Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1118 AM EDT Sat Aug 24 2019
Valid 12Z Tue Aug 27 2019 - 12Z Sat Aug 31 2019
...Overview and Guidance/Predictability Assessment...
An upper-level ridge axis is forecast to persist along the West
Coast through the medium range, with a broad upper-level trough
downstream of the ridge, across the north central and northeastern
U.S. The trough will be reinforced periodically by shortwaves
passing through an active polar jet from the Gulf of Alaska across
central Canada, toward the Great Lakes and the Northeast. Cold
fronts associated with these waves will bring periodic rounds of
cool and dry polar air into the north central U.S.
Model consensus has improved a bit with respect to an area of low
pressure expected to be a few hundred miles east of the Outer
Banks on day 3 (Tue). The ECMWF has slowed from its previous
faster progression, and the GFS has shifted the system a bit east,
farther away from the coast. The largest differences are now with
respect to whether the models deepen the system or now, with the
00Z ECMWF the most aggressive with this and the GFS/CMC the
weakest. Please refer to outlooks issued by the National Hurricane
Center for more details on this feature. Elsewhere the most
significant differences among the models were with the progression
of a northern stream shortwave initially near the Alaska Panhandle
on Tue, expected to then progress southeastward across
western/central Canada toward the north central U.S. The ECMWF
showed a significant difference from much of the other guidance
for this feature, showing a slower southeastward progression from
Tue onward. Ensemble spread is rather large and perhaps somewhat
multi-modal here, but ensemble means do support something faster
than the 00Z ECMWF, more along the lines of the 00Z UKMET or 06Z
GFS. This eventually results in significant timing differences
among the guidance for a cold front expected to cross the north
central and northeastern U.S. days 5-7 (Thu-Sat). Farther west,
spread was somewhat less with respect to an upper-level trough off
expected to remain off the coast of the Pacific Northwest through
day 7.
Given these considerations, a blend of the 00Z ECMWF/UKMET and 06Z
GFS was used as a forecast starting point for days 3-5, with more
emphasis on the UKMET/GFS solutions by days 4-5 given their more
favored northern stream solutions. By days 6-7, increased northern
stream spread warranted heavy use of 00Z ECENS/06Z GEFS ensemble
means.
...Weather Highlights/Threats...
Areas of showers and thunderstorms will accompany a cold front
across the Great Lakes/Midwest/Ohio Valley/Northeast Tue-Wed, with
moderate to locally heavy rainfall a possibility. Models show a
stronger signal for heavy rainfall potential across the Southern
Plains Tue-Wed along the stationary trailing end of the frontal
boundary, with the front and some areas of convection lingering
through later in the week. Meanwhile, an early fall-like air mass
is expected to persist from the Northern/Central Plains to the
Upper Midwest as multiple cold fronts bring reinforcing rounds of
air originating in Canada. High temperatures are forecast to be 5
to 15 deg F below average across these areas through the forecast
period, into next weekend, with highs in the 60s and 70s for many
locations. Meanwhile, the West Coast and portions of the Great
Basin will remain warm beneath the upper ridge, with highs
expected to be 5 to 15 deg above average.
Ryan
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