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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1355Z Jul 11, 2018)
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
955 AM EDT Wed Jul 11 2018

Valid 12Z Sat Jul 14 2018 - 12Z Wed Jul 18 2018

Amplified flow should remain confined to the international border
with Canada throughout the period. Broad long wave troughing will
track from Alberta to the vicinity of Hudson Bay by Tuesday with
the southern extension of height falls slipping across the Great
Lakes and Ohio River valley. In its wake, renewed amplification is
likely across western Canada into the Pacific Northwest by Day
6/7, July 17/18. Elsewhere, while an elongated upper trough
lingers over the western Atlantic, an expansive 5940 meter 500 hPa
ridge is likely to unfold over the Desert Southwest while a weaker
anticyclone is expected across the southeastern U.S.

...Guidance evaluation and preferences...
The guidance shows some issues with less predictable shortwave
details, but is otherwise in good agreement.  For pressures, 500
hPa heights, and winds, a compromise of the 00z Canadian, 00z
UKMET, 00z ECMWF, and 06z GFS was used from Saturday into Monday,
before increasing amounts of the 00z ECMWF ensemble mean and 00z
NAEFS mean is used to replace the UKMET in the blend.  The
remainder of the grids are an even blend of deterministic and
ensemble mean guidance to account for uncertainty.  A compromise
of the 12z National Blend of Models, 00z Canadian, 06z GFS, and
00z ECMWF is planned for the days 4-7 QPF.  If the 12z GFS proves
useful, it could be included into the QPF mix.

...Weather highlights/Threats...
While many locations across the nation can expect temperatures
near to above climatology, there are a few areas which will be on
the cooler side. An active southwestern U.S. monsoonal circulation
should keep these locations wet given daily thunderstorm chances
with accompanying enhanced cloud cover.  In a general sense,
expected highs should be around 5 degrees below climatology.
Meanwhile, Sunday should prove to be a rather cool day for July
standards over the Northern Rockies into the adjacent Northern
Plains as well as portions of Nebraska. 10 to 15 degree departures
will keep highs in the 70s given cold advection in the wake of the
trough passage.

Regarding rainfall threats, as mentioned, the Four Corners region
will remain wet due to monsoon considerations with activity
gradually reaching the Central Great Basin and circulating into
the Central Rockies. Farther east, widespread convection is
anticipated along the slow moving front which will migrate from
the middle of the nation toward the Eastern Seaboard from Day 4-7,
July 15-18. There is also a signal for enhanced thunderstorm
chances across the southeastern U.S. as high precipitable water
air pools across the region.


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