Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
134 AM EST Mon Dec 16 2019
Valid 12Z Thu Dec 19 2019 - 12Z Mon Dec 23 2019
...Atmospheric River for part of the West Coast later this week...
...Overview...
Upper troughing south of the Gulf of Alaska on Thursday will
slowly move eastward as undercutting waves send periods of rain
and mountain snow to the Pacific Northwest southward to Northern
California through the weekend. Northern stream across the
US/Canadian border will remain quasi-zonal with a drier WNW flow
and little precipitation east of the Rockies and north of the
Mid-Mississippi and Tennessee Valleys. A somewhat unusual system
in the southern stream may dip through the Gulf/Florida this
weekend with an uncertain future by next Monday.
...Guidance/Predictability Assessment...
For the West, consensus approach sufficed to deal with some
timing/trajectory uncertainties, with the exception of the 12Z
Canadian (not preferred downstream as well). GFS remained on the
slightly quicker side by Sat as heights begin to lower, but was
not unreasonably even at the edge of the ensemble spread in light
of the past few days of runs. Trended toward 50/50
ensembles/deterministic by the end of the period as there remains
a difference between the ECMWF-based and GFS-based ensembles
related to Pacific troughing near the Papahanaumokuakea Marine
National Monument.
For the eastern half of the CONUS, forecast revolves around how to
handle the southern stream upper low (assuming enough
northern/southern stream separation). Trend seems to favor a more
separated and southerly track, eventually encouraging cyclogenesis
in the Gulf around Saturday. Though the models have wavered quite
a bit, the ensembles provide a much steadier, though blurrier,
forecast. Thus, continue to stick with what the pattern may likely
support with compensating lower latitude negative height anomalies
(the western one off Baja CA). This is supported by
teleconnections from the dipole couplet over Banks Island
(northern Canada) but not as much from other anomaly centers
(e.g., troughing stretched in the eastern North Atlantic and
strong upper ridging over the Sahara). Given that the 12Z GFS and
ECMWF were relatively close to their ensembles (with their typical
fast/slow relative biases, respectively), blended approach with
their ensemble means provided an adequate starting point.
...Weather Highlights/Threats...
Lead system on Thursday should weaken over BC with another right
behind it, focused into WA and northern OR with that round of
precipitation. As trailing upper troughing sharpens, moisture
plume should lift northward Friday as the cold front approaches
before sinking southward through coastal OR/NorCal as the front
moves onshore Saturday. Several inches of precipitation are
expected but over a 2-3 day period with a likely break in between
for a given location. Daytime temperatures will be near to above
average overall with a trend cooler as the front moves inland this
weekend. East of the Rockies, temperatures will be above average
with dry conditions and upper ridging overhead. Quick cold shot
will skirt through the Northeast on Thursday (10s/20s for highs
and -0s/0s/10s for lows) with a moderation thereafter.
Coastal/Gulf system will provide some rainfall for the Southeast,
depending on its evolution, with snow possible on its northern
fringe in marginally colder air at generally higher elevations.
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