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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 0704Z Jun 18, 2024)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
303 AM EDT Tue Jun 18 2024

Valid 12Z Fri Jun 21 2024 - 12Z Tue Jun 25 2024

...Record-breaking heat from the Midwest to the East Coast expected
to moderate by early next week...

...Heavy rainfall threat from the northern Plains into Upper
Great Lakes into Friday-Saturday...


Latest models and ensembles continue to show the large scale
pattern transitioning toward a more typical early summer regime
late weekend and early next week, as initially strong ridging over
the eastern half of the country gives way to a leading trough
crossing the northern tier U.S. and southern Canada (reaching the
East Coast by around next Tuesday) while a system reaching the
Northeast Pacific and West Coast this weekend continues eastward
thereafter. Although weakening, the eastern upper ridge will still
support a broad area of hazardous heat with potential for daily
records extending from the Ohio Valley/Lower Great Lakes into the
Mid-Atlantic Friday into the weekend. The northern Plains into
Upper Great Lakes should see a continued threat for heavy rainfall
through Friday-Saturday as developing northern Plains into Great
Lakes/Ontario low pressure and associated fronts interact with
anomalous moisture already in place. Some of this rainfall will
extend into the Northeast. A trailing Northeast Pacific system will
likely bring a well-defined front into the West by Sunday,
continuing east thereafter. To the south of the upper ridge
ultimately settling over the far southern tier, Potential Tropical
Cyclone One (see NHC for updates) should dissipate over Mexico by
Friday while NHC is monitoring potential for another feature over
the western Gulf mid-late period.

...Guidance/Predictability Assessment...

The latest forecast based on 12Z/18Z guidance started with an
operational model composite early in the period and then gradually
increased ensemble input (18Z GEFS and 12Z ECens/CMCens) to a total
of 60 percent by the end of the period next Tuesday. ECMWF input
was split between the 12Z and 00Z/17 run mid-late period due to
some questionable details in the 12Z run.

At least in principle, an average of dynamical and ECMWF machine
learning (ML) guidance agreed fairly well with the upper ridge
settling over the far southern tier while a fairly vigorous system
develops/tracks along the northern tier/southern Canada through the
period, leading to a moderate upper trough/leading cold front near
the East Coast by next Tuesday. The mean of ML models was close to
the dynamical guidance average for the surface system's depth,
while displaying some detail differences after most guidance agreed
to an embedded upper low aloft as of Sunday. With the trailing
Pacific system, sporadic GFS runs (like the 12Z and new 00Z
versions) have been shearing some upper energy while leaving the
main upper low behind near the British Columbia coast. The 18Z GFS
looked more like consensus. Meanwhile the ML models are generally
showing somewhat higher heights and less potential for a weakness
at the surface and aloft over or near the Gulf of Mexico versus
some GFS/ECMWF runs.

...Weather/Hazards Highlights...

The most prominent focus for heavy rainfall during the period will
be from the northern Plains into Upper Great Lakes during Friday-
Saturday as upper dynamics emerging from western North America
encourage northern Plains into Great Lakes/Ontario surface
development. This system and associated fronts will interact with
anomalous moisture that has been persisting over the region for
multiple days. The Day 4 Excessive Rainfall Outlook for Friday
maintains a Slight Risk area from parts of the Dakotas into the
U.P. of Michigan (close to continuity) while the Day 5 ERO for
Saturday proposes a Slight Risk area from northeastern
Minnesota/northern Wisconsin into northern Michigan. Surrounding
Marginal Risk areas account for less extreme but still locally
significant rainfall potential over the northern Plains (Day 4) and
over the Midwest along the trailing cold front (Day 5). Some areas
across the northern tier will be sensitive to additional rainfall
due to already wet ground heading into Friday.

Elsewhere, the new Day 4 ERO maintains continuity with a Marginal
Risk area over parts of the southern Rockies/Four Corners region
due to the abundant moisture arriving from the east and some
forecast instability, along with some model signals for locally
enhanced rainfall. Guidance suggests that the greatest moisture
anomalies should get pushed to the south and west during Saturday,
with rainfall over the region likely to be lighter and more
scattered--thus not meriting a risk area for Day 5. Meanwhile the
Northeast continues to merit monitoring from Friday into the
weekend. The combination of moisture and instability along with an
east-west front settling over the region could support some
locally enhanced rainfall rates and training/repeat activity.
However antecedent conditions should be rather dry by that time,
stream flows are already near to below normal, and model QPF is
not exceptionally heavy thus far. Therefore the outlooks continue
to depict no risk area at this time.

Continued progression of the system that should be near the Great
Lakes as of early Sunday will bring the trailing cold front through
the eastern U.S. and trailing back into the Plains by the start of
next week, bringing areas of showers and thunderstorms of varying
intensity. The front reaching the Northwest this weekend may bring
some light/scattered rainfall over northern areas. Continued
progress of this front may generate some showers/thunderstorms over
the central U.S. by the first half of next week. Some scattered
diurnal convection may persist over the southern Rockies.
Occasional showers/storms are possible along the Gulf Coast,
but with increasing uncertainty over specifics of the Gulf of
Mexico pattern at the surface and aloft by early next week keeping
confidence low regarding any potential increase in moisture at that

With a cold front reaching New England by Friday and providing a
cooling trend there, the greatest temperature anomalies from late
this week into the weekend should extend from the Midwest and Ohio
Valley/Lower Great Lakes into the Mid-Atlantic and Northeast, with
readings tending to be 10-15F above normal. This would translate
to highs well into the 90s, along with lows in the upper 60s to
upper 70s providing little overnight heat relief. Daily records for
max/warm min temperatures will be possible within the above areas,
up through warm mins early Monday (if they hold on through the
calendar day). Meanwhile much of the West will trend warmer/hotter
late this week into the weekend with highs reaching 10F or more
above normal for a couple days or so. The front reaching the
Northwest should bring cooler air to that region by Sunday-Monday,
while some of the leading western heat should reach the northern-
central High Plains at that time--connecting with lingering heat
over parts of the Plains into southern half of the East. Clouds
and rainfall will support below normal highs over the southern
Rockies/High Plains late this week.


Additional 3-7 Day Hazard information can be found on the WPC
range hazards outlook chart at:

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: