High Wind in California, December 21 to 22, 2019

Area Forecast Discussion
National Weather Service Medford OR
1007 PM PST Fri Dec 20 2019

Updated AVIATION Section

.DISCUSSION...21/00Z NAM in. 21/00Z GFS in through 186hr.

A breezy and mostly dry evening over most of the Medford CWA. The
biggest exception is the Mount Shasta area, which is getting light
precipitation facilitated by upslope flow.

An atmospheric river event is in progress over the Pacific
Northwest, and currently it is directed into Washington state. The
Medford CWA is currently in the warm (and windy) sector with a
cold front offshore.

Waves developing on the front are slowing its eastward progress,
but the upper trough driving it will continue to move towards the
coast, and the front will move onshore Saturday. However, wave
development will continue, and the front will continue to progress
slowly.

The most immediate concern for the Medford CWA is strong wind.
South and southeast winds gusting in the 50 to 70 mph range at the
coast will continue tonight as the frontal boundary stalls
offshore. Inland areas will see increasing winds into Saturday
morning as the front approaches the coast. A mosaic of High Wind
Warnings and Wind Advisories have been issued to address this
event.

After the winds, the next concern is snow with falling snow levels
behind the front. Snow impacts are anticipated Saturday night into
Sunday from the Southern Oregon Cascades and Siskiyous southward
above 3500 feet. While there is a significant amount of spread in
the model solutions for that time period, they do agree that the
frontal boundary will move into the area of concern and then slow
and stall as a strong wave tracks along the frontal boundary across
northern California and then northeastward across the forecast
area. This pattern supports moderate to heavy snowfall for the
area, but not all the model guidance agrees it will all come
together to be a significant event. Confidence is moderate to high
that there will be at least advisory level impacts during this
time period. Confidence in warning level amounts is moderate to
high for elevations above 4000 feet, especially from Mount
Ashland westward. Given the increased traffic due to the holidays,
there is also concern for higher impacts from lesser snowfall.
Thus the watch covers what could end up being warning level
impacts as this storm system moves through Saturday night into
Sunday. On the back side of the system models suggest snow levels
could come down to around 2000 feet Sunday.

Long term discussion from the Friday afternoon AFD...Mon 23 Dec
through Fri 27 Dec 2019. The collective model guidance suggests
Monday and Tuesday will be mostly dry across the forecast area
thanks to weak high pressure over the area. Low pressure
approaches from the west on Christmas Day, but most of the recent
model output suggests the low and most of the moisture associated
with it will move south into Central or Southern California. This
would leave Southern Oregon and Northern California mostly dry.
The GFS operational model does bring low pressure onshore
Wednesday, but it weakens quickly with little inland precipitation
depicted. We'll be keeping some precipitation in the forecast at
the coast but much lesser chances of inland rain/mountain snow
because of this.

Confidence in the forecast decreases sharply on Thursday because
ensemble members amongst the GEFS and ECE are split. Some point to a
front and valley rain/mountain snow moving in while others suggest
continued dry conditions through Friday. We are relying on the NBM
(blend of models) for now because it captures this uncertainty and
features a chance for precipitation Thursday into Friday.

Throughout the extended (Monday through Friday), the air mass will
be colder than normal for this time of year, and snow levels will
range between 2000 and 3500 feet. High temperatures are forecast to
be generally about 5 degrees below normal for this time of year.
Much of the temperature forecast in the extended will of course
depend on how long any low clouds hang around in the morning/early
afternoon. Based on current data, it looks like Tuesday morning will
be a favorable morning for morning fog/freezing fog and could keep
temperatures below what guidance suggests. Have adjusted
temperatures to reflect that in some west side valleys.

&&

.AVIATION...For the 21/06Z TAFs...Numerous aviation hazards are
expected in the next 24 hours. First, we expect low-level wind shear
(LLWS) and strong, gusty south to southeast winds. Expect LLWS in
areas where winds aren't quite surfacing and/or are very strong just
off the ground (Roseburg/North Bend). Strong winds with gusts of
30-35kt are surfacing at Medford and Klamath Falls and are expected
to continue through much of Saturday. We have Airport Weather
Warnings out for both sites.

Meanwhile, VFR prevails ahead of a strong front remaining just
offshore, but the front will move onshore overnight/around sunrise
at the coast and just inland west of the Cascades Saturday morning.
This will bring rain, heavy at times, to the coast, including North
Bend. Ceilings and visibility will lower to MVFR/IFR and coast
range and interior west side mountains will become obscured. LLWS
concerns should lessen as the strongest winds aloft push east during
the day. MVFR will likely develop at Medford as rain moves
in Saturday afternoon and continues into the evening.

Precipitation will move inland from the coast Saturday night, but
probably won't push east of the Cascades until late Saturday night
or Sunday morning. -Spilde

&&

.MARINE...Updated 815 PM PST Friday, 20 December 2019...A
strong front is bringing gales and very steep, hazardous seas to the
waters. Periods of heavy rain will affect northern areas overnight
and into Saturday morning, spreading to areas south of Cape Blanco
by Saturday morning. Seas will peak around 20-23 feet overnight.
Thereafter, conditions will begin to improve as winds ease and seas
begin to subside. However, the overall sea state is likely to remain
chaotic and hazardous through Saturday evening.

Mariners should consider altering their plans or remain in port
until this long-lived storm passes. Conditions should fall below
advisory criteria by Saturday night, with sub-advisory conditions
persisting through at least Tuesday morning. Another system will
approach the waters late Tuesday into Wednesday. In addition,
guidance is indicating another heavy, long period northwest swell
moving into the waters Christmas Day, followed by another swell
train late Thursday into Friday. Stay tuned for updates. /BR-y/Spilde

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning until 10 PM PST Saturday for ORZ030-031.
     Wind Advisory until 10 PM PST Saturday for ORZ030-031.
     Winter Weather Advisory from 4 PM Saturday to 4 PM PST Sunday
        above 4000 feet in the for ORZ028.
     High Wind Warning until 7 AM PST Saturday for ORZ021-022.
     High Surf Warning until 4 AM PST Saturday for ORZ021-022.
     Wind Advisory until 1 PM PST Saturday for ORZ026.
     High Wind Warning from 1 AM to 1 PM PST Saturday for ORZ028.
     Wind Advisory from 7 AM to 4 PM PST Saturday for ORZ029.

CA...High Wind Warning until 10 PM PST Saturday for CAZ085.
     Wind Advisory until 10 PM PST Saturday for CAZ085.
     Winter Storm Watch from Saturday afternoon through Sunday
        afternoon above 3500 feet in the for CAZ080-082.
     Wind Advisory from 7 AM to 4 PM PST Saturday for CAZ084.
     High Wind Warning from 10 PM this evening to 7 PM PST Saturday
        for CAZ081.
     Wind Advisory until 7 PM PST Saturday for CAZ081.

Pacific Coastal Waters...Small Craft Advisory from 10 AM to
     10 PM PST Saturday for PZZ350-356-370-376.
     Gale Warning until 4 AM PST Saturday for PZZ350-356-370-376.
     Hazardous Seas Warning from 4 AM to 10 AM PST Saturday for
        PZZ350-356-370-376.

15/15/18