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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 0658Z Jul 17, 2024)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Wed Jul 17 2024

Valid 12Z Sat Jul 20 2024 - 12Z Wed Jul 24 2024

...Dangerous heat and wildfire threats will build over much of the
West this week into next week...

...Monsoonal conditions continue for the Four Corners states and
heavy rains are forecast from the South-Central U.S. through the
Southeast and Southern Mid-Atlantic...


Guidance still agrees that an amplified upper pattern will be
highlighted by a strong ridge across the western U.S. and a trough
over the east- central U.S. through the medium range period. An
amplified east-northeast Pacific upper trough will work its way
toward the Pacific Northwest by early next week, but progress will
be slow due to the strong downstream ridge. Dangerous heat will
develop underneath the ridge, and temperatures may reach record
values. In addition to dangerous heat, low humidity/dry fuels and
breezy winds will result in an increased threat of wildfires by
late week into next for portions of the Pacific Northwest/West.
Moisture will focus far to the south across the Southwest/Four
Corners region and southern Rockies where monsoonal conditions will
likely continue into next week.

Underneath the trough in the east-central U.S., a slow moving
frontal boundary will sag and meander over the south-central U.S.,
Southeast and Mid-Atlantic. This wavy and well defined boundary is
forecast to stall and be a main focus for a prolonged period of
heavy rain that could result in flooding concerns given the deep
moisture pooled along and south of the boundary. Tropical moisture
is additionally slated to work up into Florida and vicinity early
next week to fuel activity. Areas north of the boundary will see a
break in the heat as cooler and drier air moves in from Canada.

...Guidance/Predictability Assessment...

Models and ensembles remain in good agreement through the medium
range period. A model blend of the deterministic
GFS/ECMWF/CMC/UKMET and the 01 UTC National Blend of Models was
used to create the forecast through the weekend into Monday. Higher
than normal agreement in the details has led to increased forecast
confidence. Details become a bit more uncertain early next week,
which is expected, so ensemble means were added to the forecast
blend to smooth out smaller differences, ideally consistent with
individual system predictability. Higher weights were applied to
the ECMWF/ECMWF ensemble mean in the WPC guidance blend to depict
a slightly more robust and resilent western U.S. upper ridge
amplitude over time recent trends and historical ridge bias.

...Weather/Hazards Highlights...

High heat will ramp up this weekend into early next week over much
of the West, with Major to Extreme HeatRisk values expected for
many interior locations. High temperatures in the 90s and 100s will
be common, and warm overnight lows will only provide limited
relief. If the current forecast holds, temperatures may reach
record high values this weekend. Per the Climate Prediction Center,
significantly above normal temperatures are forecast to persist
across portions of the interior West into the week two period. This
prolonged period of dry heat could result in enhanced wildfire
danger in the West as well.

Meanwhile, monsoonal moisture will linger over the Four Corners
region, producing daily rounds of precipitation. Isolated to
scattered instances of flash flooding will be possible, especially
near steep terrain and burn scars. A Marginal Risk remains in place
for this region in the WPC Day 4 and 5 (Saturday and Sunday)
Excessive Rainfall Outlooks. The Marginal Risk area also includes
portions of the southern/central Rockies and adjacent High Plains.

In the central and eastern U.S., a well-defined, wavy surface
front will slowly meander, providing a focus for multiple rounds
of heavy showers and thunderstorms each day through the weekend
into early-mid next week. With ample moisture and instability in
place, locally heavy rainfall will be possible and may cause
instances of flash flooding, especially in urban and poor drainage
areas and areas with repeat/training convection. Marginal Risk
areas have been introduced for Day 4 and 5 (Saturday and Sunday).
These excessive Rainfall Outlooks stretch from the south-central
plains as energies dig to the lee of the western ridge in upper
diffluent flow states through the moist and unstable
to the southern Mid-Atlantic in a region with favorable upper jet
support, and as soils cumulatively saturate from forecast rainfall
in an ongoing wet pattern. An addional northwestward influx of
tropical moisture into Florida early next week may also fuel rains.


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: