shortwave/possibly closed low dropping across the Great Lakes
region early next week, and there is some notable disagreement in
the southern extent/amplitude of the shortwave through the region.
Recent runs of the GFS have indicated a sharper shortwave upstream
across central Canada, but is an outlier compared to the better
consensus. Farther west, mean ridging looks to rebuild in the
interior Northwest, but models like the ECMWF and CMC have shown
more shortwaves disrupting the ridge at times. These smaller
details may not be resolved for a while.

With the general model agreement at least on the larger scale, a
blend of deterministic models slightly favoring the ECMWF and GFS
was used early in the period. Introduced ensemble mean guidance
later in the period, along with the ECMWF and CMC, to help mitigate
the large scale model periods.

...Weather/Hazards Highlights...

Increasing moisture will feed into the Four Corners states in a
monsoonal pattern over the weekend and beyond, as the upper ridge
finally weakens some and drifts east. A Marginal Risk remains in
place for much of Arizona into northwestern New Mexico and
southwestern Colorado on Day 4/Saturday. A Marginal Risk expands
somewhat to cover much of the Four Corners states by Day 5/Sunday
as moisture continues to stream in and increase. Sunday's risk
includes more sensitive areas like the slot canyons of southern
Utah and the Sacramento Mountains of south-central New Mexico that
have recent burn scars. This moisture plume is forecast to expand
farther north into early next week for increasing rain/thunderstorm
chances for the Rockies to High Plains.

Through the weekend and early next week, the combination of
northern tier U.S./southern Canada shortwaves and a couple surface
fronts will promote one or more clusters of showers/storms from the
Dakotas/Upper Midwest into the Great Lakes. GIven sensitivity of
this region after recent weeks of heavy rain, and ample moisture
and instability, marginal risks were introduced on the Days 4 and 5
ERO for today's update. Storm motions should be fairly progressive
though, so this may limit the overall flash flood potential.
Rounds of rain/storms will also progress into the
Appalachians/Northeast by early next week.

Lingering rain is possible across the Eastern Seaboard on
Saturday, but the heaviest rainfall should be over by then compared
to the short range period. Expect scattered shower and
thunderstorm chances to continue and expand through the
southeastern U.S. (Lower Mississippi Valley into the
Southeast/Florida and the Carolinas) over the weekend into early
next week, with locally heavy rain possible.

The dangerous heat wave that has been ongoing across the West
should be easing somewhat by the medium range period, but still
remaining hot. Temperatures in the 110s are likely to continue
across the Desert Southwest, a few degrees above average. Above
normal heat will progress eastward from the central/northern Great
Basin into the central U.S./Midwest early next week. The northern
areas are likely to moderate to near or slightly below average for
highs after a cold front passes, but high heat indices are likely
Monday-Tuesday from the central Plains and Mid-Mississippi Valley
southward. Areas across the Ohio Valley into the Appalachians can
expect above normal temperatures this weekend and expanding into
the Eastern Seaboard/Gulf Coast early next week. Overall, most
areas of the lower 48 will have at least some heat concerns during
the period.


Additional 3-7 Day Hazard information can be found on the WPC
range hazards outlook chart at:

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: