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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1526Z Jul 18, 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
1126 AM EDT Wed Jul 18 2018

Valid 12Z Sat Jul 21 2018 - 12Z Wed Jul 25 2018

...Dangerous multi-day heat wave expected for the southern

...16Z update...

Minor changes from the overnight package as the models/ensembles
remained in rather good agreement. Refreshed the 500mb and surface
fronts/pressures with the 00Z/06Z guidance, using a deterministic
blend to start (GFS/ECMWF/UKMET/Canadian) trending toward a 50/50
deterministic/ensemble blend by next Wednesday. The 00Z
ECMWF/Canadian forged the best cluster with the ensemble means
across southern Canada early next week as the 00Z/06Z GFS runs
appeared too far north with the sfc low, but were within tolerance
farther south with the frontal timing. Upper low in the east
should linger through the period with no place to go, blocked by
downstream building upper ridging in the Atlantic. The intense
heat centered over Texas will be the biggest threat during the
period with many days near/above record high temperatures (and
record warm minimum temperatures).


Previous discussion below...


Confidence is high that significant upper-level flow amplification
will occur across the CONUS by day 3 (Sat), with a strong and
expansive upper ridge taking hold from the Great Basin to the
southern plains and lower Mississippi Valley, and a deepening
trough axis across the Great Lakes/Ohio Valley. A surface low
pressure system is expected to accompany the amplifying trough
across the eastern third of the country Sat-Mon, before eventually
becoming stationary and washing out. Additional shortwave energy
crossing the Pacific Northwest and western Canada on Sat will
bring a Pacific cold front into the northern Great Basin and
Rockies, reaching the plains by early next week. Models show
general consensus that this wave will undergo additional
amplification Tue-Wed as it reaches the Upper Midwest and Great
Lakes, setting the stage for an amplified trough across the
eastern U.S. once again. Hemispheric teleconnections continue to
suggest that this pattern change will not be short-lived, with a
persistent Rex block across eastern Asia favoring upper ridging
across the southwest and south central U.S., a relatively strong
vortex across the Arctic, and an equatorward extension in the form
of favored troughing across Hudson Bay and the Great Lakes.

Models/ensembles showed good consensus with respect to the large
scale pattern, and differences with respect to smaller scale
features were primarily confined to timing/amplitude through much
of the forecast period. As a result, a multi-model deterministic
blend (including the ECMWF/GFS/UKMET/CMC) served as a basis for
the forecast during days 3-5 (Sat-Mon). The 12Z CMC was too weak
with the southern tier upper ridge relative to consensus by days
4-5, resulting in the amplification of shortwave energy/lowering
of heights across the Pacific Northwest - and this solution was
weighted less relative to the other guidance after day 3 as a
result. During days 6-7, as spread gradually increased among the
guidance, weighting of ensemble means (ECENS/NAEFS) was boosted to
comprise a slight majority of the forecast. Models differ as to
exactly how quickly the upper trough will begin to lift out of the
eastern U.S. ahead of the next system crossing the the northern
tier. Additionally, the ECMWF became a bit faster than consensus
with the progression of the second wave across the Upper Midwest
Tue-Wed, and a solution a bit closer to the GFS and ensemble means
was preferred.

...Weather Highlights/Threats...

Scattered to numerous showers and thunderstorms with locally heavy
rainfall will be possible Sat-Sun from the Great Lakes to the
Southeast and southern Mid-Atlantic region in association with the
amplifying upper trough and surface frontal system. 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 Sat
will gain access to deeper moisture by the time it reaches the
northern plains Sun-Tue, bringing locally heavy convection to
portions of the plains and 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 Sat into next week, with a
number of record high temperatures and record high minimum
temperatures potentially in jeopardy. Relatively high dew points
(near 70 deg F for some areas) will combine with the hot
temperatures to produce dangerous heat index values 110-115 deg F
for many locations. Low temperatures near or even above 80 deg for
many areas will amplify the potentially hazardous impacts of the
heat. The potential arrival of a cold front across the southern
plains by next Tue-Wed could bring some slight relief from the


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