Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
151 AM EST Wed Dec 15 2021
Valid 12Z Sat Dec 18 2021 - 12Z Wed Dec 22 2021
...Guidance/Predictability Assessment...
Models and ensembles remain in good agreement this weekend into
Mon in a pattern with above average forecast confidence and
continuity. Forecast spread and continuity issues become
increasingly evident with the timing of embedded weather features
and stream interactions next week. The WPC medium range product
suite was primarily comprised of a blend of best clustered
solutions from the 18 UTC GFS/GEFS mean and the 12 UTC ECWMF/ECMWF
ensemble mean. Blend weighting favors the models this weekend
before transition to mainly the compatible ensemble means next
week amid growing system uncertainty and run to run variances.
WPC preferred guidance and latest 00 UTC runs overall have
suppressed southward to off the Southeast a threat for coastal low
genesis early-mid next week.
...Weather/Hazards Highlights...
A developing frontal low has potential to bring a swath of
plowable snow as enhanced by terrain from the upper OH Valley
through especially the interior Northeast this weekend. An
enhanced rainfall focus with the trailing front will meanwhile
sink across the Gulf Coast and Southeast as post-frontal cooling
high pressure builds across the Plains and to the East. Guidance
has been inconsistent with the prospect of subsequent frontal wave
developments and coastal low genesis early next week up the East
Coast in part from northern stream energies and/or with latest
guidance trends now favoring ejection of a separated southern
stream trough that leads to southward suppressed wet lows from the
Gulf to offshore the Southeast.
Well upstream, dynamic but progressive upper trough and surface
system passage will build moisture to support moderate to terrain
enhanced heavy precipitation over the Northwest this weekend.
Amplified upper trough digging over the eastern Pacific early-mid
next week in the wake of this system offers potential to focus
ingredients for moderate to heavy precipitation into north-central
CA with slow system approach.
Schichtel
Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook 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