357
FXUS02 KWNH 180647
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
246 AM EDT Mon Aug 18 2025

Valid 12Z Thu Aug 21 2025 - 12Z Mon Aug 25 2025

...Dangerous Hurricane Erin passing offshore this week to 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. This trough will help keep Erin on a
track offshore and away from the Eastern Seaboard, but high surf
and rip currents will be a threat mid to late week across parts of
the coast. A shortwave moving over Canada will help to reinforce
and amplify troughing over the East next weekend. Shortwave
troughing into the Northwest around next Monday may help to break
down the western ridge a bit. Heat threats across the Intermountain
West and especially the Southwest will build through the week,
while rainfall focuses mainly across the South and East along with
monsoonal activity over the Southwest/Rockies.


...Guidance/Predictability Assessment...

The mid-large scale flow pattern continues to show above average
agreement and predictability through the upcoming medium range
period. The guidance also continues to come into better alignment
regarding the speed and placement of Erin with high confidence it
will track safely offshore the East Coast. Please consult NHC
products for the latest information on the track and intensity
forecasts for Hurricane Erin. Otherwise, differences with the
Canadian shortwave are confined to mainly the details which of
course have implications on QPF across the north-central U.S. later
this week and the Northeast next weekend. Greater differences
arise late period across the Northwest regarding the evolution and
degree of shortwave energy into the West. Despite, there is pretty
good consensus this may help break down the ridge, at least a bit.

The WPC forecast tonight was based on a general model compromise
days 3-5 amidst excellent model agreement. Increased ensemble mean
input up to 50 percent by Day 7 to help with differences in the
Northwest early next week.


...Weather/Hazards Highlights...

An NHC forecast track for powerful Hurricane Erin offshore the
East Coast will bring high waves and an increased and multi- day
threat for rip currents along much of the East Coast this week to
monitor. Gusty winds may also be possible for parts of the Outer
Banks of North Carolina on Thursday. The heaviest rainfall should
remain well offshore and not a hazard.

A frontal boundary lingering in the Southern tier will be the
focus for heavy rainfall much of the period, aided by above normal
PW values and instability. A broad Marginal Risk on the Day
4/Thursday and 5/Friday Excessive Rainfall Outlooks are highlighted
from the Southern Plains to the central Appalachians. Another
front moving into the region behind this one next weekend will
bring renewed moisture and thunderstorm threats, with rain chances
returning towards the Mid- Atlantic and Northeast ahead of this
front/upper trough.

The strong shortwave skirting the Canada/US border will support
showers and thunderstorms ahead of a cold front that swings through
the north-central states. This front should be progressive, but
still, anomalous moisture and instability could result in isolated
flash flooding. As such, a marginal risk was added to the Day
4/Thursday ERO.

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 valid for Thursday and Friday. Favorable flow may also spread
activity into the central Rockies/Plains next weekend.

Heat will mainly focus across the Western U.S. into next weekend,
with major to localized extreme HeatRisk likely across the
Southwest and daytime temperatures exceeding 110F in some
locations. Farther north into the interior West and California
valley, moderate to locally major HeatRisk is forecast.

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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