Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1135 AM EDT Tue Aug 21 2018
Valid 12Z Fri Aug 24 2018 - 12Z Tue Aug 28 2018
00Z/06Z guidance suite was relatively stable compared to the
previous shift and saw no reason to deviate much. Refreshed the
fronts/isobars with about a 50% weighting of new guidance with 50%
continuity. Previous discussion is below.
...Pattern Overview and Guidance Evaluation/Preferences...
Model guidance continues to show strong support for a
strengthening upper ridge across the northeast Pacific during the
medium range, which supports a downstream mean trough across the
northwestern U.S., and an expanding ridge from the
Lower/Mid-Mississippi Valley to the Mid-Atlantic. Deterministic
solutions showed relatively modest timing/amplitude differences
with several shortwaves traversing the northern tier Fri-Sun, and
a multi-model deterministic blend (including the
ECMWF/GFS/UKMET/CMC) served as a basis for the forecast for days
3-5. By days 6-7 (Mon-Tue), models continue to show some spread as
to the degree of western U.S. trough amplification that will
occur, and also with the downstream ridge amplification across the
Mississippi Valley. Given the degree of flow amplification
expected across the North Pacific, as well as the extratropical
transition of Typhoon Soulik across eastern Asia, would expect a
greater degree of flow amplification to occur across the CONUS as
well. Given the relative increase in model spread by days 6-7,
weighting of ECENS/GEFS means was boosted to a majority of the
forecast during that time period.
...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 across the mid/upper 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 4 to 8 deg F
below average, 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 central U.S.
toward the eastern U.S. during the weekend. Finally, height falls
across the Northwest will lead to a cooling trend with
temperatures falling to more than 10 deg below average in the
interior by Sunday into Monday.
WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found at: