Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EST Sat Dec 28 2019
Valid 12Z Tue Dec 31 2019 - 12Z Sat Jan 04 2020
...Significant winter storm expected to continue Tue across
northern New England...
...Heavy rainfall potential over the Lower Mississippi Valley next
Thursday...
...Overview...
A lead upper low will move along the US/Canadian border Tue/Wed
which will spread a swath of snow through northern New York and
New England. In the West, a deep upper low will swing through Baja
California and old Mexico Wed-Fri before eventually lifting
northeastward. This may support a heavy rainfall potential from
eastern Texas into Louisiana/Arkansas and perhaps farther east
late in the week. In the Pacific Northwest, a couple frontal
passages will bring in modest precipitation to the region.
...Guidance/Predictability Assessment...
Models and ensembles showed good clustering on the first couple
days of the forecast, and a deterministic blend of the 12Z/18Z GFS
and 12Z ECMWF/Canadian/UKMET served as a good starting point. The
quicker GFS runs in the east were offset by a slower progression
in the West/Southwest/Mexico compared to the ECMWF/Canadian/UKMET.
By about next Thursday, the largest differences among the guidance
were over Texas/Mexico with the upper low. The 18Z/12Z GFS were
slower than the UKMET which was slower than the Canadian. The 12Z
ECMWF was quickest of the deterministic models and even among the
quickest 1/5th or so of ensemble members (and quicker than its
previous two runs). Given the strength of the system over Mexico,
was reluctant to believe the quickest solution was plausible. The
ensemble means were just a bit quicker than the GFS and somewhat
like the Canadian, and formed the starting point for that feature
next Thu-Sat as it lifts northeastward. Building ridging over
Florida (500mb height anomalies over +2 sigma near Cuba) should
favor a slower solution as well.
Back to the West, 12Z GFS was not preferred as the ensembles
showed increased troughing into the West with a cold front
progressing eastward and southward. 18Z GFS was a bit lower with
heights but still higher than the ensemble means. The 12Z ECMWF
was more aggressive with height falls so an intermediate solutions
was preferred given the uncertainty leading up to next Sat.
...Weather Highlights/Threats...
Exiting surface low off New England (and trailing parent low over
the Great Lakes) will leave a trail of snow for parts of northern
NY and much of NH/VT into Maine. Temperatures will trend upwards
after the storm departs as heights rise from the west. Focus will
then be on the Pacific Northwest with and incoming front on
Tuesday pushing through the northern Rockies Wednesday and onto
the Plains by early Thursday, where it may be rather
moisture-starved. Highest precipitation will be over coastal areas
and the Cascades with additional snow along the Divide. To the
south, upper low will lift out of Mexico toward the Lower MS
Valley. Cold front will link up with the Gulf warm front on the
tail-end of the departing New England front which may help tap
Gulf moisture and bring in an area modest to locally heavy
rainfall by Thursday. After that, it remains unclear how resilient
this system may be but at least some rainfall will spread eastward
through the Southeast on Friday. By then, another front will
likely enter the Pacific Northwest which will bring in another
round of lowland rain and mountain snows, along with cooler
temperatures behind it.
Fracasso
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