Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
209 AM EDT Tue Sep 10 2019
Valid 12Z Fri Sep 13 2019 - 12Z Tue Sep 17 2019
...Overview and Guidance Evaluation/Preferences...
An upper-level ridge is forecast to remain entrenched from the
lower Mississippi Valley to the Southeast through much of the
medium range. Meanwhile, a stream of relatively vigorous upper
shortwaves are forecast to traverse the CONUS northern tier.
Farther south, an easterly wave is forecast to approach Florida,
and move west into the Gulf of Mexico by the weekend, traversing
the southern periphery of the expansive upper ridge.
Model consensus was relatively good at the start of the medium
range (Fri), and a blend of the 12Z ECMWF/CMC/18Z GFS served as a
good starting point initially for the low pressure system crossing
the Upper Midwest/Great Lakes, and a leading shortwave reaching
the Pacific Northwest on day 3. After that, the GFS has continued
to struggle with the evolution of additional shortwave energy, and
the potential amplification of a large scale upper trough across
the western U.S. days 5-7 (Sun-Tue). Prior runs of the GFS have
continued to show much flatter flow off the Pacific Northwest into
early next week, keeping any significant height falls confined
relatively far north, while the ECMWF and CMC have consistently
amplified a large upper trough (or perhaps even a closed low)
across the northwestern U.S. The 00Z run of the GFS (which was not
available for this forecast package), has finally come around to
the ECMWF/CMC idea, now showing a broad upper trough amplifying
across the western U.S. Sun-Tue. The forecast for days 4-7
(Sat-Tue) was based heavily on a blend of the 12Z ECMWF/CMC, with
gradually increasing weight placed on ensemble means (ECENS/NAEFS)
through time. This solution should bring a strong cold front
across the Great Basin/northern Rockies Sun-Mon, emerging into the
Northern Plains by Mon night-Tue.
...Weather Highlights/Threats...
Scattered showers and thunderstorms will be possible from the
Southern/Central Plains to the Upper Midwest, Ohio Valley, and
Great Lakes Fri-Sat as the surface low pressure system moves from
the north central U.S. into Canada, with the cold front trailing
into the Mississippi/Ohio Valleys and the Southern Plains. Farther
south, showers and thunderstorms are expected to become fairly
widespread across the Gulf Coast states and the Southeast from
late this week into early next week in association with an
easterly wave propagating from the tropics across Florida and into
the Gulf of Mexico. This system may produce localized areas of
heavy rainfall. Meanwhile, the upper trough digging into the
Northwest by Sun-Mon is expected to bring an increase in
precipitation, especially to the mountain ranges of the Pacific
Northwest and northern Rockies. Sufficient cold air may accompany
this upper trough to bring the first round of snow to the higher
peaks across the region. Finally, an increase in monsoonal
moisture across the Southwest Sun-Mon is expected to bring
scattered showers and thunderstorms to portions of the Four
Corners region.
High temperatures are forecast to be 5 to 15 deg F above average
from the Ohio Valley to the Southeast and lower Mississippi Valley
on Fri, with many areas reaching well into the 90s. Some slight
moderation in temperatures is possible by the weekend, but highs
are expected to remain 5 to 10 deg above average beneath the upper
ridge. The upper trough reaching the western U.S. Sun-Mon will
bring much cooler temperatures, with highs 5 to 15 deg below
average expected to overspread the Northwest and northern Great
Basin. Meanwhile, temperatures ahead of the trough will warm
across the Plains and the Upper Midwest, where highs may reach 5
to 15 deg above average by early next week.
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