Heavy Snow in Oregon, March 4 to 6, 2017

Area Forecast Discussion
National Weather Service Medford OR
420 PM PST Sat Mar 4 2017

Updated AVIATION and MARINE Sections

.SHORT TERM...Through Tuesday morning...
We expect snow impacts to greatly increase tonight through Sunday
as snow levels fall and precipitation increases across the
forecast area. Snow impacts tonight into Sunday morning are then
likely to continue into Monday for lower elevation areas. As has
previously been indicated by preceding shifts, the influence of
the March sun and ground temperatures will limit valley snow
impacts, especially during the afternoon hours Sunday and Monday.
This does leave a period of maximum concern Sunday night into
Monday morning, however.

Showers moving across the area today have briefly lowered snow
levels to between 2500 and 3500 feet. Early this evening a
secondary cold front will bring snow levels down further as it
moves through, likely to between 500 and 1500 feet by midnight.
Snow accumulations with the next round of precipitation with this
front will be a bit tricky for elevations below 2000 feet, as
most of the precipitation should move through before snow levels
drop that low. Additionally, recent mild temperatures and warmer
ground and road temperatures will cause any snow that does fall
below even 3000 feet to be melt quickly. Above 2000 feet expect
some impacts on the passes at least briefly as this moves through.
Winter weather advisories remain in effect for this.

Sunday through Monday snow levels will remain generally below
2000 feet, and mainly at 1000 feet or lower. Some winter storm
watch areas have been scaled back, but not because impacting snow
is not expected. Impacting snowfall is still expected, but the
watch has been adjusted to account for areas where impact is
expected to be to warning levels. Advisories will be issued as
needed for other areas. Altogether, waves of vigorous showers will
move onto the coast and through the area Sunday through Monday.
The greatest concern remains Sunday evening through Monday morning
as a couple of stronger impulses move through under the colder
cover of darkness, meaning air, road, and surface temperatures
will be more conducive to accumulation and resultant roadway
impacts.

Late Monday into Tuesday 1000-700mb thicknesses and 850mb temps
indicate snow levels are likely to hover near 2000 feet as the
next frontal system moves in. Eventually, these snow levels
should rise above 3kft, but impacts will still be a concern with
above that level. BTL

.LONG TERM...Tuesday through Saturday...Models continue to show good
agreement on Tuesday with a Pacific low moving into the area from
the west followed by a continued wet pattern through next wee. On
Tuesday, the low pressure system will move into the coast and bring
another round of precipitation with snow levels increasing Tuesday
into Tuesday night as heights build. Current models show snow levels
starting out around 3000 feet Tuesday morning then rising to 5000 to
6500 feet by late Tuesday afternoon. Showers are expected linger
over the area Tuesday night as a shortwave disturbance moves into
the area.

Models then show broad agreement on a continued wet pattern
Wednesday through Saturday but show considerable variability on the
details and timing of additional systems moving into the region. On
Wednesday, the GFS and ECMWF indicate an increased westerly jet
stream aloft over the northeastern Pacific. These models bring
another moist front towards the Pacific Northwest on Wednesday with
a secondary disturbance moving along the front as it gradually pushes
inland Thursday into Thursday night. Snow levels are expected to be
high on Wednesday, around 7000 feet elevation or higher then will
lower Thursday night and Friday. Models show variability on the
timing and intensity of this frontal system as it moves into the
area Wednesday and Thursday and show variability on how far snow
levels will lower behind the front Thursday night through Friday. So
there is low confidence on the details of this system.

For Saturday, model solutions bring a wet but much cooler pattern in
place with an upper trough dipping down into Pacific Northwest and
bringing widespread showers. The ECMWF shows a slightly less cold
pattern with the trough being positioned further north compared to
the GFS. GFS ensemble solutions also show  variability on the
placement and strength of this trough with the mean solution similar
to the ECMWF. Will need to monitor this period since current models
support showers with snow levels lowering to near lower pass levels
and possibly lower. /CC

&&

.AVIATION...05/00Z TAF CYCLE...Gusty southwest winds should ease in
the next few hours. Cold air aloft will move in over the area
tonight with freezing levels lowering down to 1000-1500 feet west of
the Cascades. Showers will continue with widespread MVFR ceilings
with terrain obscurations. Local IFR is possible late tonight into
Sunday morning, especially where rain showers turn to snow. This may
occur at KRBG and KMFR. There is a slight chance for thunderstorms
from the coastal waters to the coastal range tonight through Sunday
evening. Overall, on Sunday, expect a mix of VFR and MVFR in
rain/snow showers and freezing levels only rising to 2500 feet.
Gusty southwest to west winds are expected to develop again late
Sunday morning/early Sunday afternoon. -Spilde

&&

.MARINE...Updated 400 PM PST Saturday 4 March 2017...Moderate to
gusty southwest winds and steep seas will persist over the waters
tonight into Sunday morning. Wind gusts to gale force are expected
over the outer waters north of Cape Blanco late Sunday morning
through Sunday afternoon with steep to very steep seas around 15 ft
at 12 seconds for all areas north of Cape Blanco. We have issued
gale/hazardous seas warnings for PZZ370 (North Outer) with a
hazardous seas warning for PZZ350 (North Inner). Small craft
advisories are in effect elsewhere. Gusty winds and steep seas will
continue Sunday night through Monday. Then, models are showing yet
another stronger front moving into the waters on Tuesday with the
potential for gales and very steep seas. A watch may be necessary
tonight or tomorrow. Additional disturbances will bring periods of
moderate or higher winds with steep to very steep seas through the
week. -Spilde

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory until 4 PM PST Sunday for ORZ027>031.
     Winter Storm Watch from Sunday afternoon through Monday
     afternoon for ORZ029>031.
     Winter Weather Advisory until 4 PM PST Sunday ABOVE 1500 FEET
     for for ORZ021>026.
     Winter Storm Watch from Sunday afternoon through Monday
     afternoon for ORZ021>026.
     Winter Storm Watch from Sunday afternoon through Monday
     afternoon for ORZ027-028.

CA...Winter Weather Advisory until 4 PM PST Sunday for CAZ080>083.
     Winter Storm Watch from Sunday afternoon through Monday
     afternoon for CAZ080-082-083.
     Winter Weather Advisory until 4 PM PST Sunday for CAZ084-085.

Pacific Coastal Waters...Small Craft Advisory until 4 AM PST Tuesday for
     PZZ350-356-370-376.
     Hazardous Seas Warning from 10 AM to 7 PM PST Sunday for
     PZZ350-370.
     Gale Warning from 10 AM to 4 PM PST Sunday for PZZ370.

BTL/CC/MAS