Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
121 PM 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...
Through the medium range period /Dec 6-10/ the synoptic pattern
will evolve from relative zonal flow across the CONUS to a highly
amplified pattern featuring a narrow ridge along the West Coast
with deepening longwave troughing over much of the central U.S.
and Great Lakes region. At the surface, a West Coast low will drop
through southern California and eventually move across the
southern/central Plains before lifting through the Great Lakes and
Northeast U.S. by the end of the forecast period. Cold Canadian
high pressure settles in behind that system.
For this update, model guidance confidence/agreement remained at
or above average for the large scale pattern. The 00Z CMC
continued to be a fast outlier with the progression of the
shortwave energy across the western/central U.S. while the ECMWF
continued to show a slowing trend (from its 02.12Z to 03.00Z run).
However, the 12Z GFS trended a bit faster with the overall
progression of the system, so by Day 6/7 confidence drops to
average for the time period.
The model blend preference was largely an equal blend of the
deterministic ECMWF/GFS/UKMET for Day 4-5 followed by increasing
weight/percentage of the deterministic ECMWF/GFS and their
respective ensemble means toward Day 7. 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.
...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. The highest probabilities for
heavy rainfall appear to be across the Tennessee Valley,
southern/central Appalachians, and portions of the Ohio Valley.
Temperatures look to be cold enough to support some snowfall to
northern areas from 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/Taylor
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