Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
406 PM EDT Thu Aug 08 2019
Valid 12Z Sun Aug 11 2019 - 12Z Thu Aug 15 2019
...16Z Update...
No major changes were needed to the previous forecast. The 06Z GFS
and GEFS seemed to come into better agreement with the model
consensus with a couple of features. First, while there remains
some issue with hints of ridging in the Northeast U.S. on Sun as
described in the previous discussion, the 06Z GFS places the main
upper low in central Canada farther north than its older runs,
which is in better alignment. More significantly, by Wed/Thu the
06Z GFS and GEFS mean develop more troughing than previous runs in
the eastern Pacific approaching the Northwest U.S., aligning more
with the last several runs of the ECMWF as well as the CMC. Given
the ridging centered in the Gulf of Alaska/northeastern Pacific,
it seems prudent to have a higher degree of troughing downstream
over the Northwest U.S. (as teleconnections suggest). So this was
a favorable trend in the GFS/GEFS. Overall did lean away from the
operational GFS early on in the period for continuity and because
of the ridging issue described above, but was able to utilize the
06Z GEFS mean later on. The 00Z ECMWF and ensemble mean as well as
the 00Z CMC and UKMET were incorporated in the forecast as well.
For more details and discussion of sensible weather, please see
the previous discussion.
Tate
...Previous Discussion...
...Overview and Guidance/Predictability Assessment...
Models continue to struggle with the zone of relatively
progressive mid/upper-level flow across southern Canada and the
northern U.S. during the medium range. The GFS continues to be
nearly out of phase with the flow from central Canada to the
Northeast U.S. even at the beginning of the forecast period (Day
3/Sun). The 12Z ECMWF/UKMET/CMC showed upper troughing across New
England Sun morning while the GFS continues to have the trough
weaker and offshore with ridging across New England. Meanwhile,
timing differences with a deep upper low crossing near southern
Hudson Bay at the same time also result in 500-hPa heights that
are nearly out of phase between the GFS and the other mentioned
guidance. Given that the GEFS ensemble mean continued to look at
least somewhat more like the non-GFS solutions in these areas,
decided to steer away from the GFS similar to last night. This
shortwave should move a cold front east across the Midwest/Great
Lakes Sun-Mon and toward the Eastern Seaboard by Tue, with a
trailing wavy frontal boundary extending back into the
Mid-Mississippi Valley and the Southern/Central Plains. Farther
west, model consensus is much improved with a shortwave across the
Pacific Northwest on Sun, which moves east skirting the northern
Rockies on Mon. Models begin to diverge with respect to this
system by Tue as it starts to interact with the stronger westerly
flow across Canada, resulting in increased timing differences as
the shortwave begins to shear eastward.
Given these considerations, the forecast was initially based
heavily on a blend of the 12Z ECMWF/CMC during days 3-4 (Sun-Mon).
During days 5-7 (Tue-Thu), a blend of the ECMWF and ECENS/GEFS
ensemble means was used, with weighting of ensemble means
gradually increasing through time.
...Weather Highlights/Threats...
Multiple areas of showers and thunderstorms are expected to focus
across the Northern/Central Plains and east into portions of the
Midwest Sun-Mon as the polar cold front moves southeast across
these areas and the Pacific shortwave moves east across the
Rockies. Models show some signal for potentially heavy rainfall
across the Northern Plains, especially on Sunday, although given
model differences confidence in the specifics of heavy rain
location is moderate at best. By Tue-Thu, expect showers and
thunderstorms to spread eastward in association with a wave of low
pressure expected to move along a front from the Ohio Valley to
the Mid-Atlantic. As the trailing frontal boundary moves into the
Southeast, extending back into the Southern Plains by next
Wed-Thu, expect showers and storms to increase in coverage across
that region as well. Locally heavy rainfall will be possible in
any of these areas.
Below normal temperatures are expected across the northern Great
Basin and northern Rockies as the upper trough moves overhead
Sun-Mon, with high temperatures forecast to be 5 to 15 deg below
normal. These cool temperatures may also spread east into the
Northern Plains and portions of the Upper Midwest by Mon-Tue.
Farther south, the Southern Plains will remain hot through much of
the period beneath the upper ridge, with the heat spreading east
toward the Southeast as well. Highs are forecast to be 5 to 10 deg
above normal through much of the forecast period across these
areas.
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
Hazards:
- Heavy rain across portions of the Northern Plains, Sun, Aug 11.
- Heavy rain across portions of the Central Plains, the Middle
Mississippi Valley, the Great Lakes,
the Upper Mississippi Valley, and the Northern Plains, Mon, Aug 12.
- Flooding occurring or imminent across portions of the Northern
Plains.
- Flooding likely across portions of the Central Plains, the
Middle Mississippi Valley, and the
Northern Plains.
- Excessive heat across portions of the Central Plains, the Lower
Mississippi Valley, the Tennessee
Valley, the Middle Mississippi Valley, the Southeast, the Southern
Plains, and the Ohio Valley,
Sun-Wed, Aug 11-Aug 14.
- Excessive heat across portions of the Southeast, the
Mid-Atlantic, the Southern Appalachians, and
the Tennessee Valley, Tue-Wed, Aug 13-Aug 14.
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