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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0657Z Aug 10, 2022)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 AM EDT Wed Aug 10 2022

Valid 12Z Sat Aug 13 2022 - 12Z Wed Aug 17 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...

A fairly stagnant trough-ridge-trough pattern is forecast this
weekend across the contiguous U.S., as shortwaves deepen and then
maintain a trough in the East, an upper low off the West Coast
lifts northeastward into British Columbia, and a warm upper upper
high meanders near the central/southern High Plains. Upper ridging
is forecast to expand into the West by Tuesday-Wednesday of next
week, increasing temperatures to well above average particularly
in the Northwest. The ridge aloft will also continue to support
monsoonal moisture streaming into the Southwest and into the
Intermountain West and Rockies through much of next week. A
lingering frontal boundary over Florida will provide a focus for
rainfall there, while possible shortwave energy rounding the ridge
could lead to rounds of showers/thunderstorms across the
Midwest/Upper Great Lakes this weekend, with rain chances
expanding south and east early next week.


...Guidance Evaluation/Predictability Assessment...

Model guidance remains steady in indicating the central U.S. upper
high will separate the eastern trough and a compact upper low
tracking across British Columbia this weekend (which has finally
showed better agreement in track and timing). Less certain aspects
of the forecast include small-scale shortwaves rounding the
northern side of the ridge, which may not cause considerable
differences on the larger scale, but do lead to variety in
placement of storms/possible MCSs that develop in and around the
Midwest in the northwest flow. There is also some variability with
the details of energy distribution within the eastern trough, even
as the period begins Saturday with the 12Z ECMWF showing a closed
low within the trough. This initial energy looks to lift northeast
while additional shortwaves dig to maintain the trough through the
middle of next week, with perhaps better agreement for a closed
upper low to form around Tuesday-Wednesday among deterministic
models, but the placement of the upper low center varies. Among
12/18Z guidance the ECMWF was the farthest north into Canada and
the 12Z GFS was farthest south over the Great Lakes, with the 12Z
CMC leaning south as well while the 18Z GFS was in between. The
newer 00Z guidance trended a bit toward the southern side. Despite
the differences, no models seemed questionable enough to be called
outliers. Thus the WPC forecast was based on an initial blend of
the 18Z GFS and 12Z ECMWF/UKMET/CMC, with the blend serving to
smooth out the small scale differences. Gradually incorporated the
EC and GEFS ensemble means day 5-7, which were agreeable, to try
to produce a middle ground forecast. The main change to the
previous WPC forecast was regarding timing of a cold front
tracking eastward through the Great Lakes to Northeast as the
eastern trough seems to sharpen up, while the main change to the
QPF from the National Blend of Models was to increase coverage of
QPF in the east-central U.S. given shortwave energy could cause
showers and storms there.


...Weather/Hazards Highlights...

Parts of the West have seen persistent monsoonal moisture over the
past couple of months, and the rest of this week and into next
week will continue that pattern. In fact, notable moisture
anomalies are forecast to stream into parts of the Great Basin and
the Rockies and produce rain and thunderstorms. Rain totals may
increase by Monday-Tuesday for the northern/central Rockies in
particular. Meanwhile, a moist airmass and a lingering frontal
boundary will cause showers and thunderstorms across the Florida
Peninsula into early next week. Farther north, shortwave impulses
rounding the ridge could lead to rain and storms across the
Midwest/Upper Great Lakes into the Ohio Valley this weekend, but
with low confidence on the specifics. Then rain chances could
increase for the Ohio Valley to Appalachians and the East Coast
early next week along a cold front, but the details remain
uncertain there as well.

Very warm to hot temperatures around 10-15F above average, though
mainly not to record-breaking levels, are forecast across the
Northern and especially the Central Plains over the weekend, with
some gradual moderation and shift southward through the workweek,
though highs could still reach 100F in the Southern Plains. As the
upper ridge expands into the Northwest, this will warm up
temperatures there and cause highs 10-20F above normal by
Tuesday-Wednesday. Meanwhile, below normal highs are possible 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