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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0600Z Jul 19, 2018)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
200 AM EDT Thu Jul 19 2018

Valid 12Z Sun Jul 22 2018 - 12Z Thu Jul 26 2018

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

...Heavy rains possible across portions of the eastern U.S. Sunday
into next week...


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

The upper-level pattern is expected to remain relatively unchanged
through the medium range, with persistent upper ridging from the
Southwest to the southern plains, and the tendency for the
development of deep troughing in the East. Indications remain that
this pattern change will not be short-lived, with significant
hemispheric teleconnections between the evolving flow pattern
across North America and a persistent Rex block across eastern
Asia (which shows no signs of breaking down any time soon).

After wavering in recent days, models have resumed their trend
toward further amplification of the upper trough across the
eastern U.S. Sun into next week, now even suggesting an increasing
probability for some energy to cutoff across the Southeast by next
Tue-Wed. At the higher latitudes, models continue to vary in terms
of timing and intensity with additional shortwave energy crossing
Canada and skirting the U.S. northern tier Sun-Tue, and with the
associated surface low pressure system. The wave then appears set
to undergo additional amplification across the Great Lakes
Wed-Thu, renewing the troughing across the eastern U.S.

In general, consensus among the deterministic guidance was
sufficient to use a multi-model deterministic blend (including the
ECMWF/GFS/UKMET) as a basis for the forecast for days 3-5
(Sun-Tue). The CMC was excluded from the blend as it continues to
be too weak with the southwest/south central U.S. ridge, which in
turn allows heights to fall across the Pacific Northwest as some
energy separates from the higher latitude wave. For days 6-7
(Wed-Thu) weighting of ensemble means (ECENS/NAEFS) was increased
to comprise a slight majority of the forecast. Model solutions
differ with respect to the degree of trough amplification across
the Great Lakes/Ohio Valley by this time period, but a trend
toward ensemble means should best represent the current model
consensus.


...Weather Highlights/Threats...

The amplified upper trough, and associated decaying frontal system
in place across the eastern U.S. at the start of the forecast
period will bring scattered to numerous showers and thunderstorms
to areas from the Southeast and Ohio Valley to the Eastern
Seaboard. The trend toward greater amplification in recent
model/ensemble runs has increased the potential for areas of
heavy/excessive rainfall across areas of the Mid-Atlantic and
Northeast Sun-Thu, with the potential for more significant
northward transport of deep moisture. A number of deterministic
solutions now show the potential for areas of multi-inch rainfall
totals across these areas across multiple days, with a moderate
degree of ensemble support as well. The arrival of a second cold
front and amplifying trough in the East by the middle of next week
will only bring a continuation of the convection and locally heavy
rainfall. Farther west, the cold front trailing the low pressure
system crossing central Canada will bring scattered convection and
locally heavy rainfall to portions of the central/northern plains
Sun-Mon. The trailing end of the front appears likely to stall
from the central plains into the Rockies on Tue, bringing a
continued threat for locally heavy convection.

The other story 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 Sun perhaps into the middle of 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
heat due to increases in cloud cover and isolated convection.


Ryan


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

http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
http://www.wpc.ncep.noaa.gov/heat_index.shtml