Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
251 AM EDT Tue Jul 05 2022

Valid 12Z Fri Jul 08 2022 - 12Z Tue Jul 12 2022

...Prolonged Heatwave This Week and Upcoming Weekend Across
Southern Plains to Mid South...

...Overview...

A persistent and strong upper level high centered over the
southern U.S. through this week and into early next week will keep
very hot temperatures in place across much of the central CONUS.
Combined with the humidity, heat indices are likely to exceed
105-110F in places and with very little nighttime relief in
temperatures, a prolonged heatwave is expected across the southern
half of the Plains and Mississippi Valley. On the periphery of the
upper ridge, troughing initially across the eastern Pacific Friday
will shift eastward across the northwestern to north-central U.S.
over the weekend into early next week, while troughing over the
Northeast through the weekend may shift away around Monday.
Multiple impulses moving through the flow north of the ridge
should produce multiple episodes of potentially strong storms with
the potential of locally heavy rainfall across the Ohio/Tennessee
Valleys and Appalachians to the East Coast, as well as across the
northern Plains to Upper Midwest.


...Guidance Evaluation/Predictability Assessment...

Recent deterministic and ensemble guidance remains in good
agreement for the large scale setup and evolution for late week as
the center of the upper high drifts westward while the ridge axis
strengthens over the High Plains. Models for the most part show
troughing across the East Coast deepening a bit into the weekend
and pushing a cold front southward. The trend has been toward a
slightly faster front and thus QPF was farther south than the
previous issuance around Friday-Saturday. Differences in smaller
scale shortwaves remain within the trough, which have low
predictability likely even into the short range and will be
important for the details of rainfall amounts and placement.
Meanwhile, troughing with its axis near the West Coast Friday
should shift inland this weekend, but with some differences in the
details of timing and energy distribution within the trough as it
lifts northeast and then eastward early next week. Specifically,
GFS runs have been somewhat faster to press energy east into the
northern Plains compared to the ECMWF runs, while the CMC has been
more along the lines of the GFS, but becomes flatter than
preferred with its flow over the lower 48 by around Monday.
Preferred a middle ground solution between the faster GFS and
slower ECMWF. Model agreement seems to actually increase slightly
day 7/Tuesday given the trough could slightly amplify over the
Great Lakes region. For the WPC forecast blend, utilized a
multi-model deterministic blend early in the period, with
increasing proportions of the GEFS and ECENS means by the later
period to about 40 percent to smooth out some model differences
but maintaining GFS and ECMWF components to enhance features.


...Sensible Weather and Hazards...

Scattered showers and storms are likely across much of the U.S.
from the Rockies eastward at times this week into early next week,
with enhanced totals in the vicinity of a couple of frontal
boundaries initially stretching generally west to east across the
northern half of the country. Northwesterly flow aloft and
abundant moisture and instability will create a favorable pattern
for mesoscale convective systems and high rainfall rates across
the Ohio Valley and Appalachians to the East Coast. Areas of
excessive rainfall/flash flooding are possible through the
weekend, but considerable uncertainty with the placement of heavy
rain amounts, as well as the likely fast-moving nature of the
storms, precludes delineating Slight Risk areas in the
experimental day 4-5 Excessive Rainfall Outlook at this point. A
cold front in the East will finally begin to sink southward over
the weekend in response to slightly deepening troughing aloft.
This could provide additional support and focus for thunderstorms
across the Carolinas, Southeast, and eventually reaching the Gulf
Coast, while clearing is expected over the Northeast behind the
front. Meanwhile, frontal boundaries and upper-level energy could
lead to convection across the northern Plains to Upper Midwest
through the weekend with some potential for flash flooding.
Thunderstorms may push southeastward from those areas by
Monday-Tuesday along and ahead of a cold front. Elsewhere,
monsoonal moisture could persist through much of the week with
moist southwesterly flow/embedded shortwaves leading to mainly
diurnally driven showers and storms over the Four Corners states.
The southern Plains and California into the Great Basin should be
the most consistently dry areas during the period.

Very hot weather is expected to persist across the southern half
of the Plains/Mississippi Valley underneath the strong upper
ridge/high. High temperatures near or exceeding 100F are expected,
which is between 5 and 15 degrees above normal. Combined with the
humidity, daily maximum heat indices 105-110F+ are likely at times
across a large portion of the Mid-Mississippi Valley southward
into the southern Plains and Gulf Coast/Southeast. The repeating
days of high heat/humidity and lack of any appreciable nighttime
relief (lows in the upper 70s to low 80s for some areas) will
increase the potential for impacts from this prolonged heatwave to
the more sensitive and vulnerable populations. Some daily record
high temperatures are possible, particularly for warm lows.
Meanwhile, the West Coast could see cooler than normal conditions
late week before moderating closer to average and eventually above
average early next week, while the Desert Southwest should see
near normal temperatures becoming a few degrees above average.
Parts of the East may see slightly below normal temperatures
spreading south this weekend into Monday behind a cold front.


Tate


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, experimental excessive rainfall
outlook, winter weather outlook probabilities and heat indices are
at:

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