Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 PM EST Sun Nov 17 2019
Valid 12Z Wed Nov 20 2019 - 12Z Sun Nov 24 2019
...Heavy rain possible over parts of the Southwest and then
south-central U.S. east of the Rockies...
...Pattern Overview and Guidance/Predictability Assessment...
Guidance shows amplified and moderately progressive northern
stream flow persisting across the CONUS from midweek through next
weekend, with a tendency for periods of split flow as energy
separates and cuts off south of the strongest westerlies.
An upper-level trough will exit the eastern U.S. on Wed as a
deepening coastal low moves quickly into or near the Canadian
Maritimes. A second northern stream wave should quickly follow on
its heels, moving a low pressure system and trailing cold front
across the Midwest/Great Lakes Wed night-Thu and southeast
Canada/eastern U.S. by Fri.
Farther west the models and ensembles depict northern stream
energy digging southward along the West Coast as the medium range
period begins, with an upper low possibly closed off by around 12Z
Wed. This upper low should then slowly progress eastward across
the Southwest. This feature continues to be the most significant
forecast problem as solutions have differed considerably over
exactly how far south/west the feature will dig before moving
east, and also on the speed with which it moves eastward. Adding
further complexity is the potential interaction with northern
stream shortwave energy--plus the fact that models/ensembles are
all over the place with the ultimate evolution of upstream
shortwave energy that splits after reaching near 140W longitude
early day 4 Thu (with a compact upper low eventually reaching
close to the West Coast as per 12Z UKMET/CMC, as far west as 140W
as in the 00Z ECMWF, or somewhere between). The GFS has
consistently been quicker to move the Southwest upper low
eastward, with the ECMWF on the slower side of the spread and
latest CMC runs along with ensemble means in the middle. Earlier
CMC runs had represented the western extreme off the southern
California coast. There have now been some converging trends
though, with the 06Z GFS slower than the 00Z GFS and some of
yesterday's runs versus the 00Z ECMWF adjusting somewhat faster
than some of its previous runs. This appears similar to model
behavior over the past month or so... the ECMWF cluster being the
leader in signaling the idea of energy dropping into a closed low,
and then once the GFS comes into better agreement the
slower/closed cluster drifts somewhat faster.
Taking a look at the hemispheric pattern, there appears to be very
little in the way of significant blocking, with largely
progressive flow prevailing from Asia across the North Pacific to
North America. Additionally the teleconnections associated with
persistent positive height anomalies becoming established across
western Canada and/or eastern Pacific favor colder
conditions/lower heights across the eastern half of the CONUS,
suggesting that the flow should have a tendency to remain somewhat
progressive. These considerations and aforementioned model
trends/tendencies favor an intermediate approach for the Southwest
upper low and the ejection of its energy to the east of the
Rockies. This favored timing maintains continuity (albeit with
slightly faster progression) for the idea of a frontal wave
developing near the lower Mississippi Valley by Fri night-Sat and
then moving northeastward thereafter.
Based on the above, the updated forecast incorporated ideas from
the 00Z ECMWF/CMC/UKMET and 06Z GFS early in the period and
started to include modest 06Z GEFS/00Z ECMWF mean input by days
4-5 Thu-Fri. Diverging operational model details required
increasing ensemble mean weight considerably by days 6-7 Sat-Sun.
...Weather Highlights/Threats...
Especially in light of recent trends toward the slower part of the
spread trending faster for the Southwest upper low, expect the
heaviest precipitation over the southern half of the West to be in
the early part of the period (Wed-Thu). Highest totals should be
over favored terrain in Arizona. Snow will be possible in
mountain areas, including some of the highest elevations of
Arizona/New Mexico. Ejection of the upper low's energy beyond the
Rockies late Fri through the weekend will likely reflect at the
surface as a wave of low pressure and bring a band of locally
heavy rainfall through parts of the East, most likely from the
lower half of the Mississippi Valley into parts of the
Southeast/Appalachians/Mid-Atlantic. Much of the West will be
fairly dry behind this system though the northern Pacific
Northwest/Rockies could see a little moisture arrive toward the
end of the weekend.
The mid-late week Midwest/Great Lakes system and trailing front
will bring showers and perhaps a few thunderstorms to the
southern-central Plains and mid-Mississippi Valley with some snow
possible in the northern part of the precipitation shield across
the Upper Great Lakes/interior Northeast. Currently anticipate
highest totals with this system to extend from northern
Missouri/southern Iowa into the southwestern Great Lakes region.
Expect coolest temperature anomalies of 10-15F below normal for
highs in a narrow band from the northern High Plains southward
Wed-Fri, behind a front that drops through the central U.S.. The
upper low over the Southwest during that time will support below
normal highs but above normal morning lows in its vicinity.
Temperatures will moderate over the West next weekend as the upper
low departs and the pattern aloft becomes less amplified. The warm
sector ahead of the initial central U.S. front--which should reach
the East Coast by Fri--will provide a brief period of above normal
readings, with plus 10F or greater anomalies especially for lows.
The late week frontal passage over the East will bring a short
period of modestly below normal highs.
Rausch/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