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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1607Z Aug 15, 2018)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1207 PM EDT Wed Aug 15 2018

Valid 12Z Sat Aug 18 2018 - 12Z Wed Aug 22 2018

...Pattern Overview and Preferences...

The guidance continues to agree on an amplification of the large
scale pattern toward a central/eastern North America trough, and a
western U.S./Canada ridge. This evolution is in response to a deep
upstream upper low/trough which develops south of Alaska.
Initially, the medium range period starts out with an elongated
trough across the Eastern U.S., kicking a surface low out of New
England, and a trailing front dropping through the Mid-Atlantic.
Meanwhile, a shortwave aloft emerges into the High Plains this
weekend and spins up a defined surface wave over the central U.S.
which lifts into the upper Great Lakes by Day 6/Tuesday. Ridging
builds through the period over the Western U.S. with possible weak
shortwave energy streaking through.

While there is relatively good agreement through the entire period
on the overall synoptic setup, there remains questions with
individual details, particularly regarding the well defined
surface low lifting through the north-central states. The 00z
ECMWF continues to advertise the strongest/deepest solution (a
995mb low over the upper Great Lakes on Day 6), one of which, if
it were to verify, could be record low pressure for this region.
The remainder of the guidance is varying degrees weaker, with the
06z GFS coming in the flatest/weakest. Until a more clear path is
paved here, the preference continues to be a general model blend
between the latest GFS/ECMWF/CMC, which results in something in
between the two extremes and a solution very close to that of the
latest ensemble mean guidance and WPC continuity.

Out west, the guidance initially centers the upper ridge over the
Southwest, with likely building into the rest of the Western
states as high amplitude ridging drifts inland from the east
Pacific. There is some question in the details of a shortwave
crossing through the Pacific Northwest in the middle of the
period, with the latest run of the deterministic CMC advertising a
deep closed low centered over the region by day 7. The
deterministic GFS/ECMWF and their ensemble means are much more
modest with this shortwave racing it eastward into the northern
Intermountain West and sort of becoming lost/absorbed within the
amplifying ridge over the East. Thus the preference for this
system was a non-CMC blend, but given the spread in individual
ensemble solutions, the evolution of this remains in question. 

...Weather Highlights and Hazards...

Showers/storms across the Northeast at the end of this week move
offshore by the weekend, but should continue across the
Carolinas/Tennessee Valley as the trailing front eventually lifts
northward as a warm front. Shortwave energy that likely reaches
the Plains on Sunday will provide focus for organized convection
and locally heavy rainfall from the High Plains to the
mid-Mississippi Valley this weekend. Low pressure moving northeast
from the central Plains/MS Valley towards the Great Lakes brings
with it likely showers and storms, with precip also likely along
the trailing cold front moving through the lower
Lakes/Appalachians/Southeast by the end of the period. Monsoonal
moisture into the Southwest drives diurnally driven showers and
storms across the region and into the Rockies, with the best
chance for highest rainfall totals over parts of Arizona/New
Mexico due to greater available moisture.

The large scale troughing in the central U.S. will bring an
expanding area of below normal temperatures, with the core of the
coolest highs (10 to 15 degrees below normal) centered over the
north-central Plains Sunday-Monday. Meanwhile, temperatures over
the Interior West should trend gradually warmer with time as the
ridge aloft builds, with daytime highs of 5 to 10+ degrees above
normal possible.


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