385
FXUS02 KWBC 172025
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
425 PM EDT Tue Jun 17 2025

Valid 12Z Fri Jun 20 2025 - 12Z Tue Jun 24 2025

...First significant and dangerous heat wave of the season will
first affect the central Plains to the Midwest and lower Great
Lakes Friday into this weekend, expanding into the East Coast by
early next week...


...General Overview...

A significant, dangerous heat wave is expected to start the Summer
across much of the central/eastern U.S. late this week under an
expanding upper-level ridge as major upper troughing is forecast
to work its way across the Northwest which will eventually emerge
into the northern U.S. where a rather deep cyclone is forecast to
develop this weekend. Multiple frontal waves appear to follow as
they develop and track northeast through the northern Plains along
a slow-moving front by early next week.


...Guidance/Predictability Assessment...

Model guidance remains in excellent agreement on the
aforementioned evolution of the synoptic pattern through the
medium-range period. The ECMWF solution cluster appears to be most
consistent with a deeper cyclone to exit New England as the
forecast period begins on Friday. This leads to a higher confidence
on the EC deterministic and EC mean for the remainder of the
forecast period. For the deep cyclone that is forecast to form over
the northern U.S. this weekend, the GFS shows its typical fast and
more progressive bias. Both the 00Z and 06Z GEFS means are slower
and deeper than the GFS deterministic solutions. The slower GEFS
means are consistent with the 00Z EC/EC mean cluster, further
lending confidence to the EC solutions. The 00Z CMC/CMC mean
solutions are compatible with the GEFS and EC mean in the broad
sense but with notable differences in details.

The WPC medium-range forecast package was based on a blend of 60%
from the 00Z ECMWF/EC mean, 30% from the 06Z GFS/GEFS and 10% from
the 00Z CMC mean. The blend begins with an even distribution of
the deterministic and the ensemble mean solutions followed by
increasing use of the ensemble means heading toward Day 7.


...Weather/Hazards Highlights...

Late week/weekend will bring a major synoptic pattern change as an
amplifying upper-level ridge over the central/eastern U.S. shifts
the storm track northward and brings a period of drier conditions
to many after unsettled weather much of the week. Embedded impulses
within the upper-ridge will trigger convection, potentially
organized, across the Upper Midwest/Great Lakes Friday (day 4) and
Great Lakes/Interior Northeast Saturday (day 5). A Marginal Risk
ERO is in place for both days as a strong low level jet and high
precipitable water values (up to 2 standard deviations above the
mean) support the threat of heavy rainfall and at least isolated
flash flooding. An embedded Slight Risk looks possible based on
forecast QPF values with more confidence in areal placement of this
organized convection. Daily showers and thunderstorms are also
expected in vicinity of the Gulf Coast as a frontal boundary
lingers in the region and with possible influence of tropical
moisture for the western Gulf Coast/south Texas. This influx of
moisture also looks to bring precipitation chances further
northwest into western Texas and the southern high Plains by late
weekend/early next week.

Upstream, the energetic approach of a deep upper-trough/Pacific
system and northern stream into the West will bring increasing
precipitation chances to the Pacific Northwest and northern
Rockies/Plains Friday through the weekend in unsettled flow with
moderate to heavy rainfall potential. A Marginal Risk ERO has been
maintained for Friday (day 4) for portions of north-central
Montana given the presence of strong, very moist upslope flow to
support heavy rainfall. This Marginal Risk ERO has been extended
into Saturday (Day 5) based on latest guidance depicting a farther
southward dip of an area of heavier rainfall behind the departing
cyclone in the northern Plains. In addition, a Slight Risk has been
introduced in the final version of the Day 5 ERO near the Canadian
border within the Marginal Risk area given further increase in QPF
and southward dip from the 12Z model guidance (including all of the
ensemble means except the GFS). Eventual lead frontal system
development over the Plains will bring a return of thunderstorm
chances to portions of the north-central Plains and Midwest Sunday
and particularly Monday- Tuesday, with potential for more heavy
rainfall/flash flooding as well as severe weather.

The first significant heat wave of the season is expected across
the central Plains by Friday, the Middle/Upper Mississippi Valley
by Saturday, and the Lower Great Lakes/Ohio Valley/Mid-Atlantic by
Sunday thanks to anomalous upper-level ridging building overhead.
Summer heat will also begin to intensify across the Lower
Mississippi Valley and Southeast as the ridge expands eastward.
Widespread Major to Extreme Heat Risk (levels 3 and 4/4) is
forecast, indicating an intensity of heat that is dangerous to
anyone without adequate cooling or hydration, as heat indices climb
into the 100-110 degree range, potentially higher. In addition,
muggy overnight lows in the mid- to upper 70s will bring little
overnight relief from the heat. Unfortunately, this heat wave looks
to persist through at least the middle of next week. Highs will
begin to trend cooler and below average by as much as 10-20 degrees
across portions of the Pacific Northwest by Friday and expand into
the Great Basin and northern Rockies/High Plains by the weekend as
an upper level trough and associated surface cold front from the
northeast Pacific begin to overspread the region.

Kong/Putnam


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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