Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
131 PM EST Sun Dec 15 2019
Valid 12Z Wed Dec 18 2019 - 12Z Sun Dec 22 2019
...Overview and Guidance/Predictability Assessment...
Stormy period for the Pacific Northwest into Northern California
with successive systems moving inland and a possible atmospheric
river event type set-up. The East will be quieter for the end of
the week but a weak system in the Central Plains may become more
developed off the Southeast coast next weekend.
Models and ensembles are starting to show better agreement with
the incoming system into the West on Wednesday and how it travels
across the CONUS through at least day 5. After that, big questions
remain on interactions between northern/southern stream energies.
Recent runs, like the ECMWF, tend to suggest limited seperation of
the energy, while some like the GFS keep the southern stream
seperate with rather significant low development off the Southeast
coast next weekend. Run to run continuity in most of the
deterministic runs is quite poor by day 6-7, though ensemble
members do suggest at least the potential for low development
somewhere off the East Coast. 00z low track plots show East Coast
lows anywhere from Florida to New England at 12z next Sunday.
Given the spread, WPC preferred a mostly ensemble mean blend, with
some contribution from the ECMWF and GFS for a bit more
definition. This resulted in a rather large shift northward with
the surface low to off the Mid-Atlantic coast (which is roughly in
the middle of the spread) on this cycle of the WPC progs, though
it's important to note that the day 7 forecast is of very low
confidence at this point.
...Weather Highlights/Threats...
Precipitation will be centered on the West, particularly coastal
OR and northern California. Upper trough in the eastern Pacific
will allow a fetch of moisture from near Hawai'i to reach the
OR/CA coast with a north/south oscillation between fronts as the
trough changes shape upstream. Several inches of precipitation are
expected but over a 2-3 day period with a likely break in between.
Daytime temperatures will likely be below average with the
rainfall but otherwise near average elsewhere and near to above
average for overnight lows. East of the Rockies, temperatures may
be more variable with a quick cold shot skirting through the
Northeast on Thursday (10s/20s for highs and -0s/0s/10s for lows).
East Coast low development may bring a period of snowfall to
someone from the Mid-Atlantic to the Northeast which of course is
highly variable on actual low track/existence. At this point, WPC
winter weather probabilities show nothing more than a 10% chance
of accumulations along the I-95 corridor.
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