High Wind in California, April 11, 2018

Area Forecast Discussion
National Weather Service Medford OR
858 AM PDT Wed Apr 11 2018

.DISCUSSION...IR satellite imagery is showing the strong cold
front just offshore this morning. Some light precipitation is
showing up on radar well ahead of the front inland, but most of
it is aloft. If it hasn't started raining along the coast yet, it
will in the next couple of hours. Precipitation will spread
inland to the Cascades and Siskiyous by early afternoon, (~2-3 pm
here in Medford) then move into the Shasta region and areas east
of the Cascades mid-late this afternoon/evening. We have added a
winter weather advisory for the mountains near and north of Lake
of the Woods associated with a period of moderate to heavy snow
expected to occur with the front there this afternoon into this
evening. We think the bulk of the impacts will be above 5000 feet.
That isn't to say there won't be winter impacts elsewhere. The
snow level will drop rapidly this evening and overnight, then down
to around 2000-2500 feet by Thursday morning. While there could
be 1-3 inches of snow over some of the higher summits, like
Siskiyou/Black Butte/Snowman and many east side areas this
evening, the main impact there would be from any wet/slushy areas
to potentially freeze up by Thursday morning. So, folks traveling
through the area should be prepared to encounter wintry conditions
tonight into Thursday. The mid shift did an excellent job
highlighting the potential impacts associated with the front, so
I'll refer you to the previous discussion below for more details.
No other changes this morning. -Spilde

&&

.AVIATION...11/12Z TAF CYCLE...At the coast...the main concern
this morning will be wind speed shear at North Bend. Winds will
begin to surface as a front approaches, but CIGS will lower to MVFR.
The TAF at north Bend shows wind speed shear ending and CIGS
lowering at 19z, but the timing could off by an hour before or after
19z, so watch for updates on this. It's possible CIGs will improve
late this afternoon, then MVFR cigs are expected most hours tonight.

Inland westside...there's good agreement the lower conditions will
not develop until later in the day as the front moves inland.
Therefore kept VFR cigs at Roseburg until 21z and Medford around
23z. However could not rule out partial mountain obscurations prior
to these times. Wind speed shear will be a concern this morning at
Roseburg and Medford.

Eastside...Moderate to strong winds aloft will begin to surface
late this morning and continue through this afternoon including
Klamath Falls. VFR cigs will continue into this afternoon, then
lower to MVFR this evening into tonight. Rain could mix with snow
this evening with a reduction in visibility at Klamath Falls for a
few hours later this evening. -Petrucelli

&&

.MARINE...Updated 555 AM PDT Wednesday 11 April 2018...The latest
ASCAT pass at 6z shows winds well below gales. This looks reasonable
given the cold front is still west of the outer waters. The cold
front itself is around 130 W and is moving east. Therefore have
adjusted the start time of the gales to begin at 5 am pdt. Even then
it possible it could be delayed by an hour or two. Also the coverage
area of gales has been adjusted over the southern waters. The
general consensus is the stronger winds will be further north. The
models are in good agreement with the timing of when the front will
move inshore resulting in winds shifting to the southwest and
decreasing (which should be around 11 am pdt). However the high res
ARW is about 3 hours faster with the timing of the front, but given
the front is still west of the waters, suspect this is an outlier.

Even though winds will be decreasing, a heavy west swell will
develop behind the front, likely peaking during the latter half of
the day Thursday into Thursday evening. Prior to these times, seas
will still remain elevated enough for small craft conditions, thus
have issued a small craft for hazardous seas late this morning
through Sunday night. Please see MWWMFR for more details.

There is good agreement between the wave watch bulletin, ENPwave and
ECMWF showing wave heights peaking between 16 and 18 feet Thursday
afternoon and evening.

A couple of weak weak fronts will move through Thursday through
Friday. High pressure will build south of the waters Friday night
into Saturday, then a stronger front will move into the waters
Saturday night. -Petrucelli

&&

.PREV DISCUSSION... /Issued 556 AM PDT Wed Apr 11 2018/

Updated Marine Discussion Below.

SHORT TERM...Nothing much is going on in our area at the moment,
but a cold front is rapidly approaching the coast, and this will
bring us a variety of weather today. While this front won't be a
huge impact, there are some concerns today and tonight, and I'll
cover those individually and in order of potential impact.

Snow: Snow levels are sitting at around 4500 feet this morning
and will bump up to around 5500 feet as the front approaches
today. Snow will develop over the highest elevations (Crater Lake)
today, and then as snow levels drop this evening, we'll start to
see accumulating snow at lower elevations like Diamond Lake. The
impacts from this snow are tricky due to the strong April sun and
warm sub-surface road temperatures. During daylight hours, snow is
likely to melt off over all but the highest elevations. But as the
sun sets, we expect snow to begin sticking to roads. There will
also likely be a period of heavy snow as the front moves through
right around sunset. A winter weather advisory has been issued for
the Crater/Diamond Lake areas late this afternoon and evening. It
is really the potential for briefly high snow rates during the
frontal passage (rather than overall snow totals) that caused us
to issue the advisory. Elsewhere, significant snow impacts look
unlikely. After the front passes, cool, moist, west flow will keep
light to moderate snow going over the Cascades tonight through
Thursday. Precipitation amounts look most significant Thursday as
it becomes more unstable and convection deepens. However, then
we'll be fighting the strong April sun again, so we probably won't
see much impact.

Wind: While we expect breezy conditions in many areas as the
front moves through, this does not look like a big wind maker.
That said, winds do look at little higher than previously expected
over the usually windy areas in Lake County, and winds were bumped
up there. This has resulted in a wind advisory being issued for
this afternoon and evening.

Rainfall: The moisture stream associated with this front
technically qualifies as an Atmospheric River, and while it will
probably put down 1-2 inches of rain at the south coast and
adjacent mountain ranges, it moves right on through and doesn't
linger too long. As a result, outside of the usual ponding of
water, we don't have any hydrologic concerns today. Rains will be
beneficial elsewhere with, perhaps, a quarter of an inch of rain
in Medford. Given, as stated above, that this front doesn't bring
that much wind, it will be easier for it to rain in Medford than
with the windier fronts (less downsloping). It is a weird quirk
of the Medford area that the stronger the front, the less rain we
get in Medford, and the weaker the front, the more rain we get.
Not always true, but often.

Thunderstorms: Lightning is certainly possible as this front is
dynamic and it brings decent cold air aloft. While I wouldn't be
surprised to see a lightning strike anywhere this afternoon
through Thursday, the best chance for lightning will be over the
waters this evening as the upper trough moves through and then
again on the east side tomorrow as the backside shortwave moves
through. We don't have any concerns about this lightning other
than the usual safety concerns. Just keep an eye/ear to the sky
over the next 36 hours or so. If thunder roars, go indoors.

Cool, showery weather is expected behind the front from tonight
through Thursday and probably all the way into Friday. As usual,
showers will be most prevalent on the west side, but they can't
really be ruled out anywhere. -Wright

LONG TERM...Saturday through Tuesday night...Models are trending
towards a consistent solution beginning this weekend, with a large
scale trough pushing into the Pacific Northwest from the
northeastern Pacific ocean. Unlike the past several systems, this
would not be a fast mover, and will take several days to fully dig
in over southern Oregon and northern California. The first wave
of precipitation is expected to arrive at the coast by Saturday
afternoon, but rain and mountain snow will not fully overspread
the area until Sunday morning. The upper level trough axis arrives
by Sunday night, and remains overhead until Monday afternoon.
This amounts to an extended period of rain for the region, as once
the more steady precipitation ends with the frontal passage,
showers under the trough will continue well into Monday.

Snow levels this weekend will start relatively high, at around 5000
to 6000 feet, but will lower steadily, reaching down to 2500 to
3000 feet by Monday morning. Given the instability expected on
Monday, and these lower snow levels, a stray roll of thunder would
not be out of the question (even in areas of snow) and small hail
or graupel would be very likely with any rain showers.

The region may see a quick break on Tuesday, but this would be
short lived, as the models show another frontal system arriving
Tuesday night into Wednesday. -BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory from 2 PM this afternoon to 8 PM PDT this evening
     for ORZ031.
     Winter Weather Advisory from 2 PM this afternoon to 2 AM PDT
     Thursday above 5000 feet in the for ORZ027-028.

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 11 AM PDT this
     morning for PZZ350-356-376.
     Gale Warning until 11 AM PDT this morning for
     PZZ350-356-370-376.
     Small Craft Advisory for hazardous seas until 5 AM PDT Monday
     for PZZ350-356-370-376.

MAS/TRW/MAP/BPN