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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1911Z Aug 12, 2022)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
311 PM EDT Fri Aug 12 2022

Valid 12Z Mon Aug 15 2022 - 12Z Fri Aug 19 2022

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

...Heavy rain/Flash Flooding possible across portions of the
Central Plains/Middle Mississippi Valley next Tuesday/Wednesday...

...Excessive heat threat across the South-Central U.S. and
Northwest next week...


...Overview...

An amplified upper flow pattern for next week will feature troughs
over the northeast Pacific and east-central U.S. sandwiched around
a ridge over the West. The ridge will build northward through the
Northwest and western Canada, leading to a threat of excessive
heat, as the northeast Pacific trough shifts slowly eastward
toward the coastline. The ridge and embedded shortwave impulses
will also continue to support widespread monsoonal moisture
streaming underneath into a wet and unsettled Southwest,
Intermountain West, and Rockies through next week. The downstream
digging of shortwaves/height falls to the lee of the ridge will
periodically reinforce upper diffluent flow over a hot
south-central U.S. before feeding into a slow to dislodge
east-northeast U.S. closed low. This low along with modest coastal
low development should support multiple days of rainfall in the
Northeast and allow a break from summer temperatures down through
roughly the eastern half of the nation as a cooling front settles
far down through the east-central U.S. to the South to provide a
multi-day focus for convection with locally heavy rains.


...Guidance Evaluation/Predictability Assessment...

The overall synoptic pattern was captured reasonably well by the
current guidance with some areas of uncertainty. The
trough-ridge-trough setup will tilt farther west as the central
CONUS ridge expands and lifts into the Northwest. This was
resolved fairly well by the suite of deterministic guidance
through day 5 before the 00z EC introduces an anomalous shortwave
trough over the California coast on days 5 and 6, prompting a more
ensemble-centric approach through the rest of next week. A general
model blend of deterministic EC/GFS/UKMET/CMC being used and the
00z EC/UKMET being favored with greater weighting due to their
emphasis on an impactful shortwave impulse rotating through the
Central Plains. This same blend was continued through day 4 before
being diluted a bit with the 00z EC/CMCE and 06z GEFS to help
signal a closed low in the East that was missing in the 00z EC.
Ensembles and the 06z GFS were used on day 6 before ending with
00z ECE favored ensemble blend on day 7. GEFS 06z GEFS members
struggled to agree with the rest of the 00z/06z guidance with
respect to the eastern trough early next week and the
east-northeastern Pacific trough later in next week, as it was
generally a bit deeper.

...Weather/Hazards Highlights...

Portions 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. Collaborated WPC experimental Slight
Risks of excessive rainfall are in place for the Central Rockies
for early next week, where there is a viable guidance signal for
heavy convective rainfall rooted by digging shortwave energy/upper
diffluence and terrain lift. The heavy rain focus looks to shift
slowly southward mid-later next week into the Southern Rockies.

There are increasing chances for organized clusters of convection
with heavy downpours to spread through parts of the
Central/Southern Plains and mid-lower Mississippi Valley as the
week progresses. A slight risk of excessive rainfall is in effect
for parts of central-southeastern Missouri next Tuesday.
Elsewhere, the Gulf Coast and into Florida could see scattered
storms with a moist airmass and frontal boundaries. Then as a
surface low is currently forecast to track across the Ohio Valley
through the Mid-Atlantic to the Northeast, organized rains are
forecast for those areas. There's increasing confidence in a
multi-day rain event occurring for much of the coastal Northeast,
from New Jersey up to Maine through mid-week under closed upper
low influence and with near shore track of a coastal frontal low.

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.

Kebede/Schichtel


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