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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0555Z Aug 10, 2018)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
155 AM EDT Fri Aug 10 2018

Valid 12Z Mon Aug 13 2018 - 12Z Fri Aug 17 2018

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

A pair of stagnant upper lows will initially inhabit the map to
commence the week. These should be fixed over the Central
Appalachians and Southern High Plains, respectively. Both features
are expected to gradually evolve into open waves with gradual
eastward migration the following days. The most strongly forced
systems should remain along or north of the international border
with Canada. Strong height falls moving across the Canadian
prairies on Monday will eventually lead to amplified flow sitting
over Quebec by mid-week with possible southward extension into
upper New England. Lurking farther upstream, a return to amplified
troughing is possible by Day 7/August 17 across the Pacific
Northwest and British Columbia. However, this feature is much more
uncertain and very model dependent. Across the Desert
Southwest/Four Corners region, models advertise a return of a
594-dm mid-level ridge toward the middle/later part of next week.

Regarding the upper low sitting west of the mid-Atlantic, there is
an eastward trend in the 12Z ensemble solutions by Tuesday. The
12Z ECMWF and most its ensemble members as well as the 12Z UKMET
favor a slower progression. This would appear a bit more
reasonable given the tendency for cut-off lows to be slower than
the guidance expects. As the mentioned upper low initially over
the Southern High Plains begins its eastward migration, this will
ultimately force the mid-Atlantic system out to sea. The larger
model uncertainties lie within the active flow bordering Canada.
Regarding height falls pushing from Saskatchewan to Quebec, the
past couple of ECMWF runs have been more aggressive which would
aid in the production of a sub-1000 mb cyclone just north of Maine
by Thursday. However, the GFS solutions remain more subdued which
would cast some doubt on the validity of the ECMWF. Farther west,
ensemble spaghetti plots show tremendous scatter with phase
differences evident as well. The 12Z ECMWF ensemble mean depicts
mean ridging extending across western North America while the 18Z
GEFS/12Z NAEFS means show more zonal flow. Considering the past
four GFS runs, the previous two favored troughing while preceding
cycles showed ridging. Suffice to say, a very challenging
forecast. Farther south, while the 12Z CMC depicts a shortwave
moving into California, the 18Z GFS/12Z ECMWF beg to differ with a
return of the 594-dm mid-level ridge during the Day 6/7, August
16-17 period. With the arrival of the 00Z CMC, it has since backed
off on this low-amplitude trough scenario.

With reasonable model clustering through Day 4/Tuesday, went with
a combination of operational solutions. This consisted of a
combination of the 18Z/12Z GFS and 12Z ECMWF/CMC/UKMET with
greater weighting on the slower solutions across the east.
Thereafter, growing spread within the active southern Canadian
flow supported inclusion of ensemble means. There was a tendency
to include a bit more of the 12Z ECMWF/ECMWF ensemble mean to help
lower pressures a bit across eastern North America by the middle
of next week. Also, this would help build heights slightly more
over the western U.S. which has been the preferred ridge location
this summer.


...Weather Highlights and Hazards...

During the upcoming week, below average temperatures should
congregate across the south-central U.S. given widespread cloud
cover and rainfall in response to the slow moving upper low.
Across the Southern Plains, some locations may struggle to get out
of the upper 70s to lower 80s on Monday with departures around 10
to 15 degree below climatology. Although these anomalies should
become less impressive downstream, cooler conditions will prevail
given the continued precipitation threats. Farther north from the
Pacific Northwest eastward to the Great Lakes, intermittent
periods of above average temperatures are expected next week. In
particular, Monday as well as late in the week it could be around
10 to 15 degrees above mid-August climatology across the
north-central U.S. with highs soaring into the low 90s.

The most focused areas of rainfall should initially congregate
with the mentioned pair of upper lows across the mid-Atlantic and
south-central U.S. Eventually the bigger threat will emerge along
the slow moving cold front which tracks toward the Eastern
Seaboard during the week. Some operational models are quite
bullish with QPF across Missouri into Illinois and Indiana with 2
to 3 inch 24-hour amounts. Over the Pacific Northwest, the 12Z
ECMWF ridge would support a dry period although a great deal of
spread remains which holds discernible confidence at bay. Across
the Desert Southwest, daily chances for monsoonal thunderstorms
are likely although model amounts are on the lower side of the
spectrum.


Rubin-Oster


WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found 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