Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Sat Nov 30 2019
Valid 12Z Tue Dec 03 2019 - 12Z Sat Dec 07 2019
...Significant winter storm to affect the Northeast into Tuesday...
...Low pressure brings midweek heavy rain to southern California...
...Pattern Overview and Guidance/Predictability Assessment...
Latest guidance is showing better than average agreement for the
overall mean pattern during Tue-Sat as well as individual features
in principle, though with some ongoing detail/timing
uncertainties. From a multi-day mean perspective the flow aloft
will have a tendency toward troughing just off the West Coast
while a mean ridge prevails over the west-central U.S. and a mean
trough resides over the East. Features within the eastern mean
trough will include the strong system affecting New England into
Tue before departing, an upstream shortwave reaching the East by
Wed and supporting Atlantic low pressure that should track far
enough offshore to have limited influence on the East Coast, and
then an initial northeastern Pacific shortwave forecast to reach
southwest Canada by Wed and the Northeast by Sat. Farther west an
upper trough/closed low will move into the Southwest around
midweek, weakening as it passes through the longer term mean ridge
and further shearing downstream as it becomes increasingly
embedded in progressive flow. An upstream trough will amplify
into the eastern Pacific mid-late week with its axis likely
approaching the West Coast by Sat.
At the start of the period the system just off the New England
coast exhibits good clustering in general but with some
small-scale differences that would have some influence on New
England but have fairly low predictability three days out in time.
The upper trough reaching the East on Wed still exhibits some
detail differences with corresponding spread for developing low
pressure over the western Atlantic and into the Canadian
Maritimes. The common theme among guidance so far is a low track
sufficiently offshore to keep the system's moisture shield away
from the East Coast states. Consensus shows the next shortwave in
the series bringing a frontal wave into southwest Canada on Wed
with increasing divergence thereafter regarding low track and
strength/shape farther east. Thus far the ensembles and CMC runs
have been much weaker with the surface wave than latest/recent
GFS/ECMWF runs, while the new 00Z UKMET has trended somewhat
deeper/northward versus its prior run. The current array of
guidance argues for a defined wave but with somewhat less vigor
than the GFS/ECMWF until there is better agreement.
Over the past day there has been little change in the guidance
distribution for the system heading into the Southwest around
midweek and continuing eastward thereafter, with operational GFS
runs still faster than other solutions (including the GEFS mean)
to varying degrees. Early in the period the 18Z GFS compared
least favorably with other solutions relative to other recent runs
as it was farthest northeast with the upper low off California.
An average among non-GFS guidance still appears to be the most
likely scenario but the progressive nature of mean flow could
still leave the door open to nudging the dominant cluster a tad
faster at some point. There is still a fair degree of uncertainty
over exact evolution of the surface reflection which should become
increasingly weak/suppressed to the east of the Plains. Within
the larger-scale upper trough amplifying off the West Coast
mid-late week, confidence is steadily increasing that an upper low
will close off for a while per recent operational model runs.
Among the means the 12Z ECMWF mean finally closed off a low and
now the 00Z GEFS mean has done likewise. Potential influence from
energy to the north becomes an uncertainty by day 7 Sat with some
solutions indicating the trough could open up by then. Clustering
and trends favor primarily an operational model emphasis into days
5-6 Thu-Fri followed by a more even weight of models/means for day
7 Sat as detail confidence decreases.
Based on guidance available through the 18Z cycle, the initial
blend started with the 12Z GFS/ECMWF/CMC early-mid period and
began incorporating a very modest 20 percent total input of the
18Z GEFS/12Z ECMWF means on day 5 Thu. Ensemble weight increased
to 30-50 percent by days 6-7 respectively and GFS input switched
from the 12Z run to 18Z run as the 12Z run strayed to the fast
side with height falls reaching the West late in the period.
...Weather Highlights/Threats...
Effects from the early week storm off the Northeast coast will
continue over New England into Tue with Maine seeing the best
potential for meaningful snow. Brisk winds making conditions more
hazardous will likely taper off after Tue. Shortwave energy
immediately upstream may promote a period of lake effect snow,
most likely to the lee of Lake Erie/Lake Ontario in the Wed-Wed
night time frame. The system heading into the Southwest around
midweek should produce a brief period of enhanced precipitation
with highest totals over favored terrain inland from the southern
California coast and then a lesser maximum over central Arizona.
High elevation snow is possible from the Sierra Nevada across the
ranges of the Great Basin to the central Rockies. Expect this
system to produce an area of mostly rain over and east of the
central-southern Plains into the Southeast late in the week. Some
pockets of locally moderate-heavy rain are possible but with low
confidence in timing/location. Late in the period guidance is
maintaining the signal for potentially heavy precipitation (rain
and higher elevation snow) along parts of the West Coast with
strongest focus over northern California and the Sierra Nevada.
So far the detail differences exhibited by the models/ensemble
means seem to suggest more uncertainty with the precise onset of
the event (a little slower in the operational models versus
ensemble means) rather than the event itself.
Chilly temperatures will persist over parts of the East into
Tue-Wed. Expect the greatest anomalies over the Southeast/Florida
where some locations may be 10-15F below normal for max and/or min
readings. Morning lows could drop to near freezing as far south
as far northern Florida. Eastern U.S. temperatures should be
closer to normal late in the week but another frontal passage may
push highs back down to 5-10F below normal over the
Mid-Atlantic/Northeast next Sat. Expect the central U.S. to see
above normal temperatures on most days, more so for morning lows.
Warmest such anomalies should be over the northern Plains Tue-Wed
ahead of a cold front and across the southern tier Wed-Fri with
clouds/precip accompanying the system crossing the region. Highs
about 5-10F below normal should persist over the Great Basin into
parts of Arizona/southern California while most of the West should
see near to above normal morning lows.
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
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