Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
938 AM EST Mon Dec 02 2019
Valid 12Z Thu Dec 05 2019 - 12Z Mon Dec 09 2019
...Southern tier system to affect the Lower Mississippi
Valley/Southeast late week...
...More moisture to spread into the West late this week/weekend
and the East by next Monday...
...Pattern Overview and Guidance/Predictability Assessment...
A progressive but fairly amplified flow pattern is being dictated
by a mid-level vortex across central Canada and a longwave trough
moving into the eastern Pacific. Downstream, ridging builds near
the West Coast by early next week which acts to slow down the
progression of a trough moving through the Southwest. The 00z-06z
guidance showed very good agreement into Saturday morning before
the 00z Canadian became more progressive with the trough moving
through the Southwest, which is not supported by the broader
pattern. The pressures, 500 hPa heights, and wind grids were
driven by a compromise of the 00z ECMWF, 06z GFS, 00z Canadian,
00z UKMET, and continuity through Saturday morning before the
UKMET and Canadian are replaced by the 00z bias corrected NAEFS
mean and 00z ECMWF ensemble mean. This led to some slowing of the
trough moving through the Southwest but otherwise maintained
reasonable continuity. For weather, sky cover,
temperatures/extremes, and dew points, used a little bit of the
00z Canadian early. For QPF, tried to tame the 12z NBM and 06z
in-house ensemble bias-corrected QPF for day 7 (next Monday) as
they looked way too wet considering the model spread and the lower
deterministic QPF maxima seen on the 06z GFS, 00z ECMWF, and even
the wet 00z Canadian.
...Weather Highlights/Threats...
Expect relatively light southern Rockies precipitation to taper
off Thursday with the responsible southern tier system bringing a
brief period of locally moderate/isolated heavy rainfall to the
Lower Mississippi Valley and parts of the Southeast Thursday
night-Friday before being suppressed to the south. Moisture ahead
of the eastern Pacific system should reach the West Coast late
this week and then spread inland over the West through the
weekend. Consistent signals from the guidance point to the
greatest rainfall/higher elevation snowfall totals focusing over
favored terrain in the Sierra Nevada and across northern
California, possibly extending into the extreme southwest corner
of Oregon. The Pacific Northwest as well as windward terrain
across the northern two-thirds of the Interior West/Rockies will
see less extreme but still meaningful totals. The upper trough
and leading wavy cold front should reach far enough eastward for
precipitation (mostly rain) to expand over the eastern half of the
country by next Mon. Some activity may be locally heavy if the
orientation and timing of the upper trough allow for sufficient
input of low level Gulf moisture. Earlier in the period expect
one episode of lake effect snow to taper off to the lee of the
eastern Great Lakes on Thu followed by a modest event over the
northern/eastern Great Lakes and New England approximately
Thursday night-Friday night.
Locations near the East Coast should see near to moderately below
normal temperatures from late week into the weekend. New England
may see readings as much as 10-15F below normal Saturday into
early Sunday. Clouds/precipitation associated with the upper
trough progressing inland from the Pacific will spread above
normal morning lows across the West into the weekend while
maintaining moderately below normal highs over the Great
Basin/Southwest. Cool highs will expand into the Rockies by the
start of next week. Meanwhile expect a pronounced warming trend
over the eastern half of the country is expected Sunday into
Monday, with a broad area of 10-20F+ warm anomalies for morning
lows and pockets of 10F+ or so positive anomalies for highs.
Roth/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