Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM EST Tue Dec 03 2019
Valid 12Z Fri Dec 06 2019 - 12Z Tue Dec 10 2019
...Moisture spreading into the West late week-weekend with
heaviest rain/snow over northern California/Sierra Nevada...
...Precipitation to increase in coverage and intensity over the
East early next week...
...Pattern Overview and Guidance/Predictability Assessment...
Guidance continues to show that a narrow but very amplified ridge
aloft moving into the eastern Pacific/northwestern North America,
and eventually reaching the West Coast next week, will encourage a
deepening mean trough to develop over the central U.S. by next
Tue. Meanwhile a deep upper low initially off the West Coast will
likely open up by the weekend with the surrounding trough moving
into the West. Some of the eastern Pacific trough energy will
eventually interact with the amplifying Canadian troughing (which
will push a cold front into the northern tier during the weekend)
while the rest should lag behind across the southwestern states
into northern Mexico.
From the big-picture perspective there was better than average
agreement among most 12Z/18Z models and means through the period.
The one questionable solution was the 12Z CMC which brought the
eastern Pacific trough into the lower 48 at a much faster pace
than consensus (still a bit fast in the new 00Z run but better
than the 12Z version). Thus the updated forecast emphasized the
18Z GFS and 12Z ECMWF/UKMET for about the first half of the period
and then trended toward about even weight among the GFS/ECMWF and
their ensemble means.
Recent trends have favored slower progression of eastern Pacific
trough energy that moves into the southwestern U.S./northern
Mexico, with latest GEFS means/UKMET runs plus a slower adjustment
in the 00Z GFS adding to that theme. Then later in the period
there has been a general slower trend with the wavy front that
reaches the eastern U.S.--which is hard to dispute considering the
amplifying evolution aloft. Ongoing uncertainty over the timing
of incoming Pacific energy and typically low predictability for
important smaller scale details within the overall late-period
CONUS upper trough continue to temper confidence in frontal
specifics mid-late period. Upstream expect some energy to
approach the eastern Pacific/West Coast ridge aloft toward Tue.
Majority cluster of the GEFS/ECMWF means and GFS runs along with
the new 00Z CMC suggest that the 12Z ECMWF may be too quick to
bring the energy into the ridge.
...Weather Highlights/Threats...
The eastern Pacific system moving into the West will spread a
broad shield of moisture across the region from late this week
through the weekend. Guidance continues to highlight areas from
the extreme southwest corner of Oregon through northern California
and the Sierra Nevada for highest rainfall/high elevation snow
totals with multi-inch liquid totals likely over favored terrain.
The Pacific Northwest and locations farther inland through the
Rockies will also see a period of rain/snow but with less extreme
amounts. Much of the West should trend drier by the first part of
next week. Meanwhile expect increasing coverage and intensity of
precipitation over the eastern half of the country from about Sun
night onward as a wavy cold front approaches. Guidance currently
suggests enough low level Gulf inflow to produce at least some
areas of locally heavy rainfall. Best probability for highest
totals extends from near the Tennessee Valley into the
southern-central Appalachians/upper Ohio Valley but confidence in
specifics is not yet very high. Snow potential should be confined
to northern areas including the Great Lakes and northern/interior
New England. Finally, late this week a weakening wave near the
Gulf Coast will produce some light-moderate rain in its vicinity
as it weakens and descends into the Gulf while a vigorous
shortwave aloft and more modest surface low crossing the Great
Lakes/Northeast will produce areas of mostly light snow.
The system crossing the Northeast late this week will bring chilly
temperatures (highs 5-15F below normal) to the East Coast for Sat.
Farther westward, an area of above normal temperatures will make
its way across the country from west to east corresponding to
upper trough progression/amplification into the lower 48, followed
by a cooling trend. One exception to the warmth over the West
early in the period will be modestly below normal highs over the
Great Basin/Southwest. Greatest anomalies over the
central/eastern states from the weekend into early next week
should be for morning lows (plus 10-20F and locally higher) while
warmest highs should be 10-15F above normal. Coldest temperatures
versus normal by next Mon-Tue should settle into/near the Upper
Midwest and vicinity with some readings 10-20F below 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