Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1218 PM EST Sat Nov 30 2019
Valid 12Z Tue Dec 03 2019 - 12Z Sat Dec 07 2019
...Winter storm shifts from New England Tuesday...
...Low pressure brings midweek heavy rain risk to southern
California...
...Pattern Overview and Guidance/Predictability Assessment...
Overnight guidance is in decent agreement for the multi-day long
wave pattern across North America through the next week with
general troughing in the northeast US and troughing off/along the
West Coast and a deep Arctic low over the Canadian Archipelago.
However, the two troughs are transient and detail/timing
uncertainties remain through the Day 3-7 timeframe. The strong low
that crossed the country since Thursday departs New England
Tuesday. Troughing across the northeast CONUS then comes from a
northwest jet from the Canadian Prairies on the south side of the
sprawling Arctic low centered north of Hudson Bay. The first
trough in this pattern reaches the Northeast Wednesday by way of
the Great Lakes. This trough takes on a negative tilt Wednesday
night with the resultant low and potential heavy precipitation
well east of New England. Further shortwave troughs and impulses
then track into the northeast CONUS/Great Lakes through the rest
of the week. Farther west an upper trough/closed low off
California will eject east and into southern California Tuesday
night/Wednesday, weakening as it passes through the longer term
mean ridge and further shearing downstream as it becomes
increasingly embedded in progressive flow as it crosses the
southern tier of the CONUS. An upstream trough will amplify into
the eastern Pacific mid-late week with its axis likely affecting
the West Coast next weekend.
The trough entering the Southwest midweek is the main weather
maker for the CONUS through the medium range period. The GFS
continues to be the fastest and the ECMWF the slowest. However,
the 06Z GEFS and 00Z ECENS means were in decent agreement with
this feature through the forecast period in terms of heights and
precipitation. Therefore the blend was uncharacteristically heavy
on these two ensemble means (at least compared to this past week)
through Days 3 to 7.
...Weather Highlights/Threats...
Wrap around snow looks to lift north from Maine Tuesday. The next
shortwave trough crosses the Great Lakes Tuesday night into
Wednesday with the following northwest flow promoting lake effect
snow off, particularly off Erie and Ontario Wednesday/Wednesday
night. The system moving into the Southwest Tuesday
night/Wednesday midweek should produce a brief period of enhanced
rain and mountain snow 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. Differences exhibited by the
models/ensemble means continue to suggest more uncertainty with
the onset of the event rather than the event itself.
Below normal temperatures will persist over parts along the
Eastern Seaboard and the Southeast into Wednesday with anomalies
of 10 to 15 degrees below normal for Florida. Morning lows could
drop to near freezing as far south as far northern Florida
peninsula on Tuesday. Eastern U.S. temperatures should be closer
to normal late in the week but another frontal passage may push
highs back down to 5 to 10 degrees 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
except under surface high pressure that has a decent signal for
Friday over the northern Plains and Saturday over the Midwest. The
warmest anomalies should be over the northern Plains Tuesday and
Wednesday ahead of a cold front and across the southern tier
Wednesday to Friday with clouds/precip accompanying the system
crossing the region. Highs about 5 to 10 degrees 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.
Jackson
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