Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EST Wed Dec 04 2019
Valid 12Z Sat Dec 07 2019 - 12Z Wed Dec 11 2019
...Moisture spreading through the West during the weekend with
heaviest rain/snow over northern California/Sierra Nevada...
...Precipitation to increase in coverage and intensity over the
East early next week...
...Much below normal temperatures settle into the northern
Plains/Upper Midwest next week...
...Pattern Overview and Guidance/Predictability Assessment...
In most respects guidance is consistent and agreeable in showing
an evolution toward fairly deep upper troughing over the
central-eastern part of North America, downstream from a narrow
but very amplified upper ridge that should drift to the West Coast
by next Tue. The one notable trend in the guidance since a couple
days ago--first showing up in yesterday's 00Z cycle--has been
toward a slower and sharper ridge that allows for greater westward
extent of cyclonic flow over the continent. As initial West Coast
energy and amplifying Canadian flow head eastward, expect a cold
front to drop south from Canada during the weekend and a wavy
front to cross the eastern half of the U.S. during the first half
of next week. Toward the end of the period next Wed expect some
approaching eastern Pacific shortwave energy to push into the
western ridge but with models/individual ensemble members
diverging quite rapidly for specifics.
As for guidance details, the CMC continues to be the fast extreme
with the initial West Coast trough starting fairly early in the
period so it was not used in the forecast beyond day 3 Sat. The
UKMET has been on the amplified side with the shortwave as it
reaches the southwestern U.S. and northwestern Mexico. The 12Z
version also had a central U.S. surface evolution that differed
from consensus by day 5 Mon--eliminating that run from the
forecast after day 4 Sun. The new run looks better in that
regard. The 12Z ECMWF was slowest to eject the West Coast trough
energy and multi-day trends in some guidance allowed for partial
inclusion of the ECMWF in the forecast. Faster trend in the new
00Z ECMWF seems to validate the blend approach though. By
mid-late period a combination of the 18Z GFS/12Z ECMWF and their
ensemble means represented the large scale pattern well while
toning down smaller scale details that have lower confidence
several days out in time. Along the West Coast late in the
period, recent trends have favored slower arrival of incoming
Pacific energy but the sharpness and northern extent of the ridge
do favor some undercutting flow to extend into the West around the
end of the period. An average of latest solutions (minus the CMC
that is again fast) appeared reasonable given the detail spread
and lower predictability existing at that time.
...Weather Highlights/Threats...
The system beginning to affect the West Coast late in the week
will spread its rain and higher elevation snow across the West
during the weekend. Expect highest totals with this event to be
focused over northern California/extreme southwest corner of
Oregon and the Sierra Nevada. Favored terrain from the Pacific
Northwest to the northern/central Rockies will likely see
significant but less extreme amounts. Southern parts of the West
may see some lighter activity with greater sensitivity to exact
details of flow aloft for which models are still attempting to
resolve. The West will be relatively dry early next week while
some moisture may begin to reach the West Coast around midweek,
with low confidence in precipitation coverage/intensity.
The eastern half of the country should see a fairly broad area of
precipitation develop early next week ahead of a wavy cold front.
Overall not a lot has changed with respect to forecast details
compared to 24 hours ago. There is still potential for enough low
level Gulf inflow to enhance rainfall over some areas but
important shortwave specifics aloft are small enough in scale to
lower predictability. For the time being the most favored axis
for heaviest rainfall extends from the Tennessee Valley/southern
Appalachians northeastward across the northern Mid-Atlantic and
southern/eastern New England. Locations mainly in the northern
half of the Great Lakes should be cold enough to see snow from a
combination of the synoptic system and lake effect/enhancement.
Some snow will also be possible over northern/interior New England.
East Coast states will see a chilly start to the weekend with
highs in the Northeast up to 10-15F below normal, followed by a
warming trend. Warm flow ahead of the ejecting West Coast system
and upper trough amplifying into the central U.S. will bring well
above normal temperatures (though mainly just for morning lows
over the West) across the lower 48 from west to east during the
period. Some areas may see a day or so with morning lows 10-25F
above normal between Sun and Tue while highs should be a little
more moderate, 10-15F above normal. Cold Canadian air feeding
into the northern tier behind the front will bring readings down
to 10-25F below normal over the northern Plains/Upper Midwest
Mon-Wed.
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