Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Thu Dec 05 2019
Valid 12Z Sun Dec 08 2019 - 12Z Thu Dec 12 2019
...Rain/snow spreading through the West this weekend will taper
off early next week...
...Precipitation to increase over the East early next week but
with uncertain specifics...
...Much below normal temperatures settle into the northern
Plains/Upper Midwest early-mid week...
...Pattern Overview and Guidance/Predictability Assessment...
Latest guidance has been showing fairly good agreement with the
large scale flow but with some medium to smaller scale
discrepancies that have significant sensible weather implications.
For the most part a blend approach (more operational model input
early and more ensemble means late) continues to offer the best
way to navigate forecast uncertainty.
From day 3 Sun through the first half of the upcoming week
guidance continues to show strong upper trough amplification over
central-eastern North America in response to a sharp but very
amplified ridge extending from the eastern Pacific/West Coast
north through northwestern North America and into the Arctic. As
the upper trough amplifies, expect a strong cold front dropping
south from the northern Plains to interact with southern U.S.
flow/central Plains low pressure, with strengthening low pressure
likely to track through the Great Lakes into eastern Canada
Mon-Tue while a trailing cold front reaches the eastern states.
By days 6-7 Wed-Thu the eastern trough aloft should become more
shallow as eastern Pacific trough energy filters through the
longer term mean ridge near the West Coast (rebuilding by Thu).
One ongoing uncertainty in the forecast is the ultimate evolution
of trough energy near the West Coast as of early Sun, with
individual models and ensemble members continuing to exhibit wide
spread. Through the 12Z/18Z cycles CMC runs had been on the fast
side while ECMWF runs have tended to be slowest--with the 12Z
ECMWF mean starting to hint at a slower/more separated feature.
GFS runs have been moderately progressive with the GEFS mean
somewhat slower--a trend adopted by the 00Z GFS. The major
revelation in the new 00Z cycle thus far is a complete about-face
by the CMC to the slow scenario, but to add more confusion the new
00Z ECMWF has adjusted faster. At the same time there are
timing/amplitude differences with northern stream flow. These two
issues put together lead to significant discrepancies in
precipitation coverage and intensity/duration over the eastern
U.S., with low confidence in a specific solution.
Later in the period the issues with the initial western energy
still impact the forecast, as exhibited by the the 12Z ECMWF's
Gulf Coast system versus nearly all other guidance that has no
such system. However there are enough 12Z ECMWF members that
suggest at least a more subdued feature (whether from the initial
energy or the second bundle of energy that passes through the West
Coast ridge later in the period) to suggest some southern tier
precipitation potential. One other question mark late in the
period is the orientation of northern tier U.S./southern Canada
flow and influence on temperatures over/near the northern Plains.
Teleconnections relative to the most prominent positive height
anomaly centers in latest D+8 multi-day means (near the West Coast
and over the North Atlantic) favor a pattern more like the
ECMWF/CMC and their means that would lead to a warming trend
versus the GFS/GEFS mean that keep cold air entrenched over the
northern Plains. For now went with an intermediate approach given
the distant time frame and the occasional tendency for some
guidance to displace cold air too quickly.
The early to middle part of the period used various components of
the 18Z GFS/GEFS mean, 12Z and 00Z/04 ECMWF, and 12Z UKMET. The
low-confidence Gulf Coast low in the 12Z ECMWF required phasing it
out of the blend late in the period. By day 7 Thu the forecast
leaned about 2/3 to the GEFS/ECMWF means with a very slight tilt
in the ECMWF/ECMWF mean direction.
...Weather Highlights/Threats...
Expect areas of rain and higher elevation snow over much of the
West on Sun though with a drying trend already in progress over
parts of the Pacific Northwest. Best focus will be over favored
terrain but with amounts beginning to trend lighter than what are
expected during the late short-range period. Much of the West
will see drier conditions early next week. Shortwave energy
reaching the West Coast by Tue-Wed and then flow around the
periphery of a rebuilding Pacific ridge should tend to focus rain
and high elevation snow over the Pacific Northwest and possibly
the northern Rockies by mid-late week.
The forecast for precipitation specifics over the eastern U.S. has
become even more uncertain than in the past couple days. There
are still signals for heavy rainfall potential over some locations
within a broad area from the South northeastward into the
Mid-Atlantic and southern/eastern New England. However latest
guidance has been too diverse and variable with separate important
features aloft to have much confidence in determining where
highest precipitation totals will be and their magnitude. Best
potential for snow remains over the Upper Midwest/Upper Great
Lakes which could see a combination of synoptic snow and lake
effect/enhancement, with lesser probabilities extending into New
England. Lake effect snow should spread across the remainder of
the Great Lakes Tue onward as colder air moves into the region.
During the first half of the period the warm sector ahead of the
front advancing toward the East will lead to a day or so of well
above normal temperatures. Some areas may see plus 15-25F
anomalies for morning lows while plus 5-15F anomalies for highs
should be common. Behind the front expect cold air feeding into
the northern tier to bring readings down to as much as 15-25F
below normal to parts of the northern Plains/Upper Midwest from
Mon onward. In modified form this cold air will continue
southward/eastward with the East seeing temperatures generally
5-15F below normal by Wed-Thu.
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