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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1618Z Aug 13, 2018)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1218 PM EDT Mon Aug 13 2018

Valid 12Z Thu Aug 16 2018 - 12Z Mon Aug 20 2018

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

The beginning of the period (Thursday morning) shows some
combination of shortwave energy lifting out of the northern Plains
into a second feature/shortwave elongating across the Upper
Midwest. A surface wave ahead of this should gradually deepen as
it progresses from the Ohio Valley into New England Thursday and
Friday. Overall, the guidance is in relatively good agreement with
this feature. It is interesting to note the ECMWF and its ensemble
mean were noticeably slower with the shortwave placement on day 3,
but by day 4 and beyond, seems to come into better agreement with
the rest of the model suite.

Out west, models continue to show weak height falls spilling into
the Pacific Northwest on day 3, interacting with amplification in
the northern stream flow. This should ultimately lead to some
level of amplified flow across the middle of the country by this
weekend. The latest run of the GFS (06z on Aug 13) has backed off
on the more amplified trough noted previously, and now looks
closer in line with the rest of the guidance. Whatever feature
does emerge within the flow regime will advance downstream and
should be at a position near the Mississippi River by Day 7/August
20 (Monday morning), as suggested by the ensemble means. In its
wake, a possibly anomalous ridge builds just to the west across
the eastern Pacific into western Canada. While most pieces of
guidance do show some degree of anomalous heights, models continue
to show disagreement with how strong these may be. The latest runs
of the GFS continue to be the most amplified, but this may not be
a bad thing as a more amplified pattern upstream (with whatever
remains of T.S. Hector phasing into a larger upper west of Alaska)
would support a more amplified ridge downstream.

As for model choices with this cycle of the medium range progs,
through day 5, there was enough operational support and agreement
to constitute a majority operational 06Z GFS/00Z ECMWF blend.
Beyond, normal timing and intensity differences arise within the
smaller scale features so a larger influence of the ensemble means
(06Z GEFS/00z ECENS) was used. Did put slightly more weight on the
latest GFS/GEFS mean than the ECMWF/ECENS by day 7 to show that
slightly more amplified ridging across western Canada. The overall
blend for this cycle fit well with previous shift continuity so
only minor changes were needed to the surface fronts and pressures
and 500 progs.

...Weather Highlights and Hazards...

With the expectation of a deep upper trough pushing across the
Northern Rockies into the Northern/Central Great Plains during the
Day 5-7, August 18-20 time period, much cooler conditions can be
anticipated. With expected highs only in the 60s across the higher
terrain and 70s to near 80 across the lower elevations, such
readings would be around 10 degrees below average. Before this
takes place, it should be quite warm across the north-central U.S.
through Friday as high temperatures sit in the low/mid 90s.
Elsewhere, a building upper ridge along the West Coast will allow
temperatures to soar each day with most anomalous numbers over the
Pacific Northwest. By Day 6-7, Portland and Seattle may push into
the 90s which would be 10 to 15 degrees above mid-August

Multiple rounds of showers and thunderstorms are likely from the
middle of the country eastward into the Great Lakes and New
England. The best focus will initially be near the lifting surface
wave while vast warm sector convection should prevail to the
south. With the flow expected to amplify this weekend across the
Rockies, robust QPF amounts are noted in many model solutions with
a particular focus across the Central Plains into the Middle
Mississippi Valley. Across the Four Corners region, active
monsoonal flow will aid in daily thunderstorm chances with heavier
amounts potentially over southern Arizona where higher
precipitable water air is to reside.


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