Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
545 PM PDT sun Mar 16 2014
Discussion...adjusted the forecast for this evening to delay the
onset of precipitation west of the Cascades this evening. The
combination of the visible and radar images show the cold front
still offshore and the the NAM and GFS are a bout too fast with
the front's arrival. The rap 13 barely brings precipitation along
the coast by 8pm PDT, then pushes it inland between 8 PM PDT and 11
PM PDT. Thus have adjusted the timing to reflect the rap 13
timing. Also adjusted snow levels higher early this evening with
the big drop in snow level coming after 11 PM PDT. This along with
recent warming and warmer ground temperatures should result in
less accumulations overnight tonight. However there is still the
potential for a 4-6 hour period of moderate to heavy snow fall
with an inch per hour rate. This is the main reason for the Snow
Advisory. The previous discussion details the areas expected to
get impacted the most from snow and this still remains valid.
Please see previous discussion for details on mountain snow.
Aviation...based on the 16/18z taf cycle...MVFR ceilings should lift to
VFR this afternoon at all taf sites. Gusty south to southwest winds
are expected at all terminals as gradients ahead of a cold front
strengthen. Winds may be a bit more westerly at kmfr. The cold front
will bring rain to koth by around 01z, then krbg by 03z with ceilings
lowering down to MVFR. Mountains will become obscured. Rain will
hold off until at least 06z at kmfr where ceilings will lower, but
probably remain VFR. At klmt, a mix of rain and snow is expected at
the terminal between 07-09z with the frontal passage. Spilde
Marine...gusty south winds will accompany a cold front across the
waters this afternoon. Seas will become very steep this afternoon
through evening as a large west swell train moves through. Winds and
seas diminish tonight but seas remain elevated through Monday. North
gales and very steep seas are possible again Tuesday as high
pressure expands eastward as a thermal trough builds north into
southern Oregon. Fb/spilde
Previous discussion... /issued 313 PM PDT sun Mar 16 2014/
Short term...a fast moving front is on track to move into the
area this evening and tonight then shift east of the area Monday
morning. This front will bring a colder air mass to the area with
sharply lowering snow levels. Also strong and gusty southwest
winds are expected through tonight over the mountains and into the
Summer Lake area.
Ahead of the front, southwest winds have picked up over the area
with winds gusting to 45 miles per hour over the Cascades, Siskiyous and in
the Summer Lake area. Observations also show gusts of around 30
miles per hour in the east side valleys and along the coast. Winds will
increase further late this afternoon into this evening over the
area. Then winds along the coast will decrease in the late
evening as the front moves inland but will persist into tonight
for inland areas.
Snow levels are expected to drop as the front moves inland...with
snow levels around 4500 to 5000 feet as precipitation begins then
lowering to 3000 feet and locally down to 2500 feet tonight.
Moderate precipitation will spread inland this evening and tonight
to the Cascades with mainly light precipitation spreading into
areas east of the Cascades and over northern California overnight.
In the southern Oregon Cascades very heavy snowfall rates are
expected at times late this evening and tonight. High resolution
models indicate that snowfall rates of 1 to 2 inches can be expected
in the southern Oregon Cascades. Greatest snowfall rates are
expected over western slopes of the southern Oregon Cascades to
the Cascade crest. Areas such as Diamond Lake, Crater Lake,
Highway 140 near Lake of The Woods and dead Indian Memorial Road
Summit are expected to see periods of heavy snowfall rates
tonight. However since the front is expected to move across
the area quickly, total snowfall accumulations are expected at only
around 3 to 7 inches for locations above 4500 feet in the Cascades
with 1 to 3 inches below 4500 feet. In addition to heavy snow at
times, very gusty southwest winds will create periods of blowing
snow and very poor visibilities in the Oregon Cascades. Blowing
snow impacts will be greatest in exposed locations in the Cascades.
In the Siskiyous expect lighter amounts of 1 to 3 inches of
snowfall accumulation. A Winter Weather Advisory has been issued
for snow and blowing snow in these areas.
For coastal areas, the main impact through this evening will
concern high surf conditions as a large westerly swell moves into
the coastal waters. This will bring dangerous surf conditions and
large breakers to coastal areas.
Monday morning, behind the front expect precipitation to diminish
to light showers with best shower activity along the southern
Oregon Cascades. Then shower activity will taper off Monday
afternoon and evening a ridge begins to build into the area from
the west. Temperatures Monday will be much cooler than they have
been. Highs in the lower to middle 50s are expected in valley
locations west of the Cascades and highs in the 40s are expected
for valleys east of the Cascades.
Cooler temperatures will continue Monday night and Tuesday morning
as cold air remains in place. Lows in the upper 20s to middle 30s are
expected Tuesday morning for inland west side valleys and lows in
the low 20s are expected in east side valleys. The upper level
ridge will persist over the area and bring mainly dry conditions
Tuesday with temperatures warming into the 50s and 60s across
inland valleys. However a front is expected to move over the
ridge to the north and bring clouds into Coos and Douglas counties
Monday night and Tuesday. Also as the front moves inland to the
north, there is a slight chance for light rain along the coast
Monday night and Tuesday morning.
Then on Wednesday, models show another front is forecast to move
into the area and bring a round of light precipitation, mainly to
areas in southern Oregon from the Cascades west. Snow levels are
expected around 4000 to 5000 feet Wednesday.
Long term...Thursday through Sunday...
Model guidance is in very good agreement with respect to the
expected dry pattern setting up across the Pacific northwest later
next week into the weekend, so confidence is high in the extended
forecast. A shot of cooler air will move in behind the front
Thursday morning lowering snow levels down to about 2500 feet west
of the Cascades, but by this time, most of the moisture will be gone
and there's only a slight chance of showers along the north coast to
the Umpqua valley and the Cascades from Crater Lake northward.
Overall, the flow aloft will remain progressive with weak short
waves riding over top of a Flat Ridge Thursday night into Saturday,
but none of these disturbances will be strong enough or have enough
moisture to produce any precipitation. There will be a strong
surface high pressure system offshore Thursday night into Friday
morning with a thermal trough developing in northern California.
This will induce some gusty north-northeast or even east winds at
some of the middle slopes and ridges, especially the Coast Ranges of SW
Oregon. Friday into Saturday, pressure gradients relax a bit, so
these ridge winds should ease. By Sunday, a deepening trough over
the Gulf of Alaska will allow the flow to amplify, with a ridge
building along the West Coast. Temperatures during the period will
gradually modify toward and perhaps back above normal by the end of
the weekend. Spilde
or...high surf advisory until 11 PM PDT this evening for orz021-022.
Wind Advisory until 5 am PDT Monday for orz030-031.
Winter Weather Advisory from 9 PM this evening to 4 am PDT
Monday for orz027-028.
California...Wind Advisory until 5 am PDT Monday for caz085.
Pacific coastal waters...Small Craft Advisory for winds until 11 PM PDT
this evening for pzz350-356-370-376.
Small Craft Advisory for hazardous seas until 11 PM PDT Monday
Hazardous seas warning until 11 PM PDT this evening for
Small Craft Advisory for hazardous seas from 11 PM this evening
to 11 PM PDT Monday for pzz350-370.