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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0700Z Aug 11, 2022)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Thu Aug 11 2022

Valid 12Z Sun Aug 14 2022 - 12Z Thu Aug 18 2022

...Monsoonal moisture/rainfall threat continues for the Southwest
and Intermountain West/Rockies...

...Excessive heat threat across the Plains, with warming
temperatures in the Northwest next week...


...Overview...

The medium range period will begin Sunday with a continued
trough-ridge-trough pattern across the lower 48. As next week
progresses, there should be a trend toward the upper high/ridge
expanding into the West, leading to well above average
temperatures in the Northwest. The ridge aloft with embedded
shortwave energy will also continue to support monsoonal moisture
streaming into the Southwest, Intermountain West, and Rockies
through the week, with rain eventually spreading into portions of
the Plains and Mississippi Valley. Meanwhile the trough across the
East may deepen and close off an upper low around midweek, which
could cause multiple days of rainfall in the Northeast, but
dependent on the position of the upper low and the associated
surface low/fronts, which remain uncertain at this point.


...Guidance Evaluation/Predictability Assessment...

Model guidance remains agreeable for an upper high over the
central/southern High Plains to slowly retrograde and expand
ridging influence into the western U.S., overtaking the early week
weak troughing there. Some small-scale shortwaves embedded within
the broad ridge show some variability as they move through the
interior West, but nothing too significant on the large scale.
Farther east, models have persistently indicated troughing across
the eastern third of the U.S. through much of next week. But the
details remain inconsistent from model cycle to model cycle,
particularly by Tuesday-Thursday. In general, guidance indicates
that an upper low could close off within the trough around midweek
but with differing placement. The 12/18Z model cycle that was
available for the WPC forecast showed an upper low center farther
south than previous runs, as well as a deeper surface low. This
allowed more more QPF farther inland in the Northeast compared to
the previous cycle as deep moisture wraps around the low. WPC's
forecast trended upward from continuity but not as high as the NBM
given the run-to-run uncertainty. The newer 00Z models have
generally backed off a bit, showing a weaker surface low and a
farther north upper low, leading to less rainfall overall, though
still indicating some heavy amounts in New England (less in New
York and the Mid-Atlantic). The WPC forecast used a multi-model
deterministic blend for the early part of the medium range
forecast, incorporating some GEFS and EC ensemble means into the
blend days 5-7, so as not to go all in for the southern trend of
the upper low in the 12/18Z cycle.


...Weather/Hazards Highlights...

Parts of the West have seen persistent monsoonal moisture over the
past couple of months, and next week will continue that pattern as
anomalously high moisture streams into the Southwest, Great Basin,
and Rockies in conjunction with small-scale shortwave energy to
produce rain and thunderstorms, particularly in the afternoon and
evening hours. Slight Risks of excessive rainfall are in place for
portions of the Desert Southwest on Sunday and the Central Rockies
on Monday, where current forecasts show the best chances for heavy
rainfall. Heavy rain amounts look to continue through midweek in
the Central Rockies. There are increasing chances for rainfall to
spread into the central and southern parts of the Plains and
Mississippi Valley as the week progresses. Elsewhere, the Gulf
Coast and into Florida could see scattered storms with a moist
airmass and a couple of frontal boundaries. Then as a surface low
is currently forecast to track across the Ohio Valley through the
Mid-Atlantic toward the Northeast, rain is forecast for those
areas, but the track of the surface low (and upper low) have
varied quite a bit in model guidance so there is low confidence in
rainfall placement and amounts.

Very warm to hot temperatures around 10-15F above average, though
mainly not to record-breaking levels, are forecast across the
Central Plains Sunday into Monday, with a gradually moderating
trend Tuesday onward yielding below normal highs. As the upper
ridge expands into the Northwest, this will warm up temperatures
there and cause highs 10-20F above normal for Tuesday-Thursday.
Meanwhile, below normal highs are expected in the east-central
U.S. behind the cold front, with the most persistent cooler than
average weather in the Ohio and Tennessee Valleys toward the
Mid-Atlantic. Given the rainfall and cloudiness over the Southwest
and Great Basin into the Rockies, highs are likely to be several
degrees below normal there through much of next week.

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