Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
139 AM EST Fri Nov 29 2019
Valid 12Z Mon Dec 02 2019 - 12Z Fri Dec 06 2019
...Significant winter storm to affect the Northeast early in the
week...
...One or more precipitation events likely over the West Coast
states...
...Pattern Overview and Guidance/Predictability Assessment...
A strong upper low forecast to be over the Mid-Atlantic as of
early Mon will progress to the east and then northeast Mon-Tue.
Expect an associated low pressure system to track just offshore
the Northeast and then into the Canadian Maritimes, with
meaningful winter weather effects likely to the north/west of the
low track. Behind this system guidance depicts a tendency for
cyclonic mean flow over the eastern U.S. with the best defined
shortwave likely to pass over the East around midweek. Meanwhile
most solutions suggest that an upper low and surrounding
positively tilted trough energy initially off the West Coast will
consolidate during the first half of the week. This feature
should weaken as it moves inland over the Southwest/southern half
of the Rockies Wed-Thu with continued progression of the shortwave
thereafter. An upstream trough will likely amplify over the
eastern Pacific during the latter half of the week. This feature
should eventually spread moisture into a greater portion of the
West Coast region than the leading one whose precipitation will
focus more over southern areas.
Clustering among the guidance is decent for the storm affecting
the Northeast early in the week. Latest UKMET runs have been a
bit slower than the primary consensus including the GFS/ECMWF/CMC
and the GEFS/ECMWF means. The upstream shortwave reaching the
East by midweek will bring a modest northern Plains front
southeastward into East followed by offshore development whose
track should be far enough east to have limited effect on the East
Coast. There is good agreement in principle on the shortwave but
some details have low predictability for being a few days out in
time.
The system heading into the Southwest Wed into Thu shows decent
agreement in the guidance considering that there are at least a
couple separate pieces of energy involved. Typical timing
differences develop as the weakening feature crosses the southern
tier during the latter half of the week with the GFS runs showing
their faster tendency versus the ECMWF/CMC. The GEFS/ECMWF means
and historical verification suggest the GFS may be too fast but
the mean pattern looks sufficiently progressive for an
intermediate timing to be plausible. Through day 7 Fri the
primary uncertainty with the amplifying eastern Pacific trough is
if/where an embedded closed low may form. Thus far the
operational models are generally suggesting higher potential for a
closed low than the ensemble means, while GFS runs have been
tracking the closed low north/northeast of the 12Z ECMWF and
latest CMC runs. Over multiple runs the GFS has been somewhat
more consistent than the ECMWF with the general evolution. Based
on guidance available through the 18Z cycle a compromise among the
12Z ECMWF/CMC, 18Z GFS, and 18Z GEFS/12Z ECMWF means would provide
a good starting point to represent somewhat more flow separation
than the means without committing to lower predictability closed
low details for the time being.
...Weather Highlights/Threats...
The storm system tracking just offshore the Northeast early in the
week will bring a period of significant wintry precipitation to
areas from the extreme northern Mid-Atlantic into New England.
Winds may be sufficiently strong to make travel even more
difficult/hazardous. Parts of Great Lakes should see one or more
periods of snow but with mostly light accumulations. Over the
West, expect weekend precipitation to linger over California into
Mon followed by a brief break. Then the system moving into the
Southwest will bring an episode of focused precipitation with
highest totals most likely over favored terrain inland from the
southern California coast and a lesser maximum over central
Arizona. Higher elevation snow will be possible from the Sierra
Nevada into the central Rockies. This system may produce an area
of mostly rain over and east of the central-southern Plains late
in the week. The upper trough amplifying over the eastern Pacific
late in the week should spread moisture into the West Coast states
at that time. Timing and exact focus of heaviest precipitation
will be sensitive to lower predictability details within the
trough. Currently the best potential for highest totals exists
over northern California.
The eastern states will be chilly to start the week with some
parts of the South seeing highs 10-15F below normal on Mon.
Eastern U.S. temperatures will gravitate toward normal by the
latter half of the week. On the other hand expect the central
U.S. to see above normal temperatures for most of the period with
some pockets of plus 10F or greater anomalies for min and/or max
readings from Tue onward. The expected pattern will support
moderately below normal highs over the Great Basin and at times
into southern California while most of the West should see morning
lows closer to or above normal.
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