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fxus66 kmfr 201825 

Area forecast discussion
National Weather Service Medford or
1025 am PST Wed Nov 20 2019

Updated aviation section

Update...updated the winds south of Summer Lake and west of
Paisley, in the higher terrain. The north-northeast wind is
expected to bring gusty, drier conditions to the higher terrain.
Also had to update winds in western Siskiyou County, with some of
the weather stations at higher elevations showing winds picking
up, around 14 mph, with gusts to 30.

In the valleys, fog has settled in after upper clouds cleared
enough for some rapid cooling. A few weather stations in Josephine
and Jackson County reported freezing temperatures prior to fog
formation. There could have been a few places around the valleys
where freezing fog occured. Expecting fog in the rogue and
Illinois valleys to lift in a few hours as winds begin to become
a little breezy. In the Umpqua basin the fog is likely to take
longer to dissipate.

Will be assessing the need to bring back the air stagnation
advisory for areas in Jackson and Josephine County for the Friday
and weekend time period. Today, the mixing heights and winds
should be well above the minimum criteria of 1000 ft mixing
height and transport winds less than five knot. Thursday should
remain well mixed, at least during the afternoon, but will mark a
brief transition period back to calm conditions in southern
Oregon. After a quick check of the mfr bufr sounding, this may not
be needed. More to follow in the afternoon afd. -Miles

Aviation...for the 20/18z tafs...MVFR ceilings are impacting
Roseburg this morning with LIFR fog at Medford. The low clouds and
fog will gradually erode by 20-21z with VFR thereafter. Strong
northeast through east winds will impact the east side and the
higher terrain, including Klamath Falls, with gusts to 35 kt today.
An Airport weather warning is out for Klamath Falls for these
expected winds. Additionally, low-level wind shear is possible for
the next hour or so where surface winds are relatively light (more
sheltered valleys).

East-northeast winds will remain gusty at the ridges overnight into
Thursday morning, but as the air mass dries out and the pressre
gradient relaxes, a developing subsidence inversion will cause winds
to decouple and result in lighter winds in the valleys. Despite the
drying, there still may be enough moisture in the west side valleys
for some LIFR fog/stratus near sunrise in Medford/Roseburg. VFR will
prevail elsewhere. -Spilde


Previous discussion... /issued 319 am PST Wed Nov 20 2019/

Discussion...current satellite imagery are showing some clouds
over southern Oregon and northern California. That being said,
there is some clearing occurring as well. This could lead to fog
formation as it already has at Coos Bay, and possibly across the
rouge valley later this morning (around sunrise). The evening
shift brought temperatures up across the west side valleys, and
with kmfr reporting a dewpoint temperature of 38 degrees f, the
changes seemed reasonable.

Our airmass is overall a bit drier, and with north to northeast
winds across our area, things will begin to dry out today. Winds
east of the Cascades in our breezier spots may approach that of
Wind Advisory, it is tough to get 45 mph gusts out of a pattern
like this. Thus, will keep winds just below criteria--albeit they
will still be breezy.

This dry pattern with our area of high pressure will continue
through the end of the week. This will continue our dry
conditions. The inversion will form each morning in the valleys
and mixing heights will fall to near the surface overnight. (The
upper air data is showing a very shallow inversion this morning.)
That being said, expect the inversions to break at some point each
day with some modest mixing heights. This modest mixing should be
enough to preclude air stagnation, but will defer to the day
shift to take a look as well.

This weekend, expect a shift in the pattern as a series of
shortwave disturbances will move through southern Oregon and
northern California. These disturbances will bring increased
shower chances next week. Then on Tuesday, models are in
relatively good agreement regarding a strong frontal system with a
ample precipitation. Moisture transport (ivt) is modest at this
point, so it doesn't look incredibly strong; but have increased
precipitation chances and precipitation amounts from the previous
shift since all models and ensembles are in agreement that
something will occur.

The other interesting thing to note is snow levels. Right now,
forecast snow levels remain around 4000 feet; but the same models
and ensembles want to bring overnight low temperatures to near
freezing at the valley floors next week as the front passes
through. At this point, believe that the 4000 foot snow level is
much more reasonable and have trended the west side valley
overnight temperatures warmer. Currently, the bulk of the
precipitation is forecast to arrive during the day time, which
will help things. Regardless, the forecast will need to be
monitored as several travel impacts are expected along the mid to
higher passes (siskiyou Summit, Lake of the Woods, Cedar pass)for
Thanksgiving week. Additionally, as model data becomes clearer,
those snow levels could fall even farther for some of the lower
passes. -Schaaf

Aviation...for the 20/12z tafs...from the Cascades west...
widespread MVFR cigs and local IFR cigs/vsbys will present this
morning, clearing to VFR by Wednesday afternoon. VFR conditions will
then prevail through Wednesday night.

East of the Cascades...local MVFR cigs/vsbys in low clouds and fog
through mid-morning this morning will clear to VFR late morning.

Breezy to windy east to northeast winds today, especially east of
the Cascades and along the higher terrain. Gusty winds may make it
into the Rogue Valley, including kmfr. Winds at klmt may reach
Airport weather warning criteria late this morning into the afternoon.

Marine...updated 315 am PST Wednesday 20 November 2019...a thermal
trough near the coast will support gusty north to northeast winds
through tonight, approaching gales at times. Seas will remain very
steep and hazardous through Wednesday night in mixed wind waves,
northwest swell and fresh north swell. The trough will weaken
Thursday and conditions will improve with lighter winds and seas.
Winds will then remain below advisory criteria through at least
Saturday. Seas, however, will build again late Thursday into Friday
and remain steep through the weekend as multiple moderate long
period northwest swell trains move into the waters. A cold front
will move onshore Saturday night into Sunday, and gusty north winds
may return to the area Sunday.


Mfr watches/warnings/advisories...


Pacific coastal waters...Small Craft Advisory until 10 am PST Thursday for
Hazardous seas warning until 10 am PST Thursday for pzz356-376.
Hazardous seas warning until 10 am PST this morning for
Small Craft Advisory until 10 am PST Thursday for pzz376.

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