Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1150 AM EDT Tue Sep 03 2019
Valid 12Z Fri Sep 06 2019 - 12Z Tue Sep 10 2019
...Hurricane Dorian to possibly affect coastal New England on its
track away from the US...
...Record heat likely for the Lower Mississippi Valley Friday into
this weekend...
...16Z Update...
It remains the case that Dorian may cause some impacts for coastal
New England late in the week. Model spread on the track of Dorian
remains reasonably low, but the timing in the 00Z ECMWF
deterministic run and mean were slower than the NHC track.
Elsewhere, some model spread remains regarding troughs approaching
and progressing through the western U.S. Did lean away from the
00Z UKMET by day 5/Sun as it is more energetic than other
guidance. The general pattern of troughing in the West and ridging
shifting from the Rockies on day 5 to Mississippi Valley on day 6
does appear generally well-handled by the models. However, by day
7/Tue, operational models diverge with the details of embedded
troughs in the western/central U.S., so leaned much more heavily
on the 00Z GEFS/EC means by that point.
For more details, please see the previous discussion below.
Tate
...Overview...
Upper ridging initially over the Four Corners will slide
southeastward through the Lower Mississippi Valley and Southeast
this weekend and into Florida by next week. This will be in
response to Dorian's exit just offshore the New England coast as
troughing settles into the West and then the Plains Sun-Tue.
...Guidance Evaluation and Preferences...
Mix of the 12Z/18Z GFS and 12Z ECMWF (a bit slower) offered a good
starting point for Dorian as it passes into Nova Scotia late
Saturday. 00Z GFS was nearly aligned with the 03Z NHC forecast as
well. Please consult the forecasts from the National Hurricane
Center for the latest information on Dorian.
Fronts hung up around Dorian will eventually become enveloped with
the system as it progresses through extratropical transition. A
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 18Z GFS/12Z ECMWF with the GEFS/ECMWF ensemble means
sufficed as a starting point with relatively good agreement. In
the west, successive troughing will move into the Pac NW through
the period, favored along 120W. 18Z GEFS mean was much closer
(deeper/slower) to the 12Z ECMWF ensemble mean by the weekend
(Canadian ensembles have been much quicker with the overall
pattern as of late) and that blend with their operational
components was used to iron out some timing differences there and
downstream with low pressure moving through the High Plains
Mon-Tue.
...Sensible Weather...
Though Dorian is forecast to stay just offshore Cape Cod, the
interaction with the frontal boundary to the north may allow for
heavier rain to overspread southeastern Massachusetts and the
Islands late this week. A cold front will bring cooler
temperatures (5-10F below average) to the Mid-Atlantic and
Northeast Friday in the wake of Dorian. 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 upper 50 and 60s. The Southern tier of the CONUS from Texas
eastward will generally see well above average temperatures near
100F that are forecast to tie or break record highs. The heat will
stay through the weekend into next week as temperatures stay in
the middle 90s from Texas to Georgia.
Rainfall will spread out of the northern Rockies into the Plains
this weekend as an upper trough moves through Montana. This will
be reinvigorated as the second upper trough and surface front move
out of Montana into the Plains early next week.
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