Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
159 AM EST Wed Dec 11 2019
Valid 12Z Sat Dec 14 2019 - 12Z Wed Dec 18 2019
...Eastern U.S. heavy rain and interior wintry weather threat into
Saturday and again Monday/Tuesday-...
...Heavy mountain snows possible this this weekend from Northwest
to Rockies...
...Pattern Overview and Guidance/Predictability Assessment...
An active and complex pattern should persist through much of the
upcoming medium range period (Saturday-Wednesday) as
northern/southern stream shortwaves interact. As one main lead
shortwave/trough exits the Northeast this weekend, another dives
south and eastward from the eastern Pacific into the West, moving
into the South-Central U.S. by early next week, then to the East.
Through the period, there is fairly good agreement in the large
scale pattern, though forecast confidence remains somewhat
tempered due to run to run inconsistencies with the more complex
details.
The WPC medium range product suite was primarily derived from a
composite blend of reasonably well clustered model and ensemble
forecasts and the National Blend of Models Saturday into Monday.
Increased weighting significantly to the more run to run
consistent GEFS/ECENS ensemble means into Days 6/7 amid growing
forecast spread. Even so, the overall active pattern seems to
offer above normal predictability and decent continuity.
...Weather Highlights/Threats...
Southern stream shortwave progression and subsequent northern
stream flow interaction will support low development and track up
the East Coast into Saturday. This will bring heavy rain from the
Appalachians/Mid-Atlantic through the Northeast. System
interaction with lead/receding lower atmospheric cold air damming
and wrap-back/post-frontal flow offers an interior heavy snow/ice
threat and Lake effect snows. The degree of phasing between two
initially separate streams and baroclinic zone/coastal details
will be determining factors of eventual evolution and potential
coastal system threat.
The pattern will also become wet upstream over the Pacific
Northwest/California into Saturday as Pacific shortwave energies
punch onshore. Moderate to heavy snows will develop as the system
penetrates inland, especially over favored north-central Great
Basin/Rockies terrain through the weekend. By next Monday into
Tuesday, the exiting shortwave will eject and then interact with
Gulf moisture to spawn a rainfall focusing low pressure/frontal
system forecast to move over the Southern Plains and Deep South,
then with some added uncertainty up the East Coast. This scenario
also sets the stage for a threat of heavy snow/ice on the northern
periphery of the precipitation shield from the S-Central Plains to
the Mid-MS/OH Valleys then Northeast.
Schichtel
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