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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 0657Z Aug 12, 2022)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
257 AM 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...

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


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 inpulses
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 mid-larger scale pattern evolution for next week seems well
depicted by guidance through medium range time scales. Model
forecasts of flow embedded systems and affects overall seem
decently clustered Monday/Tuesday to provide max detail consistent
with an amplified pattern with above normal predictability. A
composite of the GFS/ECMWF/UKMET and Canadian seems to provide a
good forecast basis along with the 01 UTC National Blend of Models
(NBM). Growing small-mid scale system variance mid-later next week
led to transition to a blend of the GEFS/ECMWF ensemble means
along with most compatible model guidance of the 12 UTC GFS/ECMWF
along with the NBM. The 12 UTC GEFS was more in line with this
solution than the less amplified patterns of the 18 UTC GFS and 12
UTC Canadian at these longer time frames. The latest 00 UTC
GFS/Canadian have trended more amplified and in line with the
aforementioned prefered composite, bolstering forecast confidence.
NBM QPF forecasts seems reasonable and consistent with the pattern
(if not details) so no significant forecaster modifications were
applied for the WPC forecast package.

...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. 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, orgainzed
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.


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, experimental excessive rainfall
outlook, winter weather outlook probabilities and heat indices are