Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
324 PM EST Mon Dec 16 2019
Valid 12Z Thu Dec 19 2019 - 12Z Mon Dec 23 2019
...Atmospheric River to Fuel Heavy Precipitation for the Northwest
Thursday-Saturday...
...Overview...
Closed Gulf of Alaska upper troughing will shift eastward as
undercutting waves send protracted periods of heavy rain/mountain
snows from the Pacific Northwest to north-central California
through the weekend as fueled by an atmospheric river and potent
jet. Snow levels will gradually rise, but also work inland to
favored terrain of the Northern Great Basin/Rockies. A northern
stream will be relatively low amplitude with little precipitation
east of the Rockies. Underneath, a somewhat unusual southern
stream system may dip into the Gulf of Mexico and Florida this
weekend along with an unsettled weather and rainfall risk
Sunday/Monday for the Southeast and maritime interests along with
modest lead snow/ice potential into the s-central Mid-Atlantic.
...Guidance/Predictability Assessment...
The WPC medium range product suite was primarily derived from a
composite blend of well clustered guidance from the GFS/ECMWF and
GEFS/ECMWF ensemble means days 3-5 (Thu-Sat). While the upcoming
heavy precipitation event for the Northwest offers good
predictability, forecast confidence is only about average east of
the Rockies given uncertainty suggested by the 00/12 UTC UKMET
runs that in contrast offer less northern stream amplitude and
interaction with a more separated/less progressive southern stream
flow. In the southern stream Days 6/7, the 12 UTC GFS is now in
much better agreement with recent 00/12 UTC ECMWF and ensemble
runs, bolstering confidence with respect to potential FL low
development and wrapped precipitation in the Southeast. The 00/06
UTC GFS runs were outlier low amplitude and progressive solutions
that did not match WPC continuity.
...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/CA as the front moves
onshore Saturday. Multiple inches of precipitation are expected
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 overall dry conditions and upper ridging overhead. Quick cold
shot will skirt over the Northeast 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 some snow/ice possible on its northern fringe
in the marginally colder south-central Mid-Atlantic air at
generally higher elevations.
Fracasso/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
Hazards:
- Heavy precipitation across portions of California, the Pacific
Northwest, and the Northern Great
Basin, Thu-Sun, Dec 19-Dec 22.
- Heavy snow across portions of the Northern Rockies and the
Northern Great Basin, Thu-Fri, Dec
19-Dec 20.
- Heavy snow across portions of the Northern Rockies and the
Northern Great Basin, Thu-Sat, Dec
19-Dec 21.
- Flooding possible across portions of the Central Appalachians,
the Pacific Northwest, and the
Ohio Valley.
- Flooding occurring or imminent across portions of the Northern
Plains.
- Much below normal temperatures across portions of the Great
Lakes, the Mid-Atlantic, the
Northeast, and the Central Appalachians, Thu-Fri, Dec 19-Dec 20.
- High significant wave heights for coastal portions of California
and the Pacific Northwest,
Thu-Fri, Dec 19-Dec 20.
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