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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1600Z Jul 17, 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
1200 PM EDT Tue Jul 17 2018

Valid 12Z Fri Jul 20 2018 - 12Z Tue Jul 24 2018

...Dangerous heat wave expected for the southern plains by later
this week...

...Overview and Guidance Evaluation/Preferences...

The latest model guidance and a quick look at hemispheric
teleconnections continues to increase the confidence that a
pattern change will be ongoing at the start of the medium range
period (Friday) across the contiguous U.S.. Models continue to
show an upper-level ridge expanding eastward from the Great
Basin/Southwest to the southern Plains, and amplification of a
trough across the Great Lakes/Ohio Valley and into the central
Appalachians/Mid-Atlantic. Expect a surface low pressure system to
accompany the eastern trough, with a surface frontal boundary
trailing back into the southern Plains. Additional shortwave
energy crossing the Pacific Northwest and western Canada late this
week will drive a cold front eastward into the northern Great
Basin and Rockies, which reaches the Plains and upper Midwest by
early next week.

Models showed very good agreement with respect to the overall
large scale flow through day 7, with any differences mostly
confined to the usual timing/amplitude disagreements with
individual features mainly late in the period. There continues to
be some question with whether or not shortwave energy in the
Pacific Northwest will lower heights again out west, possibly
breaking down the Southwest ridge slightly, as shown by the last
few runs of the GFS. This cycle of the WPC medium range
500/surface progs used a blend of the latest runs of the
deterministic models (including the ECMWF/GFS/UKMET/CMC) for days
3-4, with increasing contributions from the ensemble means
(ECENS/GEFS) by days 5-7 to account for the usual late period
differences with smaller scale features.

...Weather Highlights/Threats...

The frontal system crossing the Great Lakes/Ohio Valley Fri-Sat
will be accompanied by scattered to numerous showers and
thunderstorms, some of which could produce locally heavy rainfall.
A lingering stationary/warm front will also produce convection
across the Southeast. The strongest signal among the guidance for
heavy rainfall continues to be along the immediate Southeast
coastline Fri-Sat, in close proximity to the frontal boundary. The
upper trough is expected to linger across the eastern U.S. into
early next week as the frontal boundary begins to wash out,
keeping showers and thunderstorms a possibility for much of the
East. Farther west, the frontal system crossing the northern
Rockies Fri-Sat will gain access to deeper moisture by the time it
reaches the northern plains Sun-Mon, bringing locally heavy
convection to the central/northern plains and back into the
central Rockies.

The biggest story in the medium range period continues to be the
heat across the southern Plains. With the upper ridge building
overhead and a strong subsidence inversion likely in place,
temperatures will soar well past the century mark for many areas.
High temperatures are expected to be 5 to 15 deg F above average
across much of the southern plains from Fri into next week, with a
number of record high temperatures and record high minimum
temperatures potentially in jeopardy. High dew points will combine
with the hot temperatures to produce dangerous heat index values
110-115 deg F for many locations. Low temperatures near 80 deg for
many areas will add to the potentially hazardous impacts of the


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