Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
152 PM 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.
Regarding this potential system that may reach over or just
offshore the Northeast by midweek, the multiple pieces of energy
that may be involved contribute to some depth/track/timing
differences among model/ensemble solutions, and thus somewhat
lower than average confidence for specifics. One influence may be
energy over Lake Superior and Ontario at the start of the period
on Mon with upstream energy and another that provides the primary
dynamic support for surface development. The 00Z and 06Z guidance
have started to show more agreement in the overall evolution with
less spread than earlier runs. A blended solution was preferred,
near the 06Z GFS/00Z ECMWF. This yields a potentially rapidly
deepening area of low pressure over southern New England into the
Gulf of Maine and then New Brunswick/Bay of Fundy on Wednesday.
Elsewhere, a blended solution provided a reasonable starting point
(00Z GFS/ECMWF/Canadian/UKMET).
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. Ensembles show a large amount of timing
spread as the GEFS and ECMWF EPS became out of phase. The
combination of detail uncertainties over both areas would
recommend an increased tilt toward ensemble means late in the week
(about 50%).
...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, but would favor snow to
the north/northwest of the low track it addition to the leading
edge to the northeast. Currently the best potential for light to
modest snow extends from the Lower Great Lakes/Ohio
Valley/south-central Appalachians into the Northeast. Maine would
stand the highest probability of more significant snow.
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 with possible
embedded convection. Wintry weather is possible over northern
latitudes well to the north. Precipitation may become modest to
locally heavy late in the period (next Friday and perhaps
Saturday, just beyond this forecast) as the warm front lifts
through the Southeast.
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 and even into Florida
Wednesday behind a cold front. Precise evolution of the midweek
Northeast system will influence temperatures as well.
Fracasso/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