FXUS66 KMFR 121742

Area Forecast Discussion
National Weather Service Medford OR
942 AM PST Wed Dec 12 2018

.Updated Aviation Discussion.

.UPDATE...It will be a quiet day across the area, with last
night's front departing to the east and the next front still well
offshore. Other than a few showers along the coast north of Cape
Blanco, there will be little more than fog to concern us over the
next day or so. As the current forecast illustrates this well, and
observations are will in line with expected conditions, no updates
are necessary this morning. For more information on the rest of
the forecast, see the previous discussion below.


.AVIATION...For the 12/18Z TAFs...Stable weather has returned and
moisture from recent rains has produced low clouds and patchy to
areas of fog in portions of the Umpqua Basin, including Roseburg,
Illinois valley and upper end of the Rogue Valley including Grants
Pass. Low clouds and fog should burn off late this morning, but
itr's possible it could linger into the early afternoon hours in the
Illinois Valley and Grants Pass. The TAF at Roseburg shows VFR
ceilings at 20Z but it's possible it could break out sooner. Clear
skies and residual moisture will likely produce significant valley
fog with LIFR conditions likely at KRBG and KMFR tonight. However
incoming high clouds this evening and tonight could delay the onset
of lower conditions and adjustments were made in the TAF's to
reflect this. Incoming high cloud and offshore winds makes fog less
likely, but not impossible, at the coast tonight. For now we'll keep
VFR ceilings through the TAF period at Klamath Falls, but low clouds
and patchy fog cannot be riled out around sunrise. Later shifts will
need to reevaluate. -Petrucelli


.MARINE...Updated 845 AM PST Wednesday 12 December 2018...A
brief but high and very steep swell will arrive this morning and
then drop back to small craft advisory levels this afternoon.
Conditions hazardous to small craft, primarily from seas, will
continue tonight into Thursday. However, south winds will start to
increase Thursday afternoon as a strong front approaches. The front
will bring south gales and associated wind waves Thursday night into
Friday morning. The most recent short range guidance is for most
part in good agreement with the timing of the front. However the NAM
still shows a strong surface low developing late Thursday night into
friday morning which if correct could delay the timing of the front
and also result in stronger south winds, then west winds after the
front moves through. For now we'll lean towards the common solutions
from the GFS and ECMWF. A gale watch remains in effect for the
period. This will also coincide with the arrival of another very
high and powerful swell, the combination of which could produce
extremely high seas up to 25 feet Friday.

Additional strong fronts and periods of powerful west swell are
expected this weekend into early next week. As a result, hazards
from both wind and seas are expected to continue as well. However,
in order to avoid confusion, headlines for subsequent events have
not been issued. Mariners should be prepared for occasional
dangerous weather and sea conditions well into next week.


PREV DISCUSSION... /Issued 310 AM PST Wed Dec 12 2018/


We're watching the trough push eastward tonight. Showers still
continue behind the wave and are visible on radar. Winds are still
rather breezy, especially east of the Cascades this morning.
Klamath Falls briefly reported a wind gust around 35 mph as the
trough axis moved through, so we'll continue the wind advisory to

As for today, high pressure will begin to build with partly
cloudy skies. Temperatures should actually trend warmer as warmer
air begins to push in from the west if you look at 850 mb
temperatures. Thursday is essentially the same story, but the
moist west flow will bring a slight chance for showers along
portions of the coast. The flow is rather zonal and the ridge
begins to flatter on Thursday, so a slight chance for some rain
along the coast seems reasonable.

Friday is the big day as we have a strong low deepening east of
130 W longitude. As this low deepens, it heads farther north into
Canada. A strong cold front is expected to accompany this system
and is easily visible looking at 850mb temperatures. As this front
moves inland, winds will pick up along the coast around Friday
morning. We looked at the potential for a high wind watch along
the coast, but felt that the pressure gradient wasn't strong
enough. We felt that a watch was needed east of the Cascades as
the 700mb flow was showing 70 to 75 knots on some of the models.

The rain and snow for this system is a little underwhelming. We
don't really have that southwest jet connection to Hawaii. Right
now we're forecasting around 1 inch of rain along the coast and
roughly 0.5 inches in the west side valleys with the Rogue valley
seeing less with downslope winds. East of the Cascades will only
see a few tenths of precipitation. Snow as a result will also only
produce 1-4 inches in the mountains.

Overall, the main weather concern will be when the front moves
onshore Friday and pushes eastward across Oregon. Look for strong
winds at higher elevations on Friday.


LONG TERM.../Issued at 3 pm December 11th 2018/

Saturday, Dec 15th through Tuesday night, Dec 19th...
At the beginning of this time period a strong trough will be
deepening in the Gulf of Alaska. This is likely to bring strong
southerly winds in the coastal waters Saturday night as a cold front
approaches the coast. Snow levels are expected to increase into the
5500-6500 foot range due to the strong southerly flow. At this point
it appears the front will bring gales to the coastal waters and some
advisory level southerly winds across some of the forecast area.
Moderate to heavy precipitation is expected along and near the
Coastal Mountains and in Siskiyou County, with light to moderate
amounts elsewhere.

Monday into Tuesday the jet stream will then take aim on the mouth of
Columbia River, with ridging then amplifying along the West Coast.
More precipitation is expected across the area, though details
regarding how much comes inland how fast and how fast snow levels
rise is in question. At this point there is some concern that snow
levels could rise from 6500 feet to 8000 feet Tuesday into Tuesday
night. Thereafter, the GFS indicates the front weakening before it
comes in whereas the ECMWF pushes a strong front coming in with a
sharp drop in snow levels Thursday. BTL


OR...High Surf Advisory until 4 AM PST Saturday for ORZ021-022.
High Wind Watch from Friday morning through Friday evening for


Pacific Coastal Waters...Gale Watch from late Thursday night through Friday morning for
Small Craft Advisory until 4 AM PST Friday for
Hazardous Seas Warning until 10 AM PST this morning for

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