Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
352 PM EDT Wed Aug 07 2019
Valid 12Z Sat Aug 10 2019 - 12Z Wed Aug 14 2019
...Overview and Guidance/Predictability Assessment...
For the period as a whole there is reasonable agreement on a
persistent ridge aloft over/near the southern Plains and
progressive flow across the northern tier states. There is also
good consensus regarding an upper trough/low crossing the
Northwest during the weekend. Beyond those aspects, models and
ensembles are exhibiting considerable spread and/or run-to-run
changes for details across the eastern two-thirds of Canada
(affecting frontal systems over the lower 48), ultimate path of
ejecting Northwest U.S. energy, and evolution of eastern
Pacific/West Coast flow by the latter half of the period.
One significant change in the guidance is toward the idea of
central Canada upper low/trough energy on Sat feeding more
aggressively into the longer-term eastern North America mean
trough. 00-06Z GFS runs are a bit on the fast side with leading
height falls but overall there is a pronounced trend in
model/ensemble guidance toward more energy feeding into the mean
trough than depicted in previous cycles. This change results in a
much faster southeastward progression of a cold front that reaches
the extreme northern Plains by early day 3 Sat, along with
significant adjustments for the leading front over the
central-eastern states.
Meanwhile guidance diverges for the ultimate path of the energy
crossing the Northwest during the weekend. Best clustering of
models/means (albeit only a faint signal in the means) show
continued eastward northern tier U.S. progression within the
westerlies, in contrast to the 00Z ECMWF that pulls it up into
southern Canada and merges it with other nearby energy. The
12Z/06 ECMWF was closer to consensus for this shortwave. This
feature should produce some degree of surface waviness but
confidence is low regarding the specifics.
By the latter half of the period the forecast also becomes
increasingly uncertain over parts of the eastern Pacific and
western U.S., downstream from ridging that rebuilds just
south/southwest of mainland Alaska. The fact that by day 7 Wed
the 00Z GFS (flat flow) and 00Z ECMWF (amplified trough) near the
West Coast are the exact opposite of yesterday's 12Z runs provide
reason enough for low confidence. Teleconnections relative to the
strongest height anomaly centers in the D+8 multi-day means
support some degree of mean troughing near the Pacific Northwest
but possibly not as amplified as the latest ECMWF. Thus by later
in the period prefer to tilt the forecast mostly toward the 00Z
NAEFS/ECMWF means, as the CMC mean has an intermediate degree of
troughing versus the GEFS mean's flat flow. Note that the flat
flow scenario cannot be completely ruled out as it may end up
being a transient state before mean troughing redevelops soon
after day 7.
...Weather Highlights/Threats...
The upper trough/low ejecting from the West Coast will likely
produce episodes of showers and thunderstorms with areas of
locally heavy rainfall during the period--initially over the
Northwest/northern Rockies and then into the Plains and possibly
parts of the East. Beyond the Plains confidence is low regarding
location/intensity of rainfall given guidance spread and
run-to-run variability. Locally heavy convection may also focus
along a wavy front initially extending from the Southeast U.S.
into the central Plains as well as another front pushing southeast
from the northern Plains. Monsoonal convection over the Four
Corners states may decrease in coverage somewhat over the course
of the period. For the five-day period the best signal for
highest rainfall totals exists over parts of the northern/central
Plains but potential exists for locally significant totals at
other locations within the extreme Northwest and eastern half of
the country.
The upper ridge centered over the southern Plains will keep
temperatures 5-10F above normal over the southern states through
at least early next week. Some gradual moderation is possible by
the middle of next week. Greater likelihood for
approaching/reaching daily record values will be for warm lows.
An area of well below normal highs (up to minus 10-15F anomalies)
will accompany the upper trough passing through the Northwest and
then into the northern Plains from Sat into the first half of next
week.
Rausch
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 Middle
Mississippi Valley, the Northern
Rockies, and Interior Washington, Sat-Sun, Aug 10-Aug 11.
- Heavy rain across portions of the Northern Plains, Sun-Mon, Aug
11-Aug 12.
- Heavy rain across portions of the Middle Mississippi Valley, the
Great Lakes, and the Ohio
Valley, Mon-Tue, Aug 12-Aug 13.
- Flooding possible across portions of the Central Plains and the
Middle Mississippi Valley.
- Flooding occurring or imminent across portions of the Northern
Plains.
- Excessive heat across portions of the Lower Mississippi Valley
and the Southern Plains, Sat-Tue,
Aug 10-Aug 13.
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