Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1159 AM EDT Mon Aug 20 2018

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

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

Models and ensembles this morning continue to show very good
agreement on the synoptic pattern evolution across the U.S.
through next Monday.  An upper-level trough is expected to lift
out of the eastern U.S. late this week as an upper-level trough
appears to become more amplified over the Pacific Northwest during
the weekend into Monday.  Discrepancies shown in the model
solutions are relatively small even out to Day 7 next Monday. 
Therefore, a general model compromise was adopted to generate the
WPC medium-range grid fields.  For the sea-level pressure field, a
50-50 blend of the 00Z ECMWF with the 06Z GFS was used for Day 3,
trending toward a 30%-70% blend of their ensemble means to their
deterministic solutions from Days 5 through 7.  This has kept good
continuity with the previous WPC medium-range package.


...Weather Highlights and Hazards...

A lingering stationary frontal boundary along the Southeast U.S.
coast through most of the medium range period 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 the medium range period with high temperatures 5 to 10 deg F
below average before the upper trough lifts out.  But as the upper
ridge expands overhead, a gradual warming trend will follow as
high temperatures 5 to 10 deg F above average are forecast to
expand eastward from the  the central U.S. toward the eastern U.S.
during the weekend.  Finally, height falls across the Northwest
will lead to a cool trend with temperatures falling to more than
10 deg below average in the interior by Sunday into Monday.

Kong/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