994
FXUS02 KWNH 240658
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 AM 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 initially centered over the southern Plains will
gradually expand eastward with time and intensify bringing an
increased threat for hazardous heat to portions of the Southern
U.S.. 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...

The latest suite of guidance continues to offer very good
agreement on the large scale, and typical uncertainty in the
details that could have impacts on sensible weather/QPF. Agreement
on the timing and amplitude of a trough exiting the East on
Thursday has trended better in recent days, with model agreement on
an upper ridge building back in over the Southeast by next
weekend. The next shortwave should reach the Northwest by Thursday,
with some continued uncertainty as it reaches the northern Plains
and Great Lakes regarding interaction with additional energy from
south- central Canada. This system should progress across the
Northeast early next week, with some question on timing of the
shortwave/surface front through the East at that time. A third
trough enters the West late weekend and also offers plenty of
uncertainty in the timing/details. The CMC was a little slower than
the better consensus of guidance and was not included in the late
period forecast blend.

The WPC forecast for tonight was based on a blend of the GFS,
ECMWF, CMC, and UKMET for the first half of the period amidst
sufficient model agreement. Incorporated more of the ensemble means
the second half (with the GFS and ECMWF) to help temper some of
the guidance differences by that time. Overall, this maintained
good agreement with the previous WPC forecast as well.


...Weather/Hazards Highlights...

A low pressure system into the Northern Plains on Thursday will
bring a threat for heavy rainfall across the north-central Plains
into the Upper Midwest. There remains a lot of uncertainty in the
QPF details/amounts, but did maintain a broad marginal risk on the
Day 4/Thursday Excessive Rainfall Outlook across the region given
ample moisture and instability associated with this system.
Compared to previous model runs, the signal a more targeted area of
heavier rain has decreased, but did opt to maintain a slight risk
centered across northern Iowa given very wet antecedent conditions
and sensitivity to additional rainfall. By Friday, the system will
shift east, and there is some increased heavy rainfall signal from
eastern Iowa into southern Wisconsin overlapping with areas which
have seen significant rainfall recently. A slight risk was placed
in this region on the Day 5 ERO, with a broad marginal risk
surrounding across parts of the middle to upper Mississippi Valley
and lower Great Lakes. 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.

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. A
marginal risks remains across the Four Corners states and vicinity
for the threat of flash flooding on Thursday, with burn scars and
slot canyons particularly vulnerable. Though perhaps not everywhere
in the quite broad Marginal Risk areas will see heavy rain, slow-
moving convection is a concern for these areas and warrants the
risk depiction in the ERO. 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. Rainfall coverage may
increase once again especially in New Mexico over the weekend.

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.
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.


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




$$