Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Fri Jan 03 2020
Valid 12Z Mon Jan 06 2020 - 12Z Fri Jan 10 2020
...Overview/Guidance Assessment...
The latest model/ensemble consensus displays a progressive mean
pattern aloft with embedded features of varying amplitude
extending from the short range time frame into the medium range
forecast. Then by the later half of next week guidance indicates
development of a large scale west-central U.S. mean trough as one
ridge builds over the Pacific around 140-160W and another builds
off the southeastern U.S.. This pattern is likely to extend well
beyond the end of the period in light of the CPC 8-14 day
forecast. For the medium range period there is reasonable
agreement regarding the mean flow but discrepancies exist for some
important shortwave details--in particular for a system expected
to develop over parts of the East around Tue-Wed.
Regarding this potential system that may reach over or offshore
the Northeast by midweek, the multiple pieces of energy that may
be involved contribute to significant depth/track/timing
differences among model/ensemble solutions, and thus lower than
average confidence for specifics. One influence may be energy
over Lake Superior and Ontario at the start of the period on Mon.
Recent GFS runs have tended to be the slowest and most closed with
the energy though with the 00Z run nudging a little faster.
Upstream energy that provides the primary dynamic support for
surface development involves two streams with various ideas in the
guidance for exact interaction/evolution. As a result models and
ensemble members vary widely for the system's depth and track at
any particular time. An intermediate solution appears best given
the low confidence in any specific piece of guidance.
The next meaningful shortwave in the series should approach the
West Coast on day 4 Tue with most differences generally within the
typical error range into midweek. This energy will likely support
a wavy front crossing the central/east-central U.S. by Thu-Fri
with details requiring more time to resolve. By day 7 Fri there
is a signal in some guidance for additional shortwave energy to
approach the Northwest. Latest GFS runs have been noticeably
stronger than most other solutions (aside from a couple ECMWF runs
from 1-2 days ago). The combination of detail uncertainties over
both areas would recommend an increased tilt toward ensemble means
late in the week.
In order to achieve the desired forecast over areas of interest,
the starting blend used a multi-model approach for about the first
half of the period and then steadily increased ensemble weight
(12Z NAEFS/ECMWF means) so that the means had 70 percent emphasis
by day 7 Fri. GEFS means have been on the weak side versus other
means for the northeastern system and CMC mean input helped to add
some definition for the NAEFS mean.
...Weather Highlights/Threats...
Flow coming into the Northwest will focus highest totals of
rainfall/mountain snow over favored coastal terrain along the
northern half of the West Coast and the Cascades. Some multi-inch
liquid totals for the full five-day period are possible over the
Pacific Northwest. Significant though somewhat less extreme
amounts are possible over the northern Rockies. Expect
lighter/more scattered activity over the southern half of the
West. Cooler than normal highs should prevail over parts of the
Rockies most of the period, with expansion of below normal
anomalies over the West late in the week as mean troughing aloft
becomes established.
Precipitation will likely expand in coverage over the East during
Tue-Wed as developing low pressure heads into or near the
Northeast. Guidance spread thus far has made it difficult to
resolve precise impacts, but some solutions would produce
significant precipitation and a period of strong winds. Precip
type is also uncertain at some locations. Currently the best
potential for at least some snow extends from the Lower Great
Lakes/Ohio Valley/south-central Appalachians into the Northeast.
Areas over the east-central U.S. should see increasing
precipitation by next Thu-Fri as the large scale pattern evolves
toward a west-central U.S. trough aloft with a wavy surface front
reaching the central U.S.. Low level flow from the Gulf may help
to enhance rainfall totals in the warm sector. Wintry weather is
possible over northern latitudes.
A majority of the U.S. east of the Rockies will see above normal
temperatures during the period but with some pockets of below
normal readings such as in the Upper Midwest. Precise evolution
of the midweek Northeast system will influence temperatures as
well.
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