Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1131 AM EST Thu Dec 13 2018

Valid 12Z Sun Dec 16 2018 - 12Z Thu Dec 20 2018

...Significant precipitation for the Northwest...


...Overview...

Progressive yet still amplified flow will dominate the lower 48
next week as a lead upper low exits off the East Coast Sunday. An
incoming trough into the west will separate into northern and
southern components late Monday into Tuesday. The strength of the
southern trough or closed low will modulate the amount of
precipitation midweek over Texas. In the Pacific Northwest, a
series of systems will produce several bouts of moderate to
orographically-enhanced locally heavy rain/snow.


...Guidance Evaluation/Preferences...

Models and ensembles continue to differ on the timing/track of the
embedded systems across the lower 48, but forecast clustering has
improved over the past few runs. Accordingly, the WPC medium range
product suite was primarily derived from a composite blend of
deterministic models to start (Sun-Tue) and mostly ensembles to
end the forecast (next Wed/Thu) as spread increased over
especially the central/eastern states. Uncertainty lied downstream
of a strong Pacific jet starting next Tuesday in the Southwest
with the degree of flow separation -- the GFS/GEFS indicating a
stronger southern closed low while the ECMWF/Canadian and most of
their ensemble members indicating a weaker trough -- which will
have a direct affect on the amount of precipitation over Texas
next week. Prefer a solution a bit on the slower side of the
guidance but not as slow as the GFS. To the north, amount of
northern stream troughing will be affected by the slowness of the
southern stream, but for now will split the difference between the
quicker ECMWF ensembles and slower GEFS ensembles given
unclear/conflicting up/downstream signals.


...Pattern Overview and Weather Highlights/Hazards...

A lead main Southeast/Mid-Atlantic system will shift offshore
Sunday. Lingering wrapback rains and northern fringe snow will
slowly exit by early next week as the coastal storm/maritime
threat lifts to the Canadian Maritimes. A Canadian cold front will
sweep through the Northeast Monday bringing in cooler temperatures
Tue/Wed (about 5-10 deg F below average). Much milder temperatures
(10-30 deg F above average) are forecast over the central states,
especially the High Plains, as upper ridging and occasional
downsloping will favor relatively "warm" temperatures (mainly in
the 40s to low 50s).

A heavy precipitation and high wind threat for the Pacific
Northwest/northern CA resurges next week as a succession of
energetic Pacific systems approach the region. Some deeper lower
latitude moisture infusion and favorable flow suggests locally
significant rain and higher elevation snow. This will push
eastward to the northern Great Basin/Rockies, aided by a
pronounced northern stream flow. To the south, a split/separated
southern stream trough will carve out over the Southwest
U.S./northern Mexico by early next week, but the precipitation
amount remains uncertain. That system should track east of 90W and
increasingly tap Gulf of Mexico moisture later next week to
enhance the rainfall potential over the lower Mississippi Valley
eastward.


Fracasso/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