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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1845Z Jul 16, 2024)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
245 PM EDT Tue Jul 16 2024

Valid 12Z Fri Jul 19 2024 - 12Z Tue Jul 23 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 for the South, Southeast, and Southern
Mid-Atlantic...


...Overview...

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 upper trough will work its way
into 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 for portions of the Pacific Northwest. Better 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 into the south-central U.S., Southeast,
and Mid-Atlantic. This wavy and well defined boundary is forecast
to stall and create a prolonged period of heavy rain that could
result in flooding concerns given the deep moisture pooled along
the boundary. Areas north of the boundary will see a break in the
heat as cooler and drier air moves in from Canada.


...Guidance/Predictability Assessment...

Model guidance remains in good agreement through the medium range
period. A general model blend of the deterministic
GFS/ECMWF/CMC/UKMET and the 13Z National Blend of Models was used
to create the forecast for Friday through the weekend. Higher than
normal agreement in the details has led to increased confidence
during the first half of the period. 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.


...Weather/Hazards Highlights...

As the heatwave wanes in the East, a new heatwave will take hold
of much of the West later this week. Heat will ramp up this weekend
into early next week, 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 Day 4 and 5 (Friday and Saturday) 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 shift south, providing a focus for multiple
rounds of heavy showers and thunderstorms each day through the
weekend into early 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. A Marginal Risk
area is in place in the Day 4 and 5 (Friday and Saturday) Excessive
Rainfall Outlooks from the Gulf Coast states through the Southeast
to the southern Mid-Atlantic, with an embedded Slight Risk area in
the Carolinas on Day 4 (Friday). The risk of flash flooding will
be highest in the Carolinas where favorable moisture, instability,
and upper level jet dynamics will align. Soils in this region may
also be saturated from forecast rainfall in the Day 1-3 period.


Dolan/Schichtel


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