Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Sat Jan 04 2020
Valid 12Z Tue Jan 07 2020 - 12Z Sat Jan 11 2020
...Overview/Guidance Assessment...
Today's guidance still shows a transition from a progressive
pattern to a longer term amplified western/west-central U.S. mean
trough aloft. A Pacific upper ridge between 140-160W and another
ridge off the southeastern U.S. will help to define this trough.
Climate Prediction Center 8-14 day forecasts indicate this pattern
should persist well past the end of the medium range time frame.
Expect various regions of the lower 48 to see active weather
before and while the amplified pattern takes shape, but with
enough model and ensemble spread to temper confidence in specifics
of each potential significant event.
Based on guidance through the 12Z/18Z cycles, there had been
healthy convergence regarding the developing storm system that may
affect the Northeast around midweek, with guidance trends
generally leaning in the direction of an ECMWF/ECMWF mean
multi-run average. Consensus blend yielded a surface low position
near the southern tip of Maine as of 12Z Wed. Recent GEFS means
have tended to be on the slow/weak side of the guidance spread.
However new 00Z runs highlight the low predictability given the
two streams of energy involved. While the GFS/CMC (with the GEFS
mean adjusting closer to consensus) are not far from continuity, a
couple operational models including the UKMET are almost a
complete miss for New England due to a much wider track for the
wave that tracks over the Mid-Atlantic on Tue. The ECMWF is
adjusting a little eastward from its previous run though not to
the extent of the UKMET.
Moderate detail spread exists for the Pacific shortwave that
reaches the West Tue-Tue night. Into day 5 Thu most detail/timing
differences are generally within the typical error range for the
forecast time frame, favoring a blend approach, though the new 00Z
CMC is bit questionable with how it separates southern energy for
a time. Later in the period GFS runs become questionably
progressive with ejecting western energy--to the point of bringing
height falls into a strong positive height anomaly center which
D+8 multi-day means show near the East Coast. Greater
west/northwest deflection of this energy appears more reasonable
as reflected by the GEFS mean along with the ECMWF/CMC and their
respective means.
Farther upstream guidance is starting to show more of a signal for
a fairly strong bundle of energy to travel around the top of the
Pacific ridge around Thu and then drop southeastward toward the
large scale mean trough. Some GFS/ECMWF runs had hinted at this
in recent days though generally not in the same cycle, but now the
GFS/ECMWF/CMC all have a robust shortwave with reasonable support
from the ensemble means. Latest GFS runs hold the energy a little
westward of most other solutions late in the period--a little
extreme to include in the deterministic manual forecast but not to
the point of being outside the realm of possibility. There is
still considerable spread for surface low track with individual
solutions ranging between western Canada and the Pacific
Northwest. Overall an ensemble mean average provides a reasonable
intermediate track.
Forecast preferences led to primary emphasis on an operational
model blend for the first half of the period. Question marks in
the GFS after Thu favored trending the forecast to the 12Z
ECMWF/ECMWF mean/CMC and 18Z GEFS mean through day 7 Sat.
...Weather Highlights/Threats...
Guidance is consistent in advertising an active pattern for the
Northwest during the period with coastal areas from the northwest
corner of California through Oregon/Washington as well as the
Cascades seeing highest five-day rainfall/mountain snow totals.
The best surges of moisture are likely to occur early in the
period and then again late. Depending on surface low specifics,
some areas may see a period of strong winds as well. The northern
Rockies will also see significant precipitation but with somewhat
lower totals than areas to the west. By Sat some activity should
reach into the Sierra Nevada. Other areas in the West may see one
or more periods of lighter and more scattered precipitation.
The spectrum of possible outcomes for the midweek Northeast storm
is unusually wide for a forecast only four days out, ranging from
a minimal event to a significant snowstorm with strong winds in
its wake. Important details aloft are subtle enough to make it
difficult to determine a "most confident" solution at this time.
At least from the probabilistic/ensemble perspective there has
been some consistency in the idea that best potential for
meaningful snow will extend from the Ohio Valley/Appalachians
through the extreme northern Mid-Atlantic into the Northeast, with
highest snowfall totals over Maine.
As the amplified upper pattern takes shape, locations within and
near the lower half of the Mississippi Valley as well as Tennessee
Valley and Lower Ohio Valley should see increasing coverage and
intensity of rainfall late this week as a wavy cold front
approaches from the west and a warm front lifts northward from the
Gulf Coast. Low level flow from the Gulf may help to enhance
rainfall totals in the warm sector with possible embedded
convection. The primary question mark is how much eastward
progress the cold front will make by Fri-Sat, with the slower
timing in the non-GFS consensus leading to higher rainfall totals.
This event will have to be monitored given the recent heavy
rainfall over/near this region. Some wintry weather is possible
over northern latitudes but most likely with fairly light amounts.
By Thu-Sat expect a pronounced contrast in temperature anomalies
over the country, with fairly broad coverage of highs 5-15F below
normal over northern portions of the Plains and through a majority
of the West while readings 10-25F above normal (highest for
morning lows) prevail over a majority of the eastern half of the
lower 48.
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