Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
302 PM EDT Mon Sep 02 2019
Valid 12Z Thu Sep 05 2019 - 12Z Mon Sep 09 2019
...Hurricane Dorian forecast to affect parts of the Southeast and
possibly coastal New England this week/weekend...
...16z Update...
No major changes were needed for the day shift medium range
forecast compared to the night shift. The track of Hurricane
Dorian remains similar to previous forecasts, and the 00Z and 06Z
GFS deterministic runs slowed down a bit, which made them more in
line with the National Hurricane Center track and thus usable
through day 4/Fri. Some uncertainty remains on how far west Dorian
may go close to the Eastern Seaboard, but the main surface low
spread in the 00Z EC ensemble is elongated northeast to southwest
just offshore of the Eastern Seaboard on day 4-5, suggesting
uncertainties in timing but not too much in the placement of the
track. By day 5/Sat, 00Z and 06Z GFS runs track Dorian farther
west toward Maine, but the deterministic 00Z EC is farther east
than consensus by day 6/Sun. Thus by day 5 onward, leaned toward
the GEFS and ECMWF means. This worked well elsewhere in the CONUS
too, where troughs in the eastern Pacific/northwest U.S. had
considerable run-to-run variability in the deterministic runs on
day 6-7.
Please see the previous discussion for details about sensible
weather.
Tate
...Overview...
Upper ridging initially over the Four Corners/Southern Plains and
western Atlantic will weaken over the weekend as Hurricane Dorian
lifts north-northeastward near/along the Carolina coasts Thursday
per the National Hurricane Center. Troughing in the northern
stream will tug and then pull Dorian northeastward Friday as
additional height falls push through Ontario. Dorian is forecast
to continue into Canada just east of New England Saturday.
Troughing off the west coast will slowly ease inland Thursday and
be replenished upstream this weekend as a trough settles into the
northern Great Basin.
...Guidance Evaluation and Preferences...
Consensus position around the 12Z ECMWF/UKMET formed a starting
point for Dorian Thursday along the NC/SC coasts per NHC. By late
Friday, it is expected to quickly move away from the Outer Banks
but may still pass just off Cape Cod (near 40N/70W benchmark) on
its way toward Nova Scotia and the North Atlantic. Please consult
the forecasts from the National Hurricane Center for the latest
information on Dorian.
On Thursday, a cold front off the Northeast/mid-Atlantic coast
will hang up atop Dorian and eventually become enveloped with the
system as it progresses through extratropical transition this
weekend. Southern portion of the front may linger over the
Southeast and provide a focus for some isolated showers/storms.
Another cold front will push through the Great Lakes late Friday
into Saturday and then through the Northeast/mid-Atlantic by
Sunday. A blend of the 12Z ECMWF with the GEFS/ECMWF ensemble
means sufficed as a starting point (GFS runs were quicker with
Dorian ahead of NHC). In the west, additional troughing will
approach WA/OR Saturday with typical uncertainty in the
orientation/depth/evolution. The ECMWF/Canadian were more in line
with the ensembles than the 12Z/18Z GFS runs but upstream
uncertainty was higher than average in split-flow.
...Sensible Weather...
Dorian will bring strong winds, heavy rain/flooding, and storm
surge to parts of the Southeast late this week. Effects in New
England, if any, are uncertain due to the dependence on northern
stream flow and inherent timing uncertainty at this lead time. A
cold front will bring cooler temperatures (5-10F below average) to
the Mid-Atlantic and Northeast Thu/Fri. A stronger cold front will
bring fall-like temperatures (10-20F below average) to the High
Plains/Upper Midwest this weekend as many places rise only into
the 60s. In the West, above average temperatures are favored
Thursday ahead of the lead upper trough with a cooling trend
thereafter. The South from Texas eastward will generally see above
average temperatures.
Away from Dorian, rainfall coverage should be fairly limited over
the lower 48, with the exception being in the Pacific Northwest as
the upper trough moves into WA/OR and then into the central Plains
over the weekend as a cold front sinks southward. As the second
upper trough settles into the Great Basin, rainfall may expand
along the spine of the Rockies.
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 coastal regions of the Southeast and the lower
Mid-Atlantic, Thu-Fri, Sep 5-Sep 6.
- Heavy rain across coastal regions of the Northeast, Fri, Sep 6.
- Flooding possible across portions of the Southeast and the lower
Mid-Atlantic.
- Flooding occurring or imminent across portions of the Middle
Mississippi Valley, and the Northern Plains.
- High winds across eastern portions of the Southeast and the
Mid-Atlantic, Thu-Fri, Sep 5-Sep 6.
- High winds across coastal regions of the Northeast, Fri, Sep 6.
- Much above normal temperatures across portions of the Lower
Mississippi Valley and the Southeast, Fri-Sun, Sep 6-Sep 8.
- High significant wave heights for coastal portions of the
Southeast, the Mid-Atlantic, and the Northeast, Thu-Fri, Sep 5-Sep
6.
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