Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
306 PM EDT Tue Aug 27 2019
Valid 12Z Fri Aug 30 2019 - 12Z Tue Sep 03 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. 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 into the High Plains. In addition, Dorian will
likely have impacts on Florida and parts of the Southeast U.S. by
this weekend.
...Model Evaluation...
There remains very good agreement between deterministic models and
ensemble means on the overall synoptic scale flow in the medium
range period through day 4. Beyond day 4, a few noteworthy
differences emerge regarding the shortwave near the Pacific
Northwest and its eventual progression inland. The 00z UKMET
(which drops out after day 5) is much faster to pull the shortwave
inland, with the 00z ECMWF also slightly faster than the rest of
the model consensus days 6 and 7 but still within reason. This
cycle of the WPC fronts/pressure progs used a majority blend of
the deterministic runs (06z GFS/00z ECMWF/00z CMC) with previous
WPC continuity for days 3 and 4. Increasing weights of the
ensemble means (GEFS/ECENS) was used day 5-7, with smaller
contributions from the deterministic GFS/ECMWF and previous WPC
continuity.
...Sensible Weather...
Multiple thunderstorm complexes are likely to develop near and to
the north of a stationary front over parts of the Central Plains
on Friday and into early Saturday. Although the exact axis of
heaviest rainfall remains uncertain at this point, the potential
exists for several inches of rainfall across parts of this region.
Flooding may become an issue if training convection develops near
the front, especially within stronger thunderstorms.
The latest track from the National Hurricane Center has Tropical
Storm Dorian crossing the northern Bahama islands on Saturday with
potential landfall along the central Florida coast by Sunday. The
threat for heavy rainfall and gusty winds across Florida by this
weekend is increasing. Heavy rainfall may also extend into parts
of the Gulf Coast and Southeast later this weekend and into early
next week depending on the exact track of Dorian, which remains
highly uncertain at this time. Please refer to the latest
advisories from the National Hurricane Center for additional
information regarding 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 as a Canadian surface high governs the weather
pattern through the weekend, before a moderating trend commences
with readings returning to closer to average levels by early next
week. Expect readings to be on the order of 5 to 15+ degrees below
late August averages, 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. Highs in
the lower 110s are possible across the lower elevations of the
Desert Southwest through Saturday.
Santorelli/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 and Southern Plains,
Fri-Sat, Aug 30-Aug 31.
- Heavy rain across portions of the Southeast, Sun-Tue, Sep 1-Sep
3.
- 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 Southeastern California and
the Southwest, Fri-Sat, Aug 30-Aug 31.
- High winds across portions of northeastern/coastal Florida, Sun,
Sep 1.
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