849
FXUS02 KWBC 220658
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Sat Jun 22 2024

Valid 12Z Tue Jun 25 2024 - 12Z Sat Jun 29 2024

...Hazardous heat possible for parts of the South/Plains early to
mid next week...


...Overview...

An upper level ridge will remain anchored over much of the
Southern half 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 reaches
the West Coast around Thursday spreading a defined surface system
into the northern Plains by next weekend.


...Guidance/Predictability Assessment...

The latest suite of guidance continues to offer overall good
agreement on the large scale, but plenty of uncertainty in the
details which could have larger impacts on sensible weather/QPF
across the affected regions. A shortwave skirting the Northern
Plains/Great Lakes Tuesday-Wednesday may amplify some into the
Midwest and Northeast by Thursday, with the ECMWF offering a more
amplified and slower evolution. The better consensus (including
machine learning guidance) suggests something a bit flatter and
faster. The new 00z ECMWF (available after forecast generation
time) shows a trend in this direction as well. Out West, timing
uncertainty arises early in the period, with the UKMET the slowest
and the GFS remaining on the faster side. Ensemble means/ML models
though seem to support a little faster progression though as well.

The WPC forecast tonight used a general blend of the 12z/Jun 21
ECMWF and CMC with the 18z/Jun 21 GFS for the first half of the
period, introducing more of the ensemble means later in the period
(with some GFS and ECMWF) to help temper some of the smaller scale
uncertainties. Overall, maintained fairly good continuity with the
previous WPC forecast for days 3-6.


...Weather/Hazards Highlights...

A low pressure system moving through south-central to eastern
Canada and its associated fronts will provide a focus for possibly
impactful convective rainfall. On Tuesday-Wednesday, copious
moisture and instability in a typical summertime convective pattern
south and east of the cold front should support widespread showers
and thunderstorms progressing eastward with the boundary from the
Mississippi Valley/Midwest into the East. As a result, large
Marginal Risks are highlighted on the Days 4 and 5 ERO to cover
this favorable environment. It is still too early to pin down any
potential areas of higher risk and the front should be fairly
progressive as well, limiting the overall threat.

Anomalous moisture and sufficient instability should persist over
the Four Corners states, eventually expanding into the southern
Rockies and parts of the High Plains much of the period and
potentially lead to some locally heavy rainfall, which could cause
flooding impacts particularly over burn scars. Marginal Risks are
in the outlook for Day 4 across parts of Arizona and New Mexico,
expanding into Colorado and the south-central High Plains on Day 5
where the best moisture transport is for slow- moving convection.
Elsewhere, parts of the Southeast/Florida may also see episodes of
diurnally enhanced convection, but overall dry conditions/high FFGs
precluded any excessive rainfall risk areas at this time. A front
reaching the Northwest may produce some light rainfall Wednesday
into Thursday.

For the first half of the week, above normal temperatures will
stretch from the West to the East, though the focus for hazardous
heat will be across the central Plains states into the Southeast
where daytime highs 10-15F above normal, and heat/humidity could
lead to heat indices near 110F for some, and widespread major to
extreme HeatRisk are present Tuesday-Wednesday. Hazardous heat is
also possible across the Southwest where temperatures 105-115F are
likely. Temperatures should moderate back towards normal for the
East and West states as upper troughing moves through both regions
mid to late week, but upper ridging in between should bring a
return to modestly above normal temperatures from the Plains into
the Midwest Friday-Saturday.


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




$$