Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
305 PM EDT Thu Aug 29 2019
Valid 12Z Sun Sep 01 2019 - 12Z Thu Sep 05 2019
...Hurricane Dorian forecast to affect Florida and parts of the
Southeast U.S. by this weekend/early next week...
...Expected Weather Pattern...
The large scale pattern will feature an upper level ridge
generally centered over the Four Corners region while broad
troughing should prevail over much of southern Canada and northern
tier U.S. Resolving individual shortwaves within the northern
stream flow, including a trough/upper low initially off the
Pacific Northwest coast/Vancouver Island, continues to provide a
challenge for guidance. This shortwave uncertainty affects the
forecast of a surface wave which a majority of guidance shows
tracking across the northern Plains early next week and then
continuing across the Upper Great Lakes into eastern Canada.
Meanwhile an upper ridge initially over the western Atlantic
should direct Hurricane Dorian on a track toward Florida early
next week, with a subsequent path most likely into the Southeast.
...Model Evaluation...
Broadly speaking, except for the start of the period the 06Z
GEFS/00Z ECMWF means provide the best overall template for the
forecast as most operational models offer varying
differences/continuity changes. From fairly early on the 00-06Z
GFS are more aggressive to bring height falls across the
northeastern Pacific/Gulf of Alaska versus most other guidance.
This causes the GFS to be quicker than consensus to eject the
trough/upper low initially off the Pacific Northwest coast.
Regarding this feature the GEFS means are slower than the
operational runs and in particular the 06Z GEFS mean has nudged a
bit slower than its 00Z run to line up well with the ECMWF mean.
As it is there is a somewhat faster trend versus yesterday--in
contrast to the slower trend noted from 24 hours prior. Meanwhile
the 00Z ECMWF is broader than most other guidance (including the
means) with the southern Canada shortwave that provides the main
upper support for the surface system expected to track across the
northern tier U.S. into eastern Canada. For Dorian varying
degrees of compromise between the 00Z ECMWF and 06Z GFS provided
the best approximation of the latest National Hurricane center
track, though the ECMWF strays eastward late in the period. Based
on forecast considerations the updated forecast started with an
operational model blend on day 3 Sun, trended to half models/means
by day 5 Tue and mostly means thereafter.
...Sensible Weather...
The main weather story will be Hurricane Dorian and its eventual
track toward/into Florida and the Southeast U.S. from late weekend
into next week. Expect Dorian to bring heavy rain and strong
winds, along with storm surge near the coast. Consult National
Hurricane Center products for the latest information regarding
Dorian. Away from Dorian rainfall coverage should be fairly
limited over the lower 48. Exceptions will be over the Northeast
which may see a period of rain early in the week along with the
northern tier states which should see an episode of focused
rainfall with the system crossing the region. The Southwest/Four
Corners states may see isolated to scattered rain but with
generally light amounts.
Expect highs to be 5-15F above normal over the Interior
West/Rockies Sun-Mon followed by a slight cooling trend. The
Northwest should see plus 5-10F anomalies for highs by Tue-Thu.
Much of the West will likely see morning lows 5-15F above normal
through the period. Areas from the northern-central Plains into
the Northeast will see modestly below normal temperatures on Sun
followed by a warming trend. Another front will bring readings
back down to 5-10F below normal over the northern Plains on Tue
with cool air then spreading as far south as the central Plains
and east into the Great Lakes/Northeast.
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
Hazards:
- Heavy rain across portions of the Great Lakes, the Upper
Mississippi Valley, and the Northern Plains, Mon, Sep 2.
- Heavy rain across portions of the Great Lakes and the Upper
Mississippi Valley, Thu, Sep 5.
- Heavy rain across portions of the Southeast and the
Mid-Atlantic, Sun-Thu, Sep 1-Sep 5.
- Heavy rain across portions of the Mid-Atlantic, the Northeast,
and the Great Lakes, Sun, Sep 1.
- Flooding possible across portions of the Central Plains and the
Middle Mississippi Valley.
- Flooding occurring or imminent across portions of the Central
Plains, the Middle Mississippi Valley, the Southern Plains, and
the Northern Plains.
- Excessive heat across portions of California and the Southwest,
Sun, Sep 1.
- High winds across portions of the Southeast, Sun-Thu, Sep 1-Sep
5.
- High significant wave heights for coastal portions of the
Southeast, Sun-Tue, Sep 1-Sep 3.
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