High Wind in Oregon, March 13, 2018

Area Forecast Discussion
National Weather Service Medford OR
929 PM PDT Mon Mar 12 2018

.UPDATE...
Updates this evening have been to account for a slower inland
progression of the frontal system that's offshore. Overnight lows
have been increased, as have maximum temperatures for Tuesday.
Additionally, while peak winds have been retained, some downward
adjustment has been made to wind speeds through 4am PDT based on
the latest high resolution model guidance and observations.
Precipitation probabilities and amounts for this evening have also
been scaled back, mainly in the Mount Shasta area. Rainfall has
now begun in Mount Shasta City and we do anticipate this rain is
likely to begin measuring this hour. BTL

&&

.DISCUSSION...
A vigorous area of low pressure offshore near 40 degrees North
Latitude and 130 degrees West Longitude is slowly making its way
eastward toward the coast this evening. Anticipated winds have
been locally gusty at places like Montague, California (46mph) and
Squaw Peak in the Siskiyous (62mph), but have, thus far, been
generally below forecast values. The observed pressure gradient
has been handled fairly well by the NAM12, however, and this model
indicates that winds will continue to increase overnight through
about midnight at the coast and through 4am PDT on the West Side.
East of the Cascades winds won't peak until the daylight hours on
Tuesday as the cold front from this weather system slowly chugs
it's way eastward across the forecast area. Therefore, aside from
some short term adjustments, the forecast has been left untouched
for Tuesday except for temperatures.

It should be noted that some models are indicating over a half
inch of rain in 6 hours in the Bear Creek portion of Rogue Valley
tomorrow. While not all of the guidance shows this, the point is
that, once this front arrives, especially from the Cascades
westward, it's going to be a soaker for about 6 hours. This will
lead to ponding on area roadways, so expect travel will be slower
than normal.

Tuesday night into Wednesday snow levels drop to 3,000 to 4,000
feet, so snow will become a bit of a concern. The cold core low
at the center of the trough offshore moves in Thursday, focusing
on California. This is likely to require some snow headlines in
the Mount Shasta area, though the showers could result in them
sooner. BTL

&&

.AVIATION...13/00z TAF CYCLE...Southerly winds are increasing across
the forecast area, and turbulence will increase as a result. VFR
conditions continue through the evening, except that ceilings will
lower at the coast this evening with MVFR conditions and mountain
obscuration. A band of moderate rain will move through areas west of
the Cascades very early Tuesday morning, and this will bring West
Side ceilings (and occasionally visibility) down to MVFR.
Precipitation and MVFR ceilings along with mountain obscuration,
will reach areas east of the Cascades by early to mid Tuesday
morning.

Low level wind shear will impact North Bend and Roseburg Tuesday
morning. Additionally, freezing levels will drop sharply (lowering
by about 4000 feet) behind the front Tuesday morning as the cold
front passes through. -FB

&&

.MARINE...Updated 800 PM PDT Monday 12 Mar 2018...Strong low
pressure offshore will send fronts toward the coast overnight,
resulting in south gales over the outer waters. Models have trended
towards keeping the southerly gales farther offshore, so we expect
gales to stay mostly beyond 20 NM from shore. After the gales, fresh
swell will combine with building longer period west swell and
continued wind waves to bring very steep hazardous seas to most of
the waters. Steep seas will continue from late Tuesday into
Wednesday. Broad low pressure will remain offshore, and this will
keep the weather pattern active through the week and bring more
chances for strong southerly winds. -FB

&&

.PREV DISCUSSION... /Issued 242 PM PDT Mon Mar 12 2018/

DISCUSSION...The latest visible image shows high and high level
clouds moving into the area ahead of ahead of the strong frontal
system. The one exception is in southern Siskiyou County, including
Mount Shasta where upslope flow has resulted in a lower cloud deck.
It's still possible some light precipitation could get squeezed out
near and around Mount Shasta early this evening. None of the short
range high res models are showing anything, but could not rule it
out, so we'll leave pops in.

The main impact from the above mentioned front will be moderate to
strong winds in the Shasta Valley and portions of the Rogue Valley,
including Medford through tonight although we often decouple and see
less wind here (Medford) at night. High wind warnings and advisories
remain in place for these areas (see NPWMFR for details). Winds pick
up in the mountains and east side tonight as the front gets close.
The mountains are likely to see locally very strong winds late
tonight and Tuesday morning, especially west side mountains like Mt.
Ashland. At this time it looks like the really strong winds will be
confined to the highest peaks, but we'll have to keep a close eye on
that.

Winds east of the Cascades will increase later this evening and
continue into Tuesday afternoon. The models have backed off some on
the magnitude of the winds. However, given the front is strong and
we'll have significant air mass change behind it there is a chance
stronger winds above 700mb could mix down near the surface.
Therefore no changes will be made to the coverage area of the
winds.

This front will also bing moderate to heavy rain to the area, but
its not expected to last long enough to pose a concern. The
exception will be around Mt. Shasta where prolonged moist, upslope
flow will lead to as much as 2 inches of rain before the front
finally moves on Tuesday afternoon. Snow levels will be high
tonight into Tuesday morning, so no concerns are expected along
the I-5 corridor. Snow levels will come down Tuesday afternoon and
we could see rain mixing with wet snow if precip remains heavy
enough near Mount Shasta, but roads should be warm enough to keep
snow from accumulating.

Finally this front has plenty of energy with it, therefore we'll
have to keep a close watch on thunderstorm potential. Would not be
surprised to see lightning just about anywhere tonight into Tuesday,
but the best chance for seeing lighting is expected to be along and
just ahead of the front as it approaches the coast later this
evening, then pushing east later tonight into Tuesday morning, then
east of the Cascades Tuesday afternoon as get into daytime heating,
thus increasing the instability further. Given all the dynamics and
strong winds aloft, any of these storms could produce strong,
damaging wind gusts.

The front will move through the area Tuesday night, but plenty of
showers will remain over the area as an upper trough axis
approachers the Oregon Coast with plenty of shortwave activity
Tuesday night. Snow levels will also fall quickly, down to between
2500 and 3000 feet depending on precip intensity, therefore snow
impacts are possible overnight Tuesday into Wednesday morning.

There's some indications we could catch a brief break Wednesday
afternoon into Wednesday night as the upper trough axis moves
northeast and the next upper low approaches 130W. Scattered showers
will still be around, but most should be light. The next upper low
will approach the area Thursday and the models are in good agreement
we could see periods of moderate to heavy precipitation moving into
Southwest Oregon and Northeast California mid to late Thursday
morning and afternoon. Precipitation may decrease some as the
upper low moves south off northwest California, but confidence is
low. Snow levels will be lower and could vary between 2500 and
3000 feet Thursday morning, then between 3000 and 3500 feet in the
afternoon depending on how intense the precip is. As is typical
for this time of the year, the concern for road snow accumulation
will be during the nighttime hours and less during the day.
-Petrucelli

Long term discussion...Friday through Monday...Models continue to
show a cold pattern with widespread showers across the area Friday
as an upper low and surface low move into the area. This is followed
by additional showers on Saturday as an upper level trough remains
in place over the region. Then on Sunday, the trough will gradually
shift east and a brief break in the active pattern is possible
Sunday into Monday. A weak ridge is forecast to build into the
region late Monday but another low pressure system is expected to
develop offshore.

The main concern late in the week will be for low elevation snow
showers Friday into Saturday. Model solutions support snow levels
lowering down to western valley floors Friday morning and remaining
low around 1000 to 2000 feet during the day Friday. With the upper
low aloft and a surface low moving inland Friday, expect widespread
shower across the area. These showers are expected to continue
Friday night into Saturday. Of note, the track of the surface low
varies with some GEFS members and the GFS showing it moving into
Southern Oregon and the ECMWF and other solutions showing a more
southerly track into Northern California. The more southerly track
would support showers decreasing in coverage Friday night into
Saturday as the low moves inland south of the area. A more northerly
track with with the low moving into southern Oregon would bring
additional snow to the area Friday night into Saturday. So, given
the cold air and potential for showers and accumulating snow, will
need to monitor this with future model runs. -CC

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 5 PM PDT Tuesday for ORZ029>031.
     High Wind Warning from 11 AM to 5 PM PDT Tuesday for ORZ031.
     Wind Advisory until 11 AM PDT Tuesday for ORZ026.

CA...Wind Advisory until 5 PM PDT Tuesday for CAZ084-085.
     Wind Advisory until 11 AM PDT Tuesday for CAZ081.
     High Wind Warning until 11 AM PDT Tuesday for CAZ081.

Pacific Coastal Waters...
- Small Craft Advisory for hazardous seas until 2 AM PDT Tuesday for
  PZZ350-356-370-376.
- Small Craft Advisory for hazardous seas from 5 PM Tuesday to 11
  AM PDT Wednesday for PZZ350-356-370-376.
- Hazardous Seas Warning until 5 PM PDT Tuesday for
  PZZ350-356-370-376.
- Gale Warning until 2 AM PDT Tuesday for PZZ370-376.

BTL/FJB/MAP