Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1130 AM EDT Sun Aug 25 2019
Valid 12Z Wed Aug 28 2019 - 12Z Sun Sep 01 2019
1530 UTC Update...
Overall, no major changes were made to the ongoing forecast in
this update. A blend of the 00Z ECMWF and 00Z GFS served as a
basis to the forecast through much of the forecast period, with
more emphasis on the GFS, especially during days 3-5. The 00Z GFS
was preferred over the 06Z for a couple reasons. The 00Z GFS track
was very close to the ensemble consensus with respect to the area
of low pressure expected to track off the Eastern Seaboard during
days 3-4 (Wed-Thu), while the 06Z run seemed to race the area of
low pressure northward more quickly than most other guidance.
Additionally, the 06Z GFS was much stronger than consensus with a
northern stream shortwave crossing moving southeastward from the
Gulf of Alaska across western Canada and into the north central
U.S. Thu-Sun, resulting in a relatively deep area of low pressure
that has little support among other guidance or ensemble members.
The most significant change was to slow down the system
approaching the Pacific Northwest next weekend, as a trend toward
a slower solution was noted among a majority of the model and
ensemble guidance. Weighting of ECENS/GEFS ensemble means was
increased during days 6-7 to account for increasing
spread/uncertainty among the guidance.
Ryan
Previous Discussion (issued at 0653 UTC)...
...Expected Weather Pattern...
An amplified upper level flow pattern is expected across much of
North America through much of the medium range forecast period,
with an upper level ridge situated over the Intermountain West, a
broad upper level trough over the Great Lakes and north-central
plains, and an upper ridge near Bermuda. A large upper low
initially over Ontario on Wednesday lifts northward across Hudson
Bay by the weekend with multiple shortwave perturbations pivoting
around it, providing cold frontal passages across the northern
tier states. A northeast Pacific trough approaches the Pacific
Northwest and British Columbia late in the forecast period.
...Model Evaluation...
There is above average model agreement on the synoptic scale
across the continental U.S. through Thursday with the broad trough
over the eastern U.S. and the ridge across the West, with the
UKMET being the least amplified with the trough over the Southeast
U.S. By Friday, more noteworthy model differences become apparent
across the eastern Pacific and West Coast region. The
CMC/ECMWF/UKMET are farther east with the ridge axis compared to
the GFS and its ensemble mean, and this also holds true for the
upstream trough near 140 degrees W Friday morning, and this
continues going forward into the weekend. This difference is also
apparent between the GEFS and EC means, whilst remaining similar
with the eastern U.S. trough.
Given these considerations, a blend of the 12Z ECMWF/UKMET/CMC and
some of the 18Z GFS was used as a forecast starting point for
Wednesday through Friday, with more emphasis on the ECMWF/CMC/EC
mean by the end of the week with their more favored northern
stream solutions, and then more weighting towards the 12Z EC mean
and some of the 18Z GEFS ensemble mean by Saturday and Sunday.
...Sensible Weather...
The cold front approaching the East Coast mid week should have
enough moisture channeling northward ahead of it to support a
swath of showers and thunderstorms from the Southeast U.S. to New
England, with the best prospects for 1 inch or greater rainfall
from the Mid-Atlantic region to Upstate New York. The majority of
this rainfall should exit the East Coast by Friday with some
lingering showers across New England. The airmass behind the
front will be modified quite a bit by the time it reaches the
Eastern U.S., so temperature changes should be rather minimal
behind it.
Attention turns to the central plains and vicinity by Friday and
into the weekend as multiple thunderstorm complexes are likely to
develop near and to the north of a stationary front. Although the
axis of heaviest rainfall remains uncertain at this juncture, the
potential exists for several inches of rainfall across parts of
Kansas and Oklahoma based on some of the latest deterministic
model guidance. Flooding may become an issue if training
convection develops near the front, and also strong to severe
storms.
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. Expect readings to be on the order of 5 to 15 degrees
below late August averages. The opposite should hold true across
the southern plains and extending westward to the Desert
Southwest, where excessive heat could make weather headlines with
highs exceeding 100 degrees and potentially above 110 degrees for
the normally hotter locations.
D. 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