Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 PM EDT Fri Aug 30 2019
Valid 12Z Mon Sep 02 2019 - 12Z Fri Sep 06 2019
...Hurricane Dorian forecast to affect Florida and parts of the
Southeast U.S. next week...
...Overview...
Upper pattern next week will feature ridging over the Four
Corners/Southern Plains as well as in the western Atlantic as the
jet stream remains along the U.S./Canadian border. This leaves
Hurricane Dorian to move northward over/near the Florida
peninsula, and eventually along/just inland of the Southeast
coast. In the northern stream, positively-tilted troughing out of
the northeast Pacific into WA/BC will gradually stream eastward in
weakened fashion while additional shortwaves will round the upper
low over Hudson Bay/northern Quebec.
...Guidance Evaluation and Preferences...
Larger than average differences off the Pacific Northwest and
Florida (Dorian), continues in the latest suite of model guidance.
For Dorian, a blend of the 06Z GFS, 00Z Canadian, and 00Z ECMWF
offered a reasonable starting point nearest the 15Z NHC track that
would take Dorian into south Florida then slowly northward into
coastal Georgia/South Carolina. Please consult the latest
information from NHC on Dorian.
In the Pacific Northwest, guidance continues to show variation on
how to take a shortwave or weakly closed upper low inland (i.e.,
at what strength and speed). Trend may be quicker/weaker, but
deterministic runs are showing less than stellar run-to-run
continuity even on days 3-4. The 06Z GFS has slowed down slightly
from its 00z run, more closely resembling the 00z CMC, but the 00z
ECMWF/UKMET both remain slower and deeper with the system. By
midweek, northern stream troughing will wrap through the Great
Lakes/Northeast but again there remains some timing/amplitude
differences in the ensembles. No real trend was noted in the
ensemble systems but it appears this lead trough may swing through
about a day ahead of when Dorian might be picked up. By the end of
the week, height falls may attempt to push into the Pacific
Northwest again as another shortwave digs through western/central
Canada. Models also struggle to resolve the details of this system
as well. The blend for Dorian (GFS/CMC/ECMWF) seemed to work well
for the rest of the CONUS, along with WPC continuity from the
previous shift. Increased the usage of the ensemble means
(ECENS/GEFS) by days 6-7 to help mitigate unresolved details in
the deterministic runs.
...Sensible Weather...
Dorian will bring strong winds, heavy rain/flooding, and storm
surge to parts of Florida into the Southeast per the current
forecast. Daytime maxes would be suppressed by a few degrees along
its track but the larger negative anomalies will be over the
Northern Plains into the Great Lakes Tue-Thu behind a cold front.
Away from Dorian, rainfall coverage should be fairly limited over
the lower 48, with the exception being along/ahead of the northern
tier cold front. Locally strong to severe thunderstorms are
possible along this boundary as well, particularly Monday in the
Northern Plains/Upper Midwest. The Southwest/Four Corners states
may see isolated to scattered rain but with generally light
amounts.
Santorelli/Fracasso
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
Hazards:
- Heavy rain across portions of the Great Lakes and the Upper
Mississippi Valley, Mon-Tue, Sep 2-Sep 3.
- Heavy rain across portions of the Southeast and the
Mid-Atlantic, Mon-Fri, Sep 2-Sep 6.
- Heavy rain across portions of the Northeast, Mon, Sep 2.
- Heavy rain across portions of the Northeast, Wed, Sep 4.
- Severe weather across portions of the Central Plains, the Upper
Mississippi Valley, and the Northern Plains, Mon, Sep 2.
- Flooding possible across portions of the Central Plains, the
Southeast, the Middle Mississippi Valley, and the Southern Plains.
- Flooding occurring or imminent across portions of the Northern
Plains.
- High winds across portions of the Southeast and the
Mid-Atlantic, Mon-Fri, Sep 2-Sep 6.
- High significant wave heights for coastal portions of the
Southeast, Mon-Wed, Sep 2-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