Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 190521
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
920 PM PST Wed Jan 18 2017

...Heavy, Wet Snow Expected over Siskiyou County and southern
Oregon Cascades including Siskiyou Summit Tonight...

...Discussion...Evening update. Snow continues in Northern
California with decreasing precip rates northward, and will not
update this portion of the forecast. The South Fork of the Coquille
at Powers spike quickly to near 11 feet, indicating flooding at
Myrtle Point and the Coquille at Coquille and have issued flood
warnings for those points. The Rogue at Grants Pass was also
climbing higher and the River Forecast Center adjusted the rogue
at Agness to go briefly above flood as well. Sven

.SHORT TERM...Tonight through Friday night...Ground truth
observations from spotters and web cameras are continuing to
report heavy snowfall across the Cascades and Siskiyous above 3500
feet. This is prior to the frontal passage, which satellite
imagery indicates that it is still just offshore. The lower than
forecast snow levels are largely due to the heavy precipitation
which are lowering snow levels. Observations at Mt. Shasta city
are indicative of very heavy snowfall continuing to fall, and at the
writing of this discussion, CalTrans has closed Northbound
Interstate 5 at Dunsmuir for heavy snowfall at Mount Shasta City.
With the observed snowfall, have expanded the Winter Storm Warnings
and Winter Weather Advisories to include Siskiyou summit and much of
the Cascades. Additionally, we will need to take a closer look as
the situation unfolds near Chemult, as highway 97 could also see
travel difficulties due to snow.

In addition to the plethora of winter weather headlines, strong
winds are continuing around the normal windier spots. Although
the observations are showing diminishing winds and the gradients
are starting to ease, pressure is still dropping east of the
Cascades, and our advisory and warning areas could see another
round brief strong wind gusts. Therefore, have decided to leave
the various wind advisories and high wind warnings to continue
through the evening (and overnight for the Eastside).

In addition to the heavy snows, heavy rain is still coming down
at the coast and western Siskiyou County. Have left in the
thunderstorm chances in the coastal waters and for the immediate
coast, but offno thunderstorms have actually taken shape with the
exception of the cell around 130W from earlier today. The rainfall
will start to lighten up later this evening before becoming
showery in nature tomorrow morning. There will be a relative
break in the precipitation on Thursday. The exception to this is
for Mt. Shasta area, which will see precipitation rates and
amounts lightening up, but still going due to upslope flow for
most of the day.

The next system in this very active pattern will arrive Thursday
night into Friday. The Friday system is not expected to bring as
much rain, snow, and wind to the area as today`s system. That
being said, the setup is such that upslope flow near Mount Shasta
could generate an additional foot of snow with this system.

Furthermore, winds could also become breezy in the higher
elevations near the Rogue Valley such as Ashland and for the
Cascades and east, but winds will remain below advisory levels.

The general thought process regarding the forecast for the winds
and precipitation with the Friday system is a blend of the ECMWF
and the NAM to the previous forecast while adding additional
upslope effects near the Mt. Shasta area. Friday night will bring
the next short break as this active pattern continues in the long
term. -Schaaf

.LONG TERM....Saturday through Wednesday. Yet another strong
frontal system will affect our area over the weekend. This storm
may be the strongest in the sequence in terms of winds, as models
all show deepening low pressure recurving along the PacNW coast
Saturday night into Sunday. There are differences between the
models regarding timing, strength, and track of this low pressure,
all of which would have a big impact on the weather we ultimately
experience. Nevertheless, all are impressive enough to warrant
keeping an eye on this storm.

Snow levels will be relatively low with this storm, and dynamics may
be quite high, thus the biggest impacts are expected to be from snow
and wind. Snow is expected to be heaviest over the NorCal mountains,
especially around Mt. Shasta City, where more than a foot could
easily fall. Strong winds will affect the usual spots (waters,
coast, east side, Shasta Valley), but we may also get winds down
into the west side valleys Sunday morning.

This is not to say rain won`t be an issue, because it will be heavy
along the south coast and adjacent mountains. However, low snow
levels should lock much of this liquid up in the mountains, and QPF
amounts at the coast are not all that unusual for a winter storm.

Showers may linger into early next week, and some models hint at
another wave of low pressure forming in conjunction with the
departing upper wave Monday night, but confidence with that is low.
Beyond this, a ridge is expected to build, which should give us a
much needed break from the inclement weather. -Wright


&&

.AVIATION...18/18Z TAF CYCLE...Conditions on the west side will
be a mix of VFR and MVFR in light to moderate rain as a front
moves across the area this afternoon. Occasional IFR conditions
are possible during the heaviest precipitation. IFR conditions are
more likely where precipitation falls as snow which includes the
mountains and portions of the east side tonight as snow levels
fall behind the front. Gusty south winds will continue all areas
until the front passes. -Wright


&&

.MARINE...Updated 300 PM PST Wednesday 18 January 2017...Strong
south winds will diminish late this afternoon from northwest to
southeast as a front moves onshore. Seas will take longer to
subside and they will remain hazardous through Thursday. Winds
will be relatively light Thursday, but the next front will bring
another period of gale to occasionally storm force winds and very
steep seas Thursday night into Friday morning. All major models
are suggesting gales now, and confidence has grown with this
upcoming gale event. Behind this front, long period extremely high
west swell of 25 to 30 feet is expected late Friday into Saturday.
Confidence is high for this as this is the third day in a row that
models have shown this height range of seas.

Models are suggesting a period of high pressure and lower winds and
seas by the middle of next week. CC/SK


&&

.PREV DISCUSSION... /Issued 932 AM PST Wed Jan 18 2017/

UPDATE...Heavy, Wet Snow Expected in the Mount Shasta Region
Today into Tonight...

After taking a closer look at Satellite and lightning
observations, it appears as if there is lightning around 130
degrees W. The NAM is resolving this feature relatively well, and
have added a slight chance of thunder to the coastal waters
beginning at 1 PM. If the NAM is to be believed at face value, we
could end up with a squall line off our shores this afternoon.
This will need to be closely monitored as the day progresses.

Otherwise, satellite observations show that the atmospheric river
(AR) is a bit messier than one would expect in a classic sense.
This is likely due to the AR combining with a low pressure system
centered around 47 degrees N and 138 degrees W. In addition, water
vapor satellite imagery is showing an intermediary shortwave that
is moving into southern Oregon and northern California bringing
more precipitation. Radar imagery seems to confirm this trend
today. There may be a short break in the precipitation in Oregon
before the main front comes in today. Northern California, on the
other hand, will see precipitation most of the day--if not all of
it-- since the AR will continue to affect it.

The other big news to note is that most of the high temperatures
expected for today have already occurred early this morning. The
Medford Airport reported a high temperature of 54 degrees at 4:00
AM. Temperatures may rise somewhat today, but are likely not going
to reach the highs already seen earlier this morning. Regardless
of these new features which were not well-resolved by the models,
the forecast remains on track. -Schaaf

PREV DISCUSSION... /Issued 356 AM PST Wed Jan 18 2017/

.Heavy, Wet Snow Expected in the Mount Shasta Region Today
into Tonight...

DISCUSSION...A strong frontal system will move onshore today
bringing with it plenty of hazardous weather. Strong upslope flow
into the Mount Shasta region of Siskiyou County will begin in
earnest this morning, leading to very heavy, wet snow and causing
significant travel impacts along Interstate 5 from Weed southward
to Dunsmuir. We have already received reports of snow falling in
and around Mount Shasta City at this early hour, so the
expectation is when the stronger south winds (60-65kt) at 700 mb
arrive later this morning, precipitation will become steadier and
heavier allowing snow levels to fall all the way down to 2500 feet
south of Weed. Snowfall rates could exceed 1" per hour at times
this afternoon into early this evening before snow tapers off
later this evening and overnight. We have increased snow amounts
to 12-18" since we are expecting most, if not all, of the
precipitation to fall as snow. Up to 2 feet of snow could
accumulate near Snowman Summit along highway 89. Needless to say,
travel will be extremely hazardous in this area today, so please
avoid this area if you can. Snow will also fall over the
Siskiyous and Cascades - please see WSWMFR for all the details.

Another element to this storm is wind. We have observed wind
gusts this morning along the coast in excess of 80 mph (Flynn
Prairie RAWS at 1543 feet had a gust to 86 mph around 09z). Many
other stations have reported gusts of 55 to 65 mph. Strong winds
are also occurring in the Shasta Valley near Weed. The high winds
will also impact the higher terrain and much of the east side
through this evening. Wind headlines can be viewed at NPWMFR.
Expect winds to diminish first at the coast this afternoon and
overnight over the east side as the strong upper level winds push
to the east of the forecast area.

Meanwhile, a band of very heavy precipitation associated with a
deep moist plume extends from Douglas and Coos counties
southwestward offshore. Peak rainfall rates in this band are
between 0.50 and 0.75 of an inch per hour. Have maintained the
areal flood watch for Curry, western Josephine and far western
Siskiyou Counties with the expectation there will be at least some
urban and small stream/creek flooding associated with this band of
rain this morning through late this afternoon/evening.

After today and tonight`s storm, we get a brief break Thursday.
-Spilde

Thursday night through Tuesday...A very active pattern continues.
Another strong frontal system will move through our area Thursday
night and Friday bringing heavy precipitation and strong winds.
Snow levels will be lower than preceding systems (around 3500
feet), so heavy snow is much more likely to produce impacts in the
mountains this time around. While wind, heavy rain, and snow
headlines are almost a given with this system, we will wait for
the current system to run its course before hoisting another large
set of hazard products. The moisture plume will be aimed at us
through the day Friday and into Saturday, Thus the coast will
continue to see heavy rainfall, with snow over the mountains.

There may be a brief break Saturday before the next system barrels
in Saturday night and Sunday. The models are in decent agreement
that this system could be a whopper in terms of wind with all
showing strong and deepening low pressure moving inside 130W. The
strongest by far is the Canadian, and it shows low pressure
exploding down to 965 millibars as it curves north just inside 130W
roughly off of Coos Bay. The other models have more of a double-
barreled low pressure system, which we`ve seen sap a lot of energy
out of our recurving systems before. This will bear watching as the
stronger solutions could produce a significant wind storm. We won`t
get too excited just yet, but I did ramp up winds and pops for
Sunday. Sunday`s system will probably be the last really strong one
for a while, but models do suggest cool, damp weather continuing
into next week. -Wright

AVIATION...18/12Z TAF CYCLE...Conditions are deteriorating at the
coast this morning as winds and rain increase, and MVFR/IFR ceilings
develop. At the moment,  winds are too strong to support fog from
forming at the coast and well to dry for any low level visibility
restrictions in the west side valleys.

For the remainder of the day,  there is plenty of instability and
moisture feeding into the area, so widespread rain showers are
expected at all terminals and east of the Cascades. Eventually, LMT
will see a changeover to snow Wednesday evening as cooler air moves
in. Winds will continue as the low approaches the coast today , yet
diminish in the late evening into early morning as it lifts up the
coast.

CIGS during Wednesday will remain in MVFR categories and obscure
mountains.  With the winds diminishing, and ample low level
moisture, fog will come back into the forecast Wednesday night.
However, low level clouds at some sites may prevent fog from
forming, so kept it out of the forecast at MFR and LMT for this TAF
cycle.

-Smith

MARINE...Updated 300 AM PST Wednesday 18 January 2017...Storm force
winds and very high and very steep seas  will continue this morning
into early this afternoon. Combined southerly wind waves and west
swell will bring very steep 18 to 26 ft seas this morning.
Individual waves near forty feet have been observed at buoy 229 and
when seas peak individual waves in excess of 50 feet are possible.
South winds will gradually lower from west to east across the
coastal waters this afternoon. However, areas of gale force winds
will continue to impact the waters this afternoon within 20 nm of
shore with very steep hazardous seas and small craft winds will
impact areas beyond 20 nm from shore. Winds will lower below small
craft levels this evening. Seas will gradually lower late this
afternoon but remain very steep into early this evening.  Tonight
and Thursday, expect steep seas around 14 to 16 feet dominated by
13 seconds west swell.

Another strong front is expected Thursday night and Friday morning.
This front may bring areas of strong gales, with best chances for
gales across the waters beyond 10 nm from shore and near Cape
Blanco. Additionally very high long period west swell will build
into the waters behind this front late Friday and Saturday, peaking
in the 25 to 30 foot range at a period of 17 to 18 seconds Friday
night and Saturday. This will bring extremely dangerous conditions
to the waters, bars, and coastline as these very high and very steep
seas build into the waters.

The active pattern continues into the weekend with a third strong
frontal system bringing the potential for additional strong winds
and very high and very steep seas. -CC

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Warning until 4 AM PST Thursday for ORZ030-031.
     Wind Advisory until 4 AM PST Thursday for ORZ029>031.
     Winter Storm Warning until 4 AM PST Thursday for ORZ027-028.
     High Wind Warning until 4 PM PST this afternoon for ORZ021-022.
     Flood Watch through late tonight for ORZ022-024.
     High Surf Advisory until 7 PM PST this evening for ORZ021-022.
     Wind Advisory until 6 PM PST this evening for ORZ026.

CA...High Wind Warning until 4 AM PST Thursday for CAZ085.
     Wind Advisory until 4 AM PST Thursday for CAZ084-085.
     Flood Watch through late tonight for CAZ080.
     Winter Storm Warning until 4 AM PST Thursday for CAZ082-083.
     Winter Storm Warning until 4 AM PST Thursday for CAZ080.
     High Wind Warning until 7 PM PST this evening for CAZ081.
     Winter Weather Advisory until 4 AM PST Thursday for CAZ081.
     Wind Advisory until 7 PM PST this evening for CAZ081.

Pacific Coastal Waters...Gale Watch from Thursday afternoon through
late Thursday night      for PZZ350-356-370-376.      Small Craft
Advisory for hazardous seas until 4 AM PST Friday      for PZZ350-
356-370-376.      Gale Warning until 4 PM PST this afternoon for
 PZZ350-356-370-376.      Hazardous Seas Warning until 10 PM PST
this evening for      PZZ350-356-370-376.      Hazardous Seas
Warning from 4 AM Friday to 4 AM PST Sunday for      PZZ350-356-370-
376.

$$

BMS/TRW/NSK



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