Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 251326 CCA
AFDMFR

Area Forecast Discussion...CORRECTED
National Weather Service Medford OR
525 AM PST Sun Feb 25 2018

.Updated aviation and marine discussions.

.DISCUSSION...The next frontal system in this long series of late
winter
storms is on its way, located off the coast of Washington early
this morning. This system features more moisture than the previous
one, and therefore will impact mountain passes more due to higher
snowfall rates and higher overall snow accumulations. Recent
model runs have held pretty steady with cold air and expected
precipitation compared to previous runs, so we have not made any
changes to the forecast snowfall or snow levels.

Steady, frontal precipitation arrives this morning, peaks in
intensity this afternoon and evening, then tapers off from
northwest to southeast behind the front and becomes showery Monday
morning. There are nuances to this forecast that are worth
noting. These include the expected 1-2 inch per hour snowfall
rates in the High Cascades between 1pm and 10pm today/tonight.
Given the cold air in place, precipitation will be all snow in the
mountains above 3000 feet and road surface temperatures should be
freezing through the event. The second note is that snow levels
will fall sharply this evening over lower passes along I-5 between
Grants Pass and Roseburg with the frontal passage. Timing for
this changeover from moderate-heavy rain to snow in these
locations looks to be between 7pm and 10pm. Given this is
occurring after sunset, road temperatures should be near freezing
and a quick burst of accumulating snow is likely before snow
becomes less steady and more showery. There is more uncertainty
for lower elevations in terms of any changeover to snowfall and
how much snow is expected, but it is possible that there could be
brief early morning snow impacts in West Side valleys or just
above the valleys. With that said, nothing jumps out in the data
this morning to warrant an Advisory issuance for those low
elevation locations. Model data depicts widespread impacts to
portions of Northern California and areas east of the Cascades
with this system, especially compared to yesterday`s weather. For
all the expected winter impacts, Winter Storm Warnings continue
for the Cascades, areas just east of the Cascades, Siskiyous, and
Western Siskiyou County. Winter Weather Advisories continue for
other areas, generally above 2000 feet.

On a short note about climate, average snow depth for late
February at Crater Lake National Park is 108 inches, and current
snow depth is 65 inches. While we probably won`t be catching that
number this season, we will be getting closer to it by the end of
this coming weekend, and that`s good news for area snowpack. When
taking into account how much water`s in the area snowpack, we`re
only about 40 percent of normal for Southern Oregon, but again,
we`ll be increasing that number through the week and into the
weekend.

Back to the weather, a 36 to 48 hour break in significant
precipitation is expected from Monday afternoon through Wednesday
morning. Both the operational EC and GFS show bombogenesis
occurring off the coast of British Columbia from Wednesday morning
through Wednesday night (low pressure deepening by 31mb in 24
hours). Admittedly, many GEFS members don`t show such strong
deepening, but it`s safe to say there`s potential for a very
dynamic system to affect the area Wednesday through Friday as an
upper low remains over the NE Pacific Waters and sends shortwave
energy into the forecast area. The previous long-term discussion
highlights the expected evolution of the weather pattern late next
week, due to this upper low. SK

Previous long-term discussion...Wednesday through
Saturday...Models continue to show good agreement with active
pattern over the Pacific Northwest. An upper trough and low
pressure system is forecast to shift down the Pacific Northwest
coast Wednesday and Wednesday night. With the low moving into the
Pacific Northwest, there is the potential for a strong cold front
to move inland and impact the area late Wednesday into Wednesday
night. This front shifts east of the area on Thursday with an
upper trough moving over the area and bringing a showery pattern.
Then a cold and showery pattern continues Friday into Saturday.

On Wednesday, there is general agreement that a relatively strong
and moist frontal system will approach the Pacific Northwest coast
then move inland Wednesday night into Thursday morning. Models
indicate that this front may bring abundant moisture into the area
late Wednesday through early Thursday. Areas of moderate to heavy
rain and snow are possible, especially from the Cascades west. Snow
levels are expected around 3000 to 4000 feet with this system. So,
impacts from accumulating snow are possible on higher mountain
passes and as well as for the Mount Shasta area. East of the
Cascades, expect light to moderate precipitation, mostly falling as
snow. Also models indicate that a strong mid level 700 mb jet of
around 45 to 55 kt may move into the area ahead of the front. This
jet aloft combined with an increasing surface pressure gradient will
bring the potential for areas of strong gusty winds to develop,
mainly in the Shasta Valley and from the Cascades east. However,
due to variability in the details on the strength and track of
this low pressure system, there is lower confidence on the exact
details and impacts with this storm. Will need to monitor this
system with future model runs.

Thursday into Friday, an upper trough is forecast to gradually move
inland over the region and bring widespread showers. Additional
disturbances moving around the trough and inland may bring enhanced
showers during this period. Snow levels will lower late Thursday
into Friday and are expected to lower from 2500 to 3500 feet down to
1000 to 2000 feet on Friday.

Friday night into Saturday, the trough will gradually shift eastward
and a shortwave ridge will build into the area resulting in
decreasing showers. There is significant variability on the timing
of this ridge building into the area and decreasing shower
activity. May continue to see showers across the area on Saturday
with diminishing activity Saturday evening/night. However, given
the upper ridge is weak and models show another low pressure
system developing off the coast, expect only a brief break in the
active pattern next weekend. -CC

&&

.AVIATION...25/12Z TAF CYCLE...A cold front will move
through the area today and this evening. Expect the frontal passage
through OTH at 22z, RBG at 00z, MFR at 03z, and LMT at 04z.
Precipitation will spread across the region ahead of this front with
snow expected for LMT by late afternoon. Periods of IFR and moderate
snow may occur at LMT this evening. Snow levels will drop everywhere
behind the front and RBG and MFR could see snow showers by late
Sunday night. Sandler

&&

.MARINE...Updated 520 AM PST Sunday 25 Feb 2018...A cold front
will drop from the north this afternoon and evening. Winds will
increase ahead of this front, bringing stronger southwest to west
winds and wind waves around 5 to 8 feet. In addition, heavy
northwest swell will continue today...thus expect steep seas to
continue. Winds shift to northerly behind the front tonight into
Monday and wind waves diminish...however an even steeper northwest
swell will approach, causing very steep combined seas beginning late
Sunday night. Seas will gradually subside Tuesday into Wednesday.
Then, another round of steep heavy swell will come in late Wednesday
into Thursday, with possible gales as well.
Sandler

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Weather Advisory from 4 PM this afternoon to 4 AM PST
     Monday for ORZ029>031.
     Winter Storm Warning from 4 PM this afternoon to 4 AM PST Monday
     for ORZ030.
     Winter Weather Advisory from 4 PM this afternoon to 4 AM PST
     Monday above 2000 feet in the for ORZ023-024-026.
     Winter Storm Warning until 4 AM PST Monday above 2500 feet in
     the for ORZ025.
     Winter Storm Warning from 4 PM this afternoon to 4 AM PST Monday
     for ORZ028.
     Winter Weather Advisory until 4 AM PST Monday below 2500 feet in
     the for ORZ025.
     Winter Storm Warning from 10 AM this morning to 4 AM PST Monday
     for ORZ027.
     Winter Storm Warning from 4 PM this afternoon to 4 AM PST Monday
     for ORZ029.

CA...Winter Weather Advisory from 4 PM this afternoon to 4 AM PST
     Monday for CAZ081>085.
     Winter Storm Warning from 4 PM this afternoon to 4 AM PST Monday
     above 2000 feet in the for CAZ080.

Pacific Coastal Waters...Small Craft Advisory until 4 PM PST Tuesday
for      PZZ350-356-370-376.

$$

NSK/CC/MSC



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