Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 151646
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
846 AM PST Fri Dec 15 2017

.UPDATE...Current satellite and radar observations are showing
lots of clouds over the Pacific Northwest, but limited rainfall in
southern Oregon. This will change as the day progresses and the
front dips southward. Will allow the air stagnation to expire on
time this morning. The overall forecast looks mainly on track,
and have not made any changes. Have attached the previous
discussion which details the forecast ahead.
-Schaaf

&&

.MARINE...Updated 820 AM PST Friday 15 Dec 2017...A cold front
will bring increasing north winds into this evening. A strong
surface pressure gradient will cause stronger winds south of Cape
Blanco...with strongest winds south of Gold Beach. Elsewhere,
winds will be of moderate strength across the waters this
afternoon and evening. Winds weaken slightly overnight. High
pressure offshore and a coastal thermal trough will develop
Saturday, causing gusty north winds south of Cape Blanco. The
winds will be strongest south of Gold Beach late Saturday night
into early Sunday morning. North winds will subside Sunday into
Monday ahead of a stronger cold front that is due to arrive late
Tuesday. Southerly winds will develop Tuesday with seas becoming
high and rising into Wednesday. But, confidence remains low to
moderate regarding the track and strength of the front, and thus
also the strength of winds. -DW/MSC

&&

.PREV DISCUSSION... /Issued 704 AM PST Fri Dec 15 2017/

DISCUSSION...

Latest infrared satellite imagery shows the front is on it`s way
and should be on shore around 18z or so. Nothing really has
changed from previous model runs as this trough is expected to
move through rather quickly and destabilize the air mass. In
general, most areas in Oregon should see a few hundreths of
precipitation, whereas isolated areas on the Cascades, such as
Crater Lake, could see up to .2 inches of precipitation, which
should translate to around 1 - 2 inches of snow for elevations
above 6500 feet.

The other component, which has probably been talked about in
previous discussions is a higher mixing height during Friday
evening. This should `air` out the valley and allow the pollution
to move eastward with the trough. The rain should help clean
things up a bit too.

The day shift also noted that cold air trapped in the valleys
ahead of the cold front could produce isolated freezing rain in
some areas. My argument against that is that most of the
precipitation is expected to hit the valleys around the 21Z to 00z
time range. By then, temperatures should be on their way up even
with the fog. Roseburg has been hitting 36 degrees around 21Z for
the last couple of days.

After this trough, a dirty ridge builds in with a rather active
pattern to the north near Washington through the weekend. The
mixing should be very good Saturday, but deteriorate Sunday. Some
thermal troughing can be seen on Sunday, and the Chetco effect
will be ongoing with the off shore winds. So highs near 60 can be
expected near the Brookings area Sunday and Monday afternoon.

Another system is expected around Wednesday. the ECMWF and GFS
are in pretty good agreement on the wave location and progression
during this time with a few minor disagreements with precipitation
intensity, which is standard this far out. This trough is a bit
wetter and and cooler than what we`ve seen for the last month. In
general snow levels during the cold front and trough passage will
be around 4500 feet, which is producing roughly 3-6 inches of snow
along the Cascades and 1-2 inches east of the Cascades at this
time. After front passage, snow levels will fall to 2500 feet.
Also, 700mb winds on the east side are showing 70 knots near
Summer Lake, so a wind advisory is possible if the system keeps
it`s forecasted strength.

-Smith

AVIATION...15/12Z TAF CYCLE...High clouds ahead of a cold front
have helped to limit LIFR this morning to portions of the Umpqua
Valley, including KRBG, the lower portions of the Rogue Valley
around Grants Pass, and the Illinois Valley. Conditions are VFR
elsewhere.

The front will bring rain and mainly MVFR to the coast around 17Z.
These conditions will then spread inland through the afternoon and
early evening with areas of mountain obscuration developing. By late
evening, post-frontal showers and snow showers will taper off.
Conditions will be mainly VFR late tonight into Saturday morning,
though LIFR/IFR is likely to return to the Umpqua Valley and the
Rogue Valley with low clouds rather than fog as the primary concern.
-DW

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 10 AM PST this morning for
     ORZ023-024-026-029>031.

CA...None.

Pacific Coastal Waters...Small Craft Advisory from 1 PM this afternoon to 4 AM PST Sunday
     for PZZ356-376.

$$

BMS/MSC/CZS



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