822
FXUS02 KWBC 241856
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Mon Jun 24 2024

Valid 12Z Thu Jun 27 2024 - 12Z Mon Jul 01 2024

...Hazardous heat threats to expand across parts of the South late
week into next weekend...


...Overview...

An upper ridge centered over the southern Rockies/Plains early
Thursday will expand eastward strengthen, with its core most
likely settling over the Lower Mississippi Valley by Saturday into
early Monday. This ridge will bring an increased threat for
hazardous heat to portions of the Southern U.S. from late this week
into early next week. To the north, a series of shortwaves/troughs
will progress from West to East across the northern tier into
early next week. Showers and thunderstorms will accompany the
associated frontal boundaries, with the best chance for heavy to
excessive rainfall across parts of the Midwest Thursday into
Friday.


...Guidance/Predictability Assessment...

Overall the latest dynamical and machine learning guidance offers
better than average agreement and continuity through the period,
with some lingering differences for the progressive northern
stream upper troughs and day-to-day position of the southern tier
upper ridge. The only stray solutions of note arise by next Monday
when the 06Z GFS becomes a little slower than consensus with the
upper trough reaching the Northwest (still a little slow in the new
12Z run), the 00Z CMC positions the southern ridge west of
remaining solutions (new 12Z run much closer to the majority), and
00Z GFS/CMC become more amplified with the trough reaching the East
(newer runs close to the means). Favorable comparisons of most
00Z/06Z guidance led to the latest update incorporating a composite
of operational runs during the first half of the period and then
adding in some of the 06Z GEFS/00Z ECens to temper the less
confident specifics of some model runs toward next Monday.


...Weather/Hazards Highlights...

A low pressure system and associated fronts tracking into the
Northern Plains Thursday will bring a threat for heavy rainfall
across the north-central Plains into the Upper Midwest. Some
conflicting signals remain regarding locations that may see
significant rainfall, but latest guidance is suggesting two areas
of relatively greater focus during the Day 4 Excessive Rainfall
Outlook period (Thursday-Thursday night). One is within the
existing Slight Risk area over parts of the Midwest with a modest
southwestward adjustment per latest guidance, and another farther
north across northern North Dakota and far northwestern Minnesota
where a Slight Risk area has been introduced. Slight Risk areas
within the broader Marginal Risk are subject to change based on
updated forecasts of convective evolution and location relative to
areas with greatest sensitivity due to prior heavy rainfall. By
Friday, the system will shift east, and guidance is maintaining the
potential for heavy rainfall centered over an area including parts
of Illinois/Wisconsin/Iowa/Missouri--again overlapping with areas
which have seen significant rainfall recently. With modest
adjustments from continuity, a Slight Risk area has been maintained
over this region on the Day 5 ERO, with a surrounding Marginal Risk
including parts of the middle to upper Mississippi Valley and
lower Great Lakes. Showers and thunderstorms accompanying this
system should reach into the eastern U.S. during the weekend.

Anomalous moisture with widespread precipitable water values over
the 95th percentile will persist over the southwestern U.S. into
Thursday and slowly lessen by the weekend. This should support
monsoonal-type showers and thunderstorms with heavy rain. The
combination of moisture/instability and model QPF signals, and
comparison to the event a few days ago, seem to support
introduction of a Slight Risk area over western Colorado into
northern New Mexico for the Day 4/Thursday ERO. Burn scars and
slot canyons will be particularly vulnerable in episodes of slow-
moving and heavy convection. A surrounding Marginal Risk area
encompasses a majority of the Four Corners states. Coverage of
storms should decrease by Friday but did maintain a small marginal
risk on the Day 5 ERO for southeast Arizona to southwest New
Mexico where the greatest moisture anomalies persist. Rainfall
coverage may increase once again especially in New Mexico over the
weekend.

Showers and thunderstorms associated with frontal boundaries may
also linger across the Southeast/Florida through the period, but
dry conditions/high FFGs continue to preclude any excessive
rainfall risk areas. Another northern tier system may generate
an episode of potentially heavy rainfall over parts of the
Northern Plains and Upper Midwest around next Monday. This system
will require close monitoring given the region`s sensitivity to
additional rainfall.

The focus for hazardous heat will initially be across parts of the
southern Plains on Thursday, where a long duration heat wave will
be ongoing and heat indices near 110F for some is leading to
widespread major to extreme HeatRisk. Heat currently across the
Southeast should briefly moderate on Thursday with troughing moving
through, but the upper ridge will build back into the region and
northward into the Mississippi and Ohio Valleys late week, bringing
several days of above normal temperatures (both daytime highs and
overnight lows) from the south-central Plains eastward. A fairly
broad area may challenge record warm lows, especially during the
weekend. Temperatures may be a few degrees above average in the
Desert Southwest, equating to highs 105-115F. Meanwhile the main
area of cooler temperatures will be across the northern tier, where
upper shortwaves and surface fronts provide some relief.


Rausch/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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