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Extended Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1536Z Jul 21, 2019)
 
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1136 AM EDT Sun Jul 21 2019

Valid 12Z Wed Jul 24 2019 - 12Z Sun Jul 28 2019

...Overview and Guidance/Predictability Assessment...

Amplified upper-level pattern expected to be in place through much
of the medium range, with persistent upper ridging centered across
the Four Corners region/Southwest and an upper trough,
transitioning to broad cyclonic flow, across the eastern half of
the CONUS. A cold front is expected to be off the Eastern Seaboard
on day 3 (Wed), with the trailing end of the front extending from
the Florida Panhandle/northern Peninsula to South Texas. Models
show consensus that this front should slow or occasionally stall
as a few waves of low pressure move along it through late in the
week, very slowly moving the front further south/east, with the
trailing portion of the boundary gradually washing out. Farther
west, shortwave energy reaching the Pacific Northwest on day 3 is
should move a cold front across the northern Great Basin/northern
Rockies, reaching the northern High Plains by day 4 (Thu). Models
handle this a bit differently compared to yesterday starting on
day 5 (Fri), with the consensus now keeping the draped polar cold
front the dominant feature, with the Pacific front washing out
across the Upper Midwest by day 6 (Sat). Finally, additional
shortwave energy is expected to reach the Pacific Northwest late
next week.

Deterministic solutions showed sufficient consensus during days
3-4 to use a multi-model blend (00Z ECMWF/UKMET/CMC/06Z GFS) as a
forecast starting point. Starting on day 5, timing/amplitude
differences began to grow with the shortwave crossing the Northern
Plains/Upper Midwest, and with the additional shortwave reaching
the Northwest late in the week. With both of these, ensemble means
supported a slower progression than the GFS and even the ECMWF,
somewhat closer to the CMC. Thus, the forecast during days 5-7 was
based heavily on the ECENS/GEFS ensemble means along with a
minority component of the CMC/GEM global.

...Weather Highlights/Threats...

The frontal boundary off the Eastern Seaboard into the Gulf of
Mexico is expected to focus areas of showers and thunderstorms
from the coast of the Carolinas south to Florida. Models suggest
the greatest potential for locally heavy rains will be across the
northern Florida Peninsula Wed-Fri. Showers/storms may lessen in
coverage/intensity by late next week as the surface front washes
out. The shortwave/frontal system moving from the Pacific
Northwest to the Northern Plains/Upper Midwest should bring an
increase in shower/storm chances to the Upper Midwest by late week
and across the Great Lakes next weekend. Meanwhile, daily
scattered storms will be possible across the central/southern
Rockies as some monsoonal moisture manages to spread into the
upper ridge. Below average temperatures (5 to 10 deg F below
average for both max and min temps) are expected across a large
area from the Southern Plains to the Southeast/Mid-Atlantic in the
wake of the cold front moving off the Eastern Seaboard.
Temperatures are expected to gradually moderate back towards
normal next weekend.

Ryan


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

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

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