870
FXUS02 KWBC 231858
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Sun Jun 23 2024

Valid 12Z Wed Jun 26 2024 - 12Z Sun Jun 30 2024

...Hazardous heat possible for the southern tier through the
period and expanding northward into the central Plains/Mississippi
Valley and Ohio Valley late week...


...Overview...

An upper level ridge will meander around southern parts of the
nation through the week, leading to multiple days of above normal
to possibly hazardous heat. To the north, one shortwave through the
northern tier will amplify somewhat as it crosses the Midwest/East
mid- to late week. The next system reaching the West Coast around
Thursday should push a defined surface low pressure system into the
northern Plains by late week. A third shortwave looks to enter the
West again next weekend.


...Guidance/Predictability Assessment...

The latest suite of guidance continues to offer good agreement on
the large scale, and typical uncertainty in the details that could
have impacts on sensible weather/QPF. Models continue to offer good
agreement on the first shortwave tracking from the Great Lakes
midweek into the Northeast late week. There were a bit more
differences with the next shortwave moving through the northern
U.S. by Friday and the associated surface low, especially with the
00Z CMC, but still minor and within typical spread for Day 5.
Details on energy through south-central Canada reinforcing this
trough as it moves into the Great Lakes and Northeast next weekend
remain uncertain and would impact overall timing. Models even show
reasonably agreeable timing on the next trough approaching the
Northwest by Sunday, though this may change in future cycles.

The updated WPC forecast used a blend of the GFS, ECMWF, CMC, and
UKMET for the first half of the period amidst sufficient model
agreement. Transitioned to 50 percent of the ensemble means in the
blend (with the ECMWF and GFS) late period to mitigate some of the
model differences which will take additional time to resolve.
Overall, this maintained good agreement with the previous WPC
forecast.


...Weather/Hazards Highlights...

A cold front moving into the East on Wednesday and copious
moisture and instability in a typical summertime convective pattern
will support widespread showers and thunderstorms from parts of
the Lower Mississippi Valley into the Northeast. There remains
enough model QPF discrepancies to preclude any sort of Slight
Risks, and the front should be fairly progressive as well, limiting
the overall threat. Still, given moisture and instability, high
rain rates are likely so a broad Marginal Risk seemed reasonable
for the Day 4 Excessive Rainfall Outlook. The front should push
offshore of the Northeast on Thursday but linger in the Southeast.
Along it, scattered thunderstorms are expected combining with
diurnally enhanced convection over Florida. Dry conditions/high
FFGs continue to preclude any excessive rainfall risk areas.
Scattered thunderstorms over Florida look to continue through late
week and the weekend.

Another low pressure system will reach the northern Plains by
Thursday, bringing a renewed threat for heavy rainfall across the
north-central Plains to Upper Midwest. A broad Marginal Risk is in
place there for Day 5/Thursday, with an embedded Slight Risk. There
is some uncertainty in where the heaviest rain will occur. Recent
model guidance trended toward showing the highest QPF farther south
into Iowa compared to areas just north from far eastern Nebraska
and southeast South Dakota and near the Iowa/Minnesota border that
have seen extremely high rainfall totals recently. The updated
Slight Risk encompasses both these farther north (wet antecedent
conditions) and south (better model consensus for heavy amounts)
areas to cover the threat. A heavy rainfall threat may emerge in
the Great Lakes and Middle Mississippi Valley Friday and Saturday
as the front presses farther southeast.

Anomalous moisture with widespread precipitable water values over
the 95th percentile will persist over the southwestern U.S. through
Wednesday and slowly lessen later in the week. This should support
monsoonal-type showers and thunderstorms with heavy rain. Marginal
Risks remain across the Four Corners states and vicinity for the
threat of flash flooding both Wednesday and 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. The main change to the
previous issuance was to have the Marginal Risk cover the slot
canyons of southern Utah that are quite sensitive to rainfall. One
uncertainty for any upgrading to any Slight Risk (on Day
4/Wednesday when moisture levels are highest) is whether cloud
coverage could limit instability and limit rain rates. Farther east
into the central High Plains, a stalling front combining with this
moisture advection could lead to heavy rain in a possible MCS
there on Wednesday night, so the Marginal Risk on Day 4 encompasses
that area too. Coverage of storms may decrease by Friday with a
break in the anomalous moisture, but increase once again especially
in New Mexico over the weekend.

The focus for hazardous heat by midweek will be across the south-
central Plains/Mississippi Valley and parts of the Southeast, where
a long duration heat wave will be ongoing and heat indices near
110F for some is leading to widespread major to extreme HeatRisk.
The heat 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.


Tate/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


$$