Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
105 PM EST Sat Dec 14 2019
Valid 12Z Tue Dec 17 2019 - 12Z Sat Dec 21 2019
...Overview and Guidance/Predictability Assessment...
An amplified but progressive pattern will maintain a rather stormy
period for the West Coast while in the east, a leading system will
bring some snow to the Northeast followed by a quieter period
until perhaps next weekend. For the leading system in the East,
the latest model runs show good agreement on surface low placement
as it quickly lifts from the northern Mid-Atlantic Tuesday to off
the Canadian Maritime coast by Wednesday. In the West, timing
differences with an emerging weak closed low/trough into
California starts as early as Wed/day 4. Even though models seem
to be trending faster, there is still enough run-to-run
variability on speed to not latch onto one particular solution at
this point. Amongst the deterministics, the slow 00z ECMWF appears
to be an outlier, with the CMC, UKMET (through day 5), and GFS all
faster especially as the shortwave lifts into the Central Plains
and Great Lakes late next week. However, there is enough ensemble
member support for the slower ECMWF so its hard to rule it out
completely. Given the uncertainty, opted for a weighting towards
the ensemble means which are just a hair faster than the 00z
ECMWF. This approach also fits well with previous WPC continuity.
By late Friday into Saturday, the troughing should shift into the
East with ensembles suggesting another area of low pressure trying
to organize off the Southeast coast late next Saturday. Back to
the West, yet another trough will slowly lurch toward the coast.
...Weather Highlights/Threats...
Lead system on Tuesday will bring a swath of snow to the Northeast
and parts of the Mid-Atlantic as low pressure exits off the coast
early Wednesday. Colder air (highs 5-10 deg below average) will
filter into the area midweek before a moderation next Fri/Sat. In
the West, precipitation will favor far northern California
northward for most of the period, but with some light rain/snow
into coastal NorCal and the Sierra as the upper trough moves
through. By next Fri/Sat, southwesterly upper flow will provide an
atmospheric river connection back to the subtropics near Hawai'i
for far northern CA into southwestern OR. Several inches of rain
are possible per the latest model guidance.
Santorelli/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