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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1900Z Jun 18, 2024)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 PM 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, while some heat shifts south...

...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 gradually 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 warm 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...

Model guidance remains agreeable with the main aspects of the
pattern especially through the weekend. The upper ridge should
begin Friday over the Mid-South to Mid-Atlantic, getting suppressed
south with time as southern and northern stream energy phase to
create a shortwave and likely small closed low near the Great Lakes
by Sunday. This feature shows reasonably good agreement in the
upper levels with dynamical and AI models, but at the surface there
was some spread in the low strength and track. The 00Z ECMWF
(deterministic) seemed to be an outlier with a strong surface low
and northwest track compared to other dynamical/AI models that were
farther southeast and weaker, but the 12Z EC has come into better
agreement. The shortwave and the cold front associated with the
surface low tracking northeastward should track through the
Northeast Monday-Tuesday, with typical levels of spread.

The next shortwave of note looks to come into the Northwest early
next week, with some spread in its timing and depth as it tracks
east near the U.S./Canadian border. There was a general trend
toward the shortwave and the associated surface frontal system
moving faster compared to the previous forecast. Then, attention
turns to the western Gulf of Mexico where there is a possibility of
a tropical system. Most deterministic models and many ensemble
members show a low approaching north from the Bay of Campeche, but
AI models show less potential.

The WPC forecast used a multi-model deterministic blend early in
the forecast period with inclusion of the GEFS and EC ensemble
means by Day 5 and increasing their percentage to half by Days 6-7.

...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 across
Minnesota and the northern half of Wisconsin into the U.P. of
Michigan (close to continuity). This could be a higher-end Slight
across central parts of Minnesota into northwestern Wisconsin. The
Day 5 ERO for Saturday will continue 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 updated Day 4 ERO maintains continuity with a
Marginal Risk area over parts of the southern Rockies/Four Corners
region due to the abundant moisture likely from both the Pacific
and the Gulf arriving, some forecast instability, and 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, so a Day 5 Marginal Risk still does not
seem needed. 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, streamflows 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
for Friday-Saturday. By Sunday (what is currently Day 6), forcing
increases with a shortwave and cold front approaching, so this may
warrant an eventual ERO risk.

Continued progression of the low pressure 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. Another 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 time.

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
through the weekend, up through warm minimum temperatures 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
the southern half of the East. Highs well into the 90s and high
dewpoints in the Southeast will lead to heat indices around 105F,
with widespread areas of major HeatRisk by early next week. 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: