Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
255 AM EDT Thu Jul 18 2024

Valid 12Z Sun Jul 21 2024 - 12Z Thu Jul 25 2024

...Dangerous heat and wildfire threats to build over the West...

...Multiple days of heavy rain are forecast for the Four Corners,
South-Central U.S., Southeast, and Southern Mid-Atlantic...


...Overview...

It remains the case that an amplified upper pattern with a strong
ridge over the western U.S. and a mean trough over the central and
eastern U.S. will persist through the medium range period.
Dangerous, potentially record-setting, heat will develop
underneath the ridge and persist through the weekend and into next
week. An upper trough will gradually nudge into the Pacific
Northwest early next week, which should help ease temperatures
there. Moderated temperatures will also develop across much of the
central and eastern U.S. underneath the upper trough and in the
wake of a cold front.

A multi-day pattern with daily potential for heavy rain locally is
forecast for much of the southern tier of the nation. Monsoon
conditions will persist in the Four Corners region and southern
Rockies, and a wavy/slow-moving front will stall across the south-
central U.S., Southeast and southern Mid-Atlantic to pool moisture
and instability to focus convection. The prolonged period could
result in flooding concerns, especially with slower moving and
repeat convection. Tropical moisture will also work into Florida
early next week to enhance daily shower/thunderstorm activity.

...Guidance/Predictability Assessment...

Guidance is in good agreement through much of the medium range
period. The WPC forecast blend was made from best clustered
guidance of the 18 UTC GFS/GEFS mean, 12 UTC ECMWF and
ECMWF/Canadian ensemble means and the 01 UTC National Blend of
Models. The WPC blend was shaded toward the ECMWF/ECMWF ensemble
mean for slightly more robust and resilient western U.S. upper
ridge amplitude. WPC product continuity was well maintained and
overall seems in line with latest 00 UTC cycle guidance.

...Weather/Hazards Highlights...

Dangerous heat will ramp up this weekend 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
by this weekend. Heat will wane slightly for western portions of
the Pacific Northwest as an upper trough and surface cold front
move into the region, but temperatures elsewhere in the West are
forecast to remain well above average next week. This prolonged
period of dry heat could also result in enhanced wildfire danger.

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 (Sunday and Monday)
Excessive Rainfall Outlooks. The Marginal Risk area also includes
portions of the southern/central Rockies and adjacent 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
and well into 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 remain in place from the central/southern Plains through the
Southeast to the southern Mid-Atlantic in the Day 4 and 5 (Sunday
and Monday) Excessive Rainfall Outlooks. Soils may become more
saturated in some areas from heavy rainfall in the short range
period (days 1-3), which could enhance flooding potential as well.
Additionally, an influx of tropical moisture into Florida early
next week may enhance daily showers/thunderstorms in the Southeast.

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