Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
121 AM EDT Mon Aug 20 2018

Valid 12Z Thu Aug 23 2018 - 12Z Mon Aug 27 2018

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

Models/ensembles continue to show good consensus that a pattern
change will occur during the medium range, with a shift away from
the western ridge/eastern trough flow pattern that has dominated
for several weeks. An upper-level ridge is expected to amplify
across the North Pacific, between Alaska and Hawaii, by mid-week.
The downstream effects of this will result in lowering heights
from western Canada into the Pacific Northwest, and gradually
rising heights from the Midwest/Ohio Valley to the Eastern
Seaboard as broad upper ridging expands eastward.

At the start of day 3 (Thu), an upper trough will be in the
process of lifting out of the eastern U.S., with shortwave energy
initially across the interior Northwest moving east across the
northern tier. Consensus surrounding both of these features is
high enough to justify use of a multi-model deterministic blend
(including the ECMWF/GFS/UKMET/CMC) as a starting point for the
forecast during days 3-5. By days 6-7 (Sun-Mon), spread among the
guidance begins to increase rather quickly. Additional shortwave
energy diving southward across British Columbia should begin to
lower heights significantly across the Pacific Northwest, but
models show quite a bit of variance as to exactly how much
amplification occurs. The ECMWF from 12Z Sun was the most
amplified deterministic solution along the West Coast by next Mon,
while the 12Z CMC was the least amplified, with the GFS along with
the ensemble means in the middle. Given the degree of flow
amplification expected to occur across the North Pacific, a more
amplified trough along the West Coast, such as shown by the ECMWF,
is certainly possible. Increased trough amplification along the
West Coast would also likely translate downstream to stronger
ridging across the east-central states, and this bears out among
the deterministic guidance with the ECMWF showing the strongest
upper ridge by day 7 centered over the lower Mississippi Valley.
Given the spread as well as the time frame under consideration
(days 6-7) was not confident enough to bite off on any one
solution. Therefore, went with a majority of ensemble means
(ECENS/NAEFS) in the forecast blend for days 6-7, with continued
minority components of the deterministic ECMWF and GFS.


...Weather Highlights and Hazards...

A lingering stationary frontal boundary along the Southeast U.S.
coast through most of the medium range will result in scattered to
numerous showers and thunderstorms with locally heavy rainfall
possible. Areas of potentially heavy convection will also be
possible from the northern/central plains to the Mississippi
Valley and Upper Midwest through much of the period as a couple
waves of low pressure traverse a frontal boundary across the
region. Convection across the central U.S. may be confined more to
the Midwest and Great Lakes by the weekend as upper-level heights
begin to rise across the central U.S., increasing the likelihood
of a capped environment farther south. Scattered afternoon/evening
showers and storms are also expected to persist through the week
across the Four Corners region, with monsoonal moisture remaining
in place.

Areas from the Mid-Mississippi Valley to the Mid-Atlantic will
begin to the medium range with high temperatures 5 to 10 deg F
below average before the upper trough lifts out. Temperatures
across these areas will gradually moderate by Fri-Sat, however.
Expect highs 5 to 10 deg F above average across much of the
central U.S. by the weekend as the upper ridge expands overhead.
Finally, as heights fall across the Northwest by late week into
the weekend, expect temperatures to fall to below average levels,
with highs by Sun-Mon 5 to 10 deg below average.

Ryan


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