Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 161625
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
825 AM PST Mon Jan 16 2017

.UPDATE...Fog in most areas this morning is not as widespread or
as thick as it has been in previous days, but patches in the
immediate vicinity of Medford and Grants Pass are dense. As with
yesterday, most areas should burn off by this afternoon, but some
areas, namely the Umpqua, could persist well into the afternoon
and even the evening.

Over the weekend, we have watched the midweek system transform
from a long duration heavy rain event into a shorter duration wind
and rain event with heavy snow along the Cascade crest and in the
Mount Shasta area. With the change in expected liquid precipitation,
roughly cut in half from just 3 or 4 days ago, hydrological
concerns have diminished greatly, and the current forecast does
not produce floods along any river within the forecast area. That
is not to say some local high water is not possible, but
widepsread issues are no longer expected. Instead, will be
focusing on snow and wind during today`s shift, with numerous
headlines likely by the end of the day.

For more information, see the previous discussion below. -BPN

&&

.AVIATION...17/12Z TAF CYCLE...Conditions through Tuesday morning
are expected to be very similar to those observed in the previous
24 hours. Areas of IFR/LIFR CIG/VIS in low clouds and freezing fog
over the West Side valleys will expand in the same areas overnight
through Monday morning. The Roseburg and Grants Pass areas
probably will not improve beyond IFR this afternoon once again, as
the inversion will become stronger. KMFR will be affected by the
lower conditions through this morning. Over the remainder of the
area (the coast and the east side), VFR conditions will prevail
through Monday afternoon, with some increase in mid and upper
level cloudiness through the day into Monday evening.


&&

.MARINE...Updated 830 AM PST Monday 16 January 2017...Seas will
briefly diminish today, but south winds will increase tonight as the
first in a series of strong fronts approaches. Gales are likely
Tuesday afternoon/evening through Wednesday and seas are likely to
peak in the 18 to 28 foot range. A period of storms force winds is
possible due to the formation of a low level coastal jet stream
Wednesday. West swell late in the week could reach or exceed 25
feet. Over several runs, the models continue to trend higher with
the swell Thursday into Friday, but the latest run lowered the peak
swell from 33 feet to 28 feet at buoy 15. Expect some more ups and
downs in future runs, but it still looks to be a very heavy swell
event.  It all depends on the strength of Gulf of Alaska storms
during the coming week.


&&

.PREV DISCUSSION... /Issued 347 AM PST Mon Jan 16 2017/

DISCUSSION...A major pattern change is on the way this week as the
quiet ridge pattern transitions toward another active, high-impact
series of storms. The evolution of the next storm system has been
an interesting one. Models have essentially, run by run, deepened
an upper trough offshore and increased the amplitude of the
upstream ridge. This has delayed any onset of heavy rain at the
coast, increased the southerly flow and chances for high winds at
the coast and inland areas, and increased the chance for heavy
snow in the Mount Shasta area.

Monday will remain dry for most places with patchy freezing fog in
most West Side Valleys, with greater coverage of fog near Grants
Pass and in the Illinois Valley. Fog and low clouds should lift late
morning or early afternoon, with similar timing to yesterday.

A chance for rain will be introduced at the coast on Tuesday with
most other areas remaining dry. Air Stagnation Advisory concerns
should diminish Tuesday as surface temperatures warm and mixing
winds increase.

South to north pressure gradients have steadily increased in recent
operational model runs, with the NAM/GFS/EC now generally showing
-10mb OTH-ACV gradients and -10 to -13mb MFR-RDD gradients. This,
along with deepening low pressure centers offshore and 80kt 850mb
SSW flow, has raised our eyebrows sufficiently for us to issue a
High Wind Watch at NPWMFR for the coast late Tuesday into
Wednesday. Confidence is coastal strong winds is not high yet
because of the very southerly nature of the winds and potential
for the highest winds to remain offshore. The next several runs of
models will hopefully help build confidence in a solution.

Windy conditions are expected in other areas, focused primarily over
the Shasta Valley and areas east of the Cascades. Additionally, SSE
flow over the Rogue Valley with 850mb winds of 50kt per the NAM12
suggests the potential for at least wind advisory conditions for
Medford, Ashland, and surrounding areas. Since the models have
continually evolved towards this windier scenario, it will be
helpful to see if this trend continues.

Models and Integrated Vapor Transport (IVT) tools still show a
period of heavy rain at the coast and just inland Wednesday, and
areal flooding remains a possibility, especially if rainfall rates
near the half inch per hour threshold. At this point, the best
chance for this looks like Wednesday late morning and afternoon. Any
lingering snow on the ground in low to mid elevations will melt at
least partially and will add to the possibility for small stream and
urban flooding. Specifically, we`re thinking about the Illinois
Valley. Additionally, latest river models show the Coquille River at
Coquille river potentially reaching flood stage on Thursday.
Confidence is not high enough yet to issue a Flood Watch before
running some more river models with latest forecast precipitation.

Last but not least, the latest suite of models and forecast tools
show the possibility for a foot of snow at Mount Shasta City from
Wednesday morning through Wednesday night. Strong southerly flow
could bring down snow levels sufficiently to impact roadways in the
area (I-5 and Highway 89) with moderate to heavy snow. A Winter
Storm Watch will be considered in future shifts if model trends
continue to point to this impact.

The previous discussion`s long-term portion follows...

Thursday through Sunday...The EC/GFS/DGEX operational solutions
are in fairly good agreement through the extended period, with
some minor differences in timing around Saturday. A fairly fast
progressive pattern will prevail through the period. It will
remain relatively wet over the Medford CWA with periods of gusty
winds.

A long wave trough will move onshore along the west coast Thursday.
This will be followed by a brief period of short wave ridging
Thursday night, which will then be followed by a long wave trough
moving onshore Friday evening.

Post-frontal showers will gradually diminish Thursday, then the
upstream upper trough will push a surface cold front onshore Friday
morning.  This front will not be particularly wet and windy, but the
south coast, south Coast Range, and western Siskiyou county will get
a significant amount of precipitation out of this system.

Post-frontal showers will continue into Friday night, when a weaker
secondary front will move onshore.

A broad flat ridge will build into the west coast Saturday, followed
by another trough on Sunday. The next front will move onshore
Saturday night, and once again the heaviest precipitation will occur
along the south coast, south Coast Range, and western Siskiyou
county. Post frontal showers will then persist through Sunday night.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Wind Watch from Tuesday afternoon through Wednesday
     afternoon for ORZ021-022.
     Air Stagnation Advisory until 10 AM PST Tuesday for ORZ023.
     Air Stagnation Advisory until 10 AM PST Tuesday for ORZ029.
     Air Stagnation Advisory until 10 AM PST Tuesday for ORZ024-026.

CA...None.

Pacific Coastal Waters...Gale Watch from Wednesday afternoon
     through Wednesday evening for PZZ350-356-370-376.
     Storm Watch from Tuesday evening through Wednesday
     afternoon for PZZ350-356-370-376.
     Gale Warning from 10 AM Tuesday to 4 PM PST Wednesday
     afternoon for PZZ350-356-370-376.

$$

BPN/JRS/NSK/SBN



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