Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1202 PM EST Thu Jan 09 2020
Valid 12Z Sun Jan 12 2020 - 12Z Thu Jan 16 2020
...Significant late week/weekend system affecting the eastern U.S.
to cross the Northeast on Sunday...
17 UTC Update...
A blend of the 00Z ECMWF/CMC/06Z GFS was used as a forecast
starting point initially on days 3-4 (Sun-Mon). Starting on day 5
(Tue) and onward, the GFS was not preferred as it was quite a bit
faster to move a broad upper-level trough from the Pacific
Northwest eastward across the CONUS northern tier. Given an
expected anomalous upper ridge building northward across the north
central Pacific toward the Aleutians, teleconnections would favor
a slower and more amplified system across the western U.S.
Tue-Wed, as shown by the ECMWF/CMC. Additional major differences
were noted by days 6-7 (Wed-Thu) with the GFS flattening the
subtropical ridge across the Southeast quite a bit more than the
ECMWF and the ensemble consensus. Teleconnections associated with
the aforementioned setup across the north central Pacific favor a
persistent/stronger southeastern U.S. ridge, as shown by the
ECMWF. This results in colder air associated with a strong
Canadian surface high expected to nose into the north central U.S.
week remaining a bit more confined to the northern Rockies/Upper
Midwest (whereas the GFS would spread the cold air much farther
south/east by the middle of next week). Thus, during days 5-7,
weight was gradually increased for ECENS and NAEFS ensemble means,
with some continued use of the 00Z ECMWF/CMC given their favored
solutions over the GFS. Overall, changes to forecast continuity
during this update were relatively minor.
Ryan
Previous Discussion (issued at 0700 UTC)...
...Overview/Guidance Assessment...
Latest guidance is still suggesting that during next week the
large scale pattern may begin to evolve away from the amplified
configuration consisting of a western U.S. trough and strong
east-central Pacific/western Atlantic ridges. This will occur as
the initial east-central Pacific ridge collapses in favor a ridge
building into the Bering Sea and mainland Alaska while the western
Atlantic ridge flattens while its center gravitates to near Cuba.
The gradual trend toward flatter flow across the lower 48 will
tend to keep best potential for heaviest eastern U.S.
precipitation totals over the Southeast near a persistent frontal
boundary while a series of fast-moving Pacific systems will
continue to focus the greatest western U.S. activity over the
Pacific Northwest/northwest California and northern Rockies.
Specifics of flow aloft become uncertain by the end of the period
next Thu but moisture should start sagging southward toward
central California. Low heights heights aloft crossing areas near
the western-central U.S.-Canadian border (likely anchored by an
upper low crossing southern Canada) will play a role in the much
below normal temperatures forecast over the northern Plains and
extending into the northern parts of the West to a lesser extreme.
Much above normal temperatures will persist over the eastern U.S.
but with decreasing coverage by next Thu.
Models/ensembles are finally developing a more robust consensus
for the evolution of the system crossing the Northeast on Sun.
This is due to the previously fast/sheared GFS adjusting back to
established consensus from the past day or so indicating an
occluded system over the eastern half of the Great Lakes region as
of 12Z Sun. The 18Z GFS was still a bit fast but close enough to
include as a minority input for the blend but the new 00Z GFS and
GEFS mean have completed the trend to the majority cluster.
Farther upstream the guidance depicts a rapidly moving series of
Pacific shortwaves feeding into and then ejecting from the western
U.S. mean trough aloft. There has been decent clustering for
identifying each feature as it comes into the West Coast but
models/ensembles have had a difficult timing resolving how each
impulse will reflect at the surface to the east of the Rockies.
Thus confidence remains lower than average for specifics over the
central/eastern U.S. during the first half of next week. By late
in the period a more coherent signal seems to arise, toward low
pressure crossing southeast Canada and/or New England (albeit with
some timing spread) and a trailing front reaching the
southern/eastern states.
By next Wed-Thu the main focus over the West will be a fairly
strong storm system forecast to approach the West Coast.
Currently the majority cluster includes the GFS/GEFS mean/CMC/CMC
mean/ECMWF mean which all suggest that the past couple ECMWF runs
may be too fast to bring the system inland. Looking at
larger-scale flow, teleconnections relative to the upstream
ridge's positive height anomaly center suggest that recent GFS
runs may be pulling the core of downstream troughing too far
westward in the D+8 means. This is likely to be a greater issue
after the end of the medium range period so a minority component
of the GFS could still be used as a blend component into next Thu.
While some GFS/CMC runs may have been a bit extreme, other
solutions have been trending toward their general idea of an upper
low over southwest Canada tracking close enough to the U.S. border
to bring fairly low heights across far northern parts of the
western/central U.S. during the period.
The updated forecast consisted of an operational model consensus
early in the period and then transitioned toward increasing
GEFS/ECMWF mean weight, reaching 60 percent by day 7 Thu. Due to
fast timing of the incoming Pacific system late in the period, the
forecast removed the 12Z ECMWF by Thu while maintaining some
GFS/CMC input.
...Weather Highlights/Threats...
The system expected to track across the Northeast on Sun will
bring a potential for meaningful winter weather to northern New
England and some light-moderate rainfall just to the south. The
front trailing from this system will settle over the Southeast
while the farthest southwest part of the front should lift from
the Gulf of Mexico into the southern U.S. as a warm front. This
front and modest Gulf inflow should promote periods of moderate to
locally heavy rainfall over parts of the South, with some repeat
activity possibly helping to enhance totals at some locations.
However confidence is fairly low for specifics of shortwaves
aloft/surface waves so it will take additional time to resolve
coverage and magnitude of heaviest rainfall. Meanwhile at least
one area of light to moderate precipitation (snow over northern
areas) should spread across northern areas of the central/eastern
U.S. but again with lower than average confidence in the details.
The progression of systems affecting the Northwest should provide
the best rain/mountain snow focus over favored terrain in western
Oregon and far northwest California with meaningful precipitation
also over Washington and across the northern Rockies. Central
latitudes of the Interior West/Rockies should see lighter
activity. Moisture may begin to push a little farther south
toward central California around or just after the end of the
period as favored by teleconnections relative to the upper ridge
building into the Bering Sea/Alaska. With cold air pushing
southward from Canada into northern parts of the West, there will
be increased potential for snow at very low elevations (possibly
near or at sea-level) by mid-late week. This is reflected in the
Winter Weather Outlook probabilities valid in that time frame.
The storm system which most guidance has approaching the West
Coast Wed-Thu will have to be monitored closely for the
possibility of significant inland/low elevation snow depending on
track and strength of the cold air push to the north.
Expanding influence of cold air across the northern tier will
bring readings 25-40F below normal over parts of the northern
Plains for multiple days next week while areas over and west of
the northern Rockies may be as cold as 15-30F below normal. It is
close to the climatologically coldest time of year so it may take
more extreme anomalies than these to reach daily record values at
many locations. Warmest anomalies over the East will likely be
for morning lows with fairly broad coverage of readings 20-30F
above normal. Highs of 10-20F above normal will be common with
pockets of warmer anomalies. Arrival of a cold front should begin
a cooling trend over the South and East by next 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