Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
326 PM EDT Wed Aug 28 2019
Valid 12Z Sat Aug 31 2019 - 12Z Wed Sep 04 2019
...Hurricane Dorian forecast to affect Florida and parts of the
Southeast U.S. by this weekend/early next week...
...Expected Weather Pattern...
A broad subtropical upper level ridge will likely persist across
the southern tier of the U.S. with the core of the upper high
settling over the Four Corners region during the weekend and early
next week, then drifting into the Plains. A modest upper trough
will progress from the Midwest through the Northeast early-mid
period. Another trough may amplify into eastern
Canada/northeastern quadrant of the CONUS by midweek in response
to an upstream amplified pattern highlighted by a strong Bering
Sea storm around Sun-Mon. Meanwhile a Pacific shortwave/upper low
will approach the Pacific Northwest by early next week and help to
nudge the Four Corners ridge eastward. The Southeast/Florida will
need to monitor Dorian for potential impacts by late this weekend
and into next week.
...Model Evaluation...
The two features with the most notable spread/continuity changes
are Dorian near the southeastern coast and the upper low/trough
drifting toward the Pacific Northwest. For Dorian the past couple
GFS runs have adjusted significantly to the right (though the 06Z
run has trended slower with northward progression than the 00Z run
Mon onward) while other guidance maintains varying degrees of a
farther south track than GFS runs. Complicating factors for
Dorian's forecast include specifics of the upper trough tracking
out of the Midwest and then a general weakness aloft which most
guidance suggests will develop over the southeastern states. For
the upper low/trough nearing the Pacific Northwest, the primary
trend since yesterday is toward slower progression in the
ECMWF/GFS. The slower trend shows up in GEFS mean while the ECMWF
mean has been fairly stable over the past 24-36 hours. In
contrast the CMC has a very progressive/phased solution and
individual ensemble members vary considerably as well. Thus it
would not be surprising to see further adjustments in future runs.
Toward the end of the period it appears best to put primary
emphasis on the ensemble means as the ECMWF/GFS diverge, the
00-06Z GFS keeping the upper low along/offshore the northern
California coast and the 00Z ECMWF ejecting it inland over the
Northwest. The updated forecast started with a blend of
operational guidance (majority 06Z GFS/00Z ECMWF) early in the
period, started incorporating some input from the 06Z GEFS/00Z
ECMWF means by day 5 Mon, and then reached 80 percent means by day
7 Wed. This blend also accounted for spread observed for features
affecting southern Canada and into northern U.S. east of the
Rockies.
...Sensible Weather...
As of this afternoon the National Hurricane Center has upgraded
Dorian to hurricane status. Dorian will likely make weather
headlines by late weekend into early next week across Florida and
coastal portions of the Southeast. The official NHC forecast
track depicts the storm tracking in the general direction of the
Florida Peninsula into early Monday and this will increase the
impacts from wind, heavy rains, and high surf. Model differences
are still significant so expect further refinements to the
forecast in future updates. Consult latest products from NHC for
additional information pertaining to Dorian. Elsewhere, a band of
moisture to the north of a mid-latitude front may bring rainfall
from parts of the Midwest into New England during the first half
of the period. Low pressure crossing the extreme northern Plains
may be accompanied by some rain early next week. Scattered
convection will be possible over parts of the southern half of the
West with specifics becoming sensitive to uncertain details of
flow aloft along the West Coast by the middle of next week.
In the temperature department the northern-central Plains into
Northeast should see below normal readings during the weekend
followed by a moderating trend. Some locations from the
north-central Plains into southwest Great Lakes may see minus
10-15F anomalies for highs on Sat. Expect another push of cool
air to reach the northern-central Plains next Tue-Wed. On the
other hand much of the western third of the U.S. will see max/min
temperatures running about 5-10F degrees above average with some
localized pockets of plus 10-15F anomalies. Warmer weather will
likely return to areas near the East Coast by midweek with highs
slightly above early September averages.
Rausch/Hamrick
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
- Heavy rain across portions of the Central Plains to the Middle
Mississippi Valley, Sat, Aug 31.
- Heavy rain across portions of Florida and the Southeast,
Sat-Wed, Aug 31-Sep 4.
- Flooding possible across portions of the Central Plains.
- Flooding occurring or imminent across portions of the Central
Plains and the Northern Plains.
- Excessive heat across portions of the Southwest, Sat, Aug 31.
- High winds across portions of the Southeast, Sun-Wed, Sep 1-Sep
4.
- High significant wave heights for coastal portions of the
Southeast, Sun-Wed, Sep 1-Sep 4.
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