Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
409 PM EDT Mon Aug 05 2019
Valid 12Z Thu Aug 08 2019 - 12Z Mon Aug 12 2019
...16Z Update...
Overall, no major changes were needed to the medium range suite
for the day shift update. Some differences remain in the newer
model guidance with the shortwave in the northwest to
north-central U.S. mentioned in the previous discussion. The 00Z
ECMWF and even its ensemble mean is faster with this feature than
the recent GFS/GEFS runs (the 06Z deterministic GFS and GEFS mean
are almost perfectly collocated), but may be coming into better
alignment. The shortwave positioning does affect the low position
at the surface, and did back up the low and frontal system in the
Plains westward on Mon compared to the previous forecast. The only
other main problem with the models was that the 00Z CMC
strengthened and dropped an upper low southward into west central
Canada on Fri/Sat, which was an outlier, so excluded that from the
blend.
Tate
...Previous Discussion...
...Overview and Guidance/Predictability Assessment...
An upper low is expected to drift slowly eastward across James Bay
and northeastern Canada through the medium range, with broad
cyclonic flow south of the low across the eastern third of the
CONUS. Several shortwaves will traverse this zone of cyclonic
flow, the most pronounced of which should move a cold front from
the Midwest/Great Lakes to the Eastern Seaboard Thu-Fri (days
3-4), with the trailing end of the front lingering from the
Southern/Central Plains to the Southeast into the weekend. Farther
west, an upper low/trough initially off the West Coast is forecast
to push inland by Sat, moving into the northern Great Basin and
northern Rockies by Sun, and the Northern Plains by Mon.
Meanwhile, broad upper ridging is expected to remain centered
across the south central U.S.
Model consensus through much of the forecast period was sufficient
to use a multi-model blend (12Z ECMWF/UKMET/CMC/18Z GFS) as a
basis for the forecast. The most significant differences arose by
days 6-7 (Sun-Mon) with the shortwave energy moving from the
western into the north central U.S. The ECMWF has consistently
been on the faster side of the guidance with the timing of this
feature by early next week, and the GFS was actually a bit closer
to being a middle-of-the-road solution in terms of timing based on
the ensemble spread. Thus, the forecast was trended heavily toward
ECENS/GEFS ensemble means by days 6-7, with some continued weight
on the GFS as well.
...Weather Highlights/Threats...
Models continue to show a signal for an area of organized
convection with heavy rainfall along the lingering surface front
across portions of the Central Plains and Mid-Mississippi Valley
Thu-Fri, with multi-inch rainfall totals possible. Scattered
showers and storms will also be possible along the northern extent
of the cold front from the Ohio Valley to the Northeast. The
shortwave moving into the western U.S. will bring scattered
showers and storms from the interior Northwest to the northern
Rockies late in the week into the weekend. The system should bring
an increase in convection to portions of the Northern Plains and
Upper Midwest by Sat-Sun. Monsoonal moisture will keep diurnal
showers and thunderstorms active across the Four Corners region
and the southern/central Rockies through the forecast period, with
a daily threat for locally heavy rainfall. The upper ridge will
keep hot conditions in place for the Southern Plains through much
of the period, with high temperatures forecast to be 5 to 10 deg F
above average into early next week. Cooler temperatures will
accompany the Pacific shortwave inland, with highs forecast to be
5 to 10 deg below average from the Pacific Northwest to the
northern Rockies Sat-Sun, spreading into the Northern Plains by
Mon.
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 Central Plains, the Lower
Mississippi Valley, the Tennessee Valley, the Middle Mississippi
Valley, the Southern Plains, and the Ohio Valley, Thu-Fri, Aug
8-Aug 9.
- Heavy rain across portions of the Central Plains, the Middle
Mississippi Valley, the Upper Mississippi Valley, and the Northern
Plains, Sun-Mon, Aug 11-Aug 12.
- Heavy rain across portions of the Northern Plains, Fri, Aug 9.
- Flooding occurring or imminent across portions of the Northern
Plains.
- Flooding likely across portions of the Northern Plains.
- Excessive heat across portions of the Lower Mississippi Valley
and the Southern Plains, Thu-Mon, Aug 8-Aug 12.
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