Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
112 PM EST Sat Jan 04 2020
Valid 12Z Tue Jan 07 2020 - 12Z Sat Jan 11 2020
...Heavy rain potential next Fri/Sat from the Lower Mississippi
Valley to the Ohio Valley...
...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 (500mb heigh
anomalies around +2 sigma) 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 00Z/06Z cycles, GFS/ECMWF and their
ensemble means clustered a bit quicker with the surface low near
the Gulf of Maine by Wednesday morning (Canadian a bit slower but
the UKMET well to the southeast). Sensitivity to the
northern/southern stream intereaction may lead to continued
guidance differences as the northern stream source was still over
the Gulf of Alaska. Favored the GFS/ECMWF cluster which was close
to continuity and favored more rather than less interaction.
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 (both speed and amplitude), favoring a blend
approach. Did not incorporate the 00Z Canadian as it was a bit
questionable with how it separates southern energy for a time,
though its latest 12Z run appeared more in line. 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 Canadian and the GEFS/ECMWF ensemble means. Despite
its earlier questionable evolution, the 00Z Canadian was preferred
for its timing of the upper trough (perhaps closed low) by next
Saturday but with below average confidence. This will also affect
the position/latitude of the frontal boundary in the east. A
slower ejection out of Texas would favor a farther southward
progression of the front into or through the Northeast.
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 support
has increased for a robust shortwave with reasonable support from
the ensemble means. GFS runs were a little suspect (too far west)
given the likely too quick progression in the east by then.
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 and ECMWF after Fri favored trending the
forecast to the 06Z GEFS mean/00Z ECMWF ensemble mean/00Z Canadian
into 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
but continued to favor a more defined system for now. 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 generally 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 as depicted in the WPC Winter Weather Outlook
probabilities (~10% chance of low-end snowfall late in the period).
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. East of the central Plains, readings 10-25F above
normal (highest for morning lows) will prevail over a majority of
the eastern half of the lower 48 with highest confidence from the
mid-Atlantic to the Southeast. Some record highs may be possible
in the warm sector.
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