Prognostic Meteorological Discussion
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000
FXUS02 KWBC 060657
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 AM EDT Thu Aug 06 2020

Valid 12Z Sun Aug 09 2020 - 12Z Thu Aug 13 2020

...Overview and Guidance/Predictability Assessment...

The latest cycles of most models and ensemble means maintain good
continuity and agreement for the large scale pattern.  Upper
ridging will persist from the Atlantic across the Florida
Peninsula through southern portions of the Plains/Rockies.  Around
the northern periphery of this ridge expect an upper trough
anchored by a closed or nearly closed low over Canada to progress
across the northern tier of the lower 48.  This trough will push a
cold front from the Northern Plains into the
Northeast/Mid-Atlantic over the course of the period.  Upstream
flow will amplify into a trough that settles near the West Coast
by midweek, with a leading cold front pushing into the West during
Tue-Thu.  This trough should eventually lead to inland ejection of
at least some of the weak low/trough energy initially just
offshore California.  In response to the feature reaching the
West, expect the trailing part of the central/eastern U.S. front
to stall and possibly return north as a warm front.

The upper trough crossing the northern tier U.S. has displayed
somewhat better clustering over the past couple days versus
earlier forecast, but there is still some run-to-run variability
along with detail spread.  The primary adjustment by consensus in
the 12Z/18Z cycles is to be more amplified with the upper trough
at some valid times--leading to a farther south and east extent of
the leading cold front.  This represents a solution closer to some
runs from 24-36 hours ago.

Primary differences for the developing West Coast trough during
the latter half of the period emerge by day 7 Thu.  Guidance
indicates moderate progression of the upper low that reaches
western Canada and a portion of the trough immediately to the
south, which would be consistent with the established pattern that
has displayed some degree of progression.  However solutions
diverge over how much troughing remains near the West
Coast--whether from the initial trough or upstream energy that
provides reinforcement.  The 12Z ECMWF/ECMWF mean are on the
weaker side of the spread with the lingering trough while the 18Z
GFS/GEFS mean are noticeably deeper.  CMC/CMC mean runs lean to
the deeper side but the new 00Z GFS has backed away some for the
depth.  Even relative to the slightly farther east upstream
positive height anomaly of the 12Z ECMWF/ECMWF mean,
teleconnections favor somewhat deeper troughing over the Northwest
than what their forecasts show.  This favors nudging the forecast
blend a little more in the 18Z GFS/GEFS mean direction, but
uncertainty with specific shortwaves at nearly a week out in time
tempers confidence.  At least for the time being the new 00Z ECMWF
provides support for this idea, adjusting much stronger with
energy that drops into the Pacific Northwest toward the end of the
period.

A composite of the 18Z GFS and 12Z ECMWF/UKMET/CMC provided a
reasonable starting point for the forecast early in the period.
Then the blend transitioned toward 40-60 percent total ensemble
weight by days 6-7 Wed-Thu while eliminating the 12Z CMC due to
that run becoming excessively slow/amplified with eastern
troughing aloft.  The blend tilted 60 percent toward the 18Z
GFS/GEFS mean relative to the 12Z ECMWF/ECMWF mean by day 7 Thu to
reflect West Coast preferences at that time.


...Weather/Hazard Highlights...

Multiple episodes of showers and thunderstorms will be likely
across much of the area extending from the northern half of the
Plains through most the East.  The most prominent focus for this
convection will be the cold front that travels from the Northern
Plains to Northeast/Mid-Atlantic during the period, with stalling
of the trailing part of the front leading to a continued potential
for rainfall back into the central U.S. at times.  On Sun there
will also be a weakening warm front crossing the Mississippi
Valley and another front over parts of the Upper Midwest/Upper
Great Lakes.  The Southeast/Florida will see diurnal convection of
varying intensity, with a weakening front over the Southeast
providing an added focus early in the period.  Within the broad
area of shower/thunderstorm potential there will likely be areas
of heavy rainfall but it will take additional time to refine the
regions that may receive the highest totals.  The Storm Prediction
Center is monitoring the potential for severe convection across
parts of the northern tier.  Check future outlooks as important
details are better resolved.

Above normal temperatures will spread from the Plains to the
Mid-Atlantic/Northeast ahead of the cold front progressing
eastward from the Northern Plains.  Anomalies may reach as high as
plus 10-15F over some areas from the Midwest into the Northeast,
especially for morning lows.  The southern High Plains into
southern Arizona should be consistently above normal by 5-10F
through the period under ridging aloft while portions of the
Southeast may see highs several degrees above normal Sun-Mon.  The
cool airmass over the northern Rockies/Plains on Sun will moderate
thereafter as it continues eastward.  The Northwest will see a
cooling trend by Wed-Thu as upper troughing reaches the area.

Rausch


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php


WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml

$$




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