845
FXUS02 KWBC 181857
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 PM 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 feature will help stir Erin away
from the Eastern Seaboard; however high surf and rip currents will
be a threat mid to late week across parts of the coast. Also during
this time 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 weaken
the western ridge. Hot temperatures will allow the Heat threat to
expand across the Intermountain West and especially the Southwest
regions 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 pattern has above average agreement and
predictability for the much of the extended period and has shown
continued improvement on the handling of evolution of Erin in the
Atlantic. Please continue to consult NHC products for the latest
information on the track and intensity forecasts for Hurricane
Erin.

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.

WPC started with a compromise of the ECMWF/CMC with a lesser
weighting of the GFS. The EC and CMC ensembles were included early
on and slowing increased weighting by Day 7 to help with
differences in the Northwest early next week.


...Weather/Hazards Highlights...

A powerful Hurricane Erin is forecast by NHC to be offshore the
East Coast. While some of the outer bands may ring increased
cloudiness and showers the larger impacts will be the high waves
and an increased and multi- day threat for rip currents along much
of the East Coast. 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. Follow NHC
for the latest details on Hurricane Erin.

Heavy rainfall will focus along a lingering frontal boundary over
the southern states through most of the extended period thanks to 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.

Across the north, a strong shortwave will track across southern
Canada and the north-central states supporting showers and
thunderstorms ahead of a cold front. This front is expected to be
progressive, however much of this region has been saturated of late
and soils remain sensitive. Anomalous moisture and instability
could result in isolated flash flooding. A Marginal Risk was
maintained for Day 4/Thursday.

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.

Campbell


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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