Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
255 PM 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, earlier GFS runs had remained on the fast side
of the spread (but trending slower), while the previously slow
ECMWF has sped up a bit, now more closely aligned with a cluster
of solutions near the 00Z UKMET and most ensemble members. Given
the current trends, opted to split/smooth the timing differences
between the ECMWF and GFS/GEFS camps, with a gradual trend toward
heavier weighting (50%) of 00Z ECENS/06Z GEFS ensemble means by
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 off the Mid-Atlantic coast by early Sun. Trend
has been for a bit quicker progression with the 00Z/06Z ensembles.
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, 50% ensemble weighting
by late in the extended period was warranted given the setup
despite the good agreement between the 06Z GFS and 00Z ECMWF.
...Weather Highlights/Threats...
The heavy precipitation event across the Southwest will 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/southwestern Mid-Atlantic. As the low pressure system
crosses the Appalachians and exits the Eastern Seaboard Sat
night-Sun, modest rains should spread into portions of the
Mid-Atlantic region and New England, 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/Great Lakes 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. Much of the West will see some moderation in
temperatures late this week into the weekend before additional
cooling is possible next week.
Fracasso/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 Lower Mississippi Valley,
Tennessee Valley, and Southern Appalachians, Thu-Fri, Nov 21-Nov
22.
- Flooding occurring or imminent across portions of the Northern
Plains.
- Heavy precipitation across portions of the Alaska Panhandle and
mainland Alaska, Thu-Fri, Nov 21-Nov 22.
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