Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 AM EDT Wed Aug 28 2019
Valid 12Z Sat Aug 31 2019 - 12Z Wed Sep 4 2019
...Tropical Storm Dorian forecast to affect Florida and parts of
the Southeast U.S. by this weekend...
...Expected Weather Pattern...
A broad subtropical upper level ridge is expected to persist
across the southern tier of the U.S. with the core of the upper
high expected across the Intermountain West through Monday.
Amplified troughing centered over the Great Lakes and extending
northward to Hudson Bay is expected to lift out some with a
slightly more zonal flow pattern into the medium range. A Pacific
shortwave/upper low approaches the Pacific Northwest by early next
week pushing the ridge axis eastward over the High Plains. In
addition, Dorian will likely have impacts on Florida and parts of
the Southeast U.S. by late this weekend and into Monday.
...Model Evaluation...
The models indicate good synoptic scale agreement at the beginning
of the forecast period on Saturday. However, differences become
apparent by Sunday across the northeast Pacific in regards to the
upper level low approaching the coast, with the CMC becoming more
progressive and then phasing with a northern stream trough,
becoming much more amplified than the model consensus by early
next week across the north central U.S. With respect to the
forecast track of Tropical Storm Dorian, the ECMWF, EC mean, and
CMC appear to be clustered well Monday evening, while the GFS
deviates and lifts the storm much farther up the East Coast and
the UKMET delaying the storm's arrival. Towards the end of the
forecast period midweek, forecast confidence decreases to below
average levels given the big differences noted in the northern
stream flow across southern Canada and how that affects surface
frontal timing and low placement. The GFS is depicting a surface
low Saskatchewan whilst the CMC and ECMWF are depicting more of a
surface high pressure ridge over this same general area Wednesday
morning. As a result of these differences, the EC mean and to a
lesser extent the GEFS mean and 12Z ECMWF were used as a starting
point towards the end of the forecast period.
...Sensible Weather...
Tropical Storm Dorian is expected to make weather headlines by
Sunday and 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 by late Sunday, and this will increase the impacts from
wind, heavy rains, and high surf. Model differences are still
significant, so there will likely be further refinements to the
forecast in future updates. The National Hurricane Center has
additional information pertaining to Dorian.
In the temperature department, a high quality airmass will provide
a hint of fall across much of the north-central U.S. and the Great
Lakes region this weekend as a Canadian surface high governs the
weather pattern, before a moderating trend commences with readings
returning to near or slightly above average levels by early next
week. Expect readings to be on the order of 5 to 15 degrees below
late August averages on Saturday, with the greatest departures
from normal extending from the Dakotas to Kansas. The opposite
should hold true across much of the western third of the U.S.,
with high temperatures running about 5 to 10 degrees above
average. Warmer weather will likely return to much of the East
Coast by midweek with highs slightly above early September
averages.
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
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