737
FXUS02 KWNH 170652
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
251 AM EDT Sun Aug 17 2025

Valid 12Z Wed Aug 20 2025 - 12Z Sun Aug 24 2025

...Hurricane Erin to pass safely offshore next week, but will
bring high waves and rip current threats for much of the East
Coast...


...Overview...

A Four Corners subtropical ridge will remain in place through much
of the Medium Range period, with upper troughing favored
downstream over the East, which keeps Erin on an out to sea track
and away from the Eastern coastline. A shortwave moving across
Canada will help to reinforce troughing over the East next weekend,
while weaker shortwaves periodically move into the Northwest and
ride the top of the upper ridge. Heat threats across the
Intermountain West and especially the Southwest will build through
the period, while rainfall focuses mainly across the South and
East.


...Guidance/Predictability Assessment...

The large scale pattern continues to show above average agreement
through the medium range period. Early on, the greatest differences
involve the speed of Erin as it moves well offshore the East
Coast, and how quickly it gets absorbed into the initial shortwave
through the Northeast. Guidance shows a lot of uncertainty in this
evolution, but continue to unanimously favor a track curving away
from the coast. Please consult NHC products for the latest
information on the track and intensity forecasts for Hurricane
Erin. Otherwise, there are some differences late week and into next
weekend regarding placement and strength of a shortwave/closed low
through Canada, which has late period implications on amplified
troughing across the East.

The WPC blend for the first half of the period was based on a
general model consensus, with some ensemble means included even as
early as Day 3 due to better agreement with the track of Erin
compared to the deterministic guidance. After Day 5, gradually
increased the ensemble mean contribution in the blend to help
account for greater uncertainty in the details of systems.


...Weather/Hazards Highlights...

A frontal boundary slowly dropping southward through the Southern
tier will be the focus for heavy rainfall much of next week, aided
by above normal PW values and instability. On Wednesday, a broad
Marginal Risk on the Day 4 Excessive Rainfall Outlook is
highlighted from the Southern Plains to the central Appalachians
with a slight southerly shift for Day 5/Thursday`s ERO. It is
possible for both days, embedded slight risks may be needed in the
future where the heavy rain threat ultimately becomes more
agreeable amongst the guidance. The front will continue to sag into
the Southeast and weaken with time with increased convection
continuing from the Southern Plains to the Southeast later in the
week. Moisture may lift northward towards the Mid-Atlantic and
Northeast with time next weekend ahead of another approaching
trough.

Surges of moisture into the Southwest under the base of the Four
Corners ridge combined with diurnal heating may trigger storms
capable of producing heavier rainfall over more sensitive areas
(steep terrain, in/around burn scars, dry washes/arroyos, urban
areas). A Marginal Risk ERO continues to be depicted for both Days
4-5/Wednesday into Thursday. Despite missing the East Coast by a
healthy margin, Erin will likely bring high waves and an increased
threat for Rip Currents along much of the East Coast during
portions of next week.

Significant heat across the Midwest during the short range period
will moderate by mid next week, but heat threats will increase
across much of the interior West and Southwest. Major to localized
extreme HeatRisk is likely by late week and next weekend across
much of the Southwest with daytime temperatures exceeding 110F in
some locations.

Santorelli


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



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