Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
157 AM EST Mon Nov 18 2019
Valid 12Z Thu Nov 21 2019 - 12Z Mon Nov 25 2019
...Heavy rain possible later this week for portions of the lower
Mississippi Valley and the Southeast...
...Pattern Overview and Guidance/Predictability Assessment...
Split flow will be present across the CONUS through much of the
medium range. Initially on day 3 (Thu) an upper low is expected to
be in place across the Southwest, with progressive northern stream
flow extending from the North Pacific east into Canada and the
U.S. northern tier. A northern stream shortwave and associated low
pressure system are forecast to move from the Upper Midwest/Great
Lakes on Thu quickly northeast into Quebec by Fri morning, with
the trailing frontal boundary crossing the eastern third of the
CONUS. Models consensus surrounding this feature was quite good,
with only minor timing and intensity differences noted during days
3-4. The evolution and progression of the southwestern upper low
continue to pose the biggest challenges during the medium range.
Model/ensemble consensus surrounding this feature has shown
notable improvement over the past 24-48 hours, but some
significant differences remain. These include the speed of
eastward progression, interaction (or lack thereof) with the
northern stream shortwave, and the resultant impacts on the
development of a frontal wave across the lower Mississippi Valley
by Fri-Sat. While model spread on the timing of the upper low has
reduced somewhat, the GFS has remained on the fast side of the
spread, while the previously slow ECMWF has sped up a bit, now
more closely aligned with a cluster of solutions including the
CMC/UKMET and the ensemble means. Given the current distribution
of ensemble members, opted to lean heavily on the 12Z ECMWF
starting on day 4 (Fri), with a gradual trend toward heavier
weighting of ECENS/NAEFS ensemble means in the forecast through
day 7 (Mon). This solution shows a wave of low pressure developing
along the trailing surface front across the lower Mississippi
Valley Fri/Fri night, moving northeast into the Tennessee Valley
by Sat morning, and into the Mid-Atlantic Sat night-Sun. The
arrival of the 00Z ECMWF shows some degree of consistency with its
solution, although given recent trends, would not be surprise to
see at least some further increase in forward speed in future
model cycles.
Farther west, models/ensembles show agreement on the idea that
upper-level ridging should begin to weaken over the weekend as the
primary center for positive height anomalies shifts well offshore,
allowing an active upper jet across the North Pacific to sag
southward toward the northwestern U.S. Guidance suggests that
shortwave energy traversing this flow should amplify across the
western U.S. by early next week, potentially allowing a cold front
to press into the western states. Models show a large degree of
run-to-run variability around this scenario despite at least some
agreement on the overall evolution. Thus, majority ensemble
weighting by late in the extended period was further warranted.
...Weather Highlights/Threats...
The heavy precipitation event across the Southwest will begin to
wind down by the extended period as the upper low begins to move
east, but some areas of heavy rain and mountain still will still
be possible across portions of the Four Corners region and the
southern Rockies on Thu. Farther east, rain and snow will
accompany the low pressure system crossing the Midwest/Great Lakes
on Thu and the Northeast on Fri. As the aforementioned wave of low
pressure develops along the front across the lower Mississippi
Valley Fri-Sat, a significant area of overrunning precipitation is
expected to develop, with guidance continuing to suggest the
potential for a couple inches of rain in a swath from
Arkansas/northern Louisiana eastward into portions of the
Tennessee Valley and the Southeast/southern Appalachians. As the
low pressure system crosses the Appalachians and approaches the
Eastern Seaboard Sat night-Sun, soaking rains should spread into
portions of the southern Mid-Atlantic region, although faster
forward motion of the system by that time should preclude a
significant heavy rainfall threat. Also by Sun-Mon, as lower
heights and a cold front reach the Pacific Northwest, rain and
mountain snows should begin to return to portions of the Cascades
and coastal ranges.
Below average temperatures are forecast to spread across portion
of the central and eastern U.S. in the wake of the mid/late-week
cold front. Highs are forecast to be 5 to 15 deg F below average
from the Plains Thu, reaching the Midwest on Fri. The air mass
should moderate somewhat as it spreads east, but highs 5 to 10 deg
below average are expected to reach the eastern U.S. over the
weekend.
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