Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1053 AM EST Tue Dec 11 2018

Valid 12Z Fri Dec 14 2018 - 12Z Tue Dec 18 2018

...Heavy Precipitation Threat for the Northwest...
...Heavy Rain Threat for the East...


...Overview and Guidance Evaluation/Preferences...

An increasingly strong signal for widespread precipitation
(heavier periods) for the Pacific Northwest, portions of
California and across the Southeast/Appalachian/Mid-Atlantic
region through the extended forecast. Significant timing, location
and depth differences amongst the deterministic and ensemble
guidance continued with the latest guidance, thus resulted in
below average confidence for this forecast issuance. WPC medium
range product suite was primarily derived from a composite blend
of the latest GEFS/ECMWF ensemble means with about 10% each of the
00Z GFSP/ECWMF.


...Weather Highlights and Hazards...

Precipitation/wind will substantially increase from coastal
Washington to northern California through the weekend and into the
middle of the week as a succession of approaching energetic
Pacific shortwaves/frontal systems pass through the region. Higher
Pacific Northwest terrain will see the heaviest rain/mountain snow
(with varying snow levels as temperatures rise and fall between
each system, with snows then working to the north-central Great
Basin/Rockies. Forecast precipitation up to 2 inches is
anticipated primarily for coastal areas- with additional 2 to 5
inches forecast by the 18th.

Lead shortwave energy digging into the southern Plains/lower MS
Valley Thu-Fri is expected to carve out a closed low and spawn
surface cyclogenesis/frontogenesis. Ample Gulf moisture/warm
advection increasing ahead of the deepening system will expand the
precipitation shield to the east and wrapping to the north of the
surface low. Relatively slow movement of the well organized system
and favorable dynamics support a heavy rain threat for portion of
the lower MS Valley/Mid-South to Southeast/Mid-Atlantic. Heavy
rainfall will robustly spread through the Appalachians and Eastern
Seaboard with added infusion of Atlantic moisture. Runoff problems
will be enhanced in areas with recent heavy snowfall and ongoing
river flooding. Heavy snows may also wrap into a marginally cooled
Midwest/Great Lakes to interior Northeast airmass.

Campbell/Schichtel


WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found 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