Gale Warning 26, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
905 AM PST Wed Dec 4 2019

Updated aviation discussion

.DISCUSSION...We've updated precipitation chances this morning to
add portions of Northern California in a slight chance of rain.
We've had a few reports of a hundredth of an inch as low pressure
over Central California spreads precipitation northward into
Siskiyou and Modoc Counties. Also, some dense fog is being
observed in some valleys west of the Cascades including portions
of the Illinois...Umpqua...and Rogue Valleys. We've issued a
short-term forecast (NOWCAST) and messaged on social media to
highlight this hazard. We expect conditions to improve by 10am
PST.

We'll be examining data for the next storm system today, and we'll
see what hazards need to be issued and what themes of
communication will be needed. So far this morning, it seems we'll
need some Wind Advisories and potentially a high elevation winter
weather hazards for Northern California. Please see the previous
discussion below for more details on the return to active weather.

&&

.AVIATION...For the 04/18Z TAFs...Areas of IFR cigs/vsbys in low
clouds and fog over the valleys will clear to mostly VFR by this
afternoon, but patchy IFR may linger in the Illinois and Umpqua
valley into this afternoon. The lower conditions will return to the
same areas tonight, persisting into Thursday morning before clearing
to VFR Thursday afternoon.

&&

.MARINE...Updated 830 AM PST Wednesday 4 December 2019...Weak high
pressure will persist today with swell dominated seas. South winds
will increase in the outer waters Thursday ahead of a strong front.
The front will move through Thursday night into Friday, and will
likely bring gales along with high and very steep seas. Another
front Friday night into Saturday will likely bring another round of
gales with very steep seas peaking at around 15 feet. Conditions
will improve Sunday into Monday as weak high pressure returns to the
area.

&&

.PREV DISCUSSION... /Issued 355 AM PST Wed Dec 4 2019/

DISCUSSION...Relatively quiet weather is expected for at least a
couple more days. The latest satellite image shows and upper low
south of our area that will bring occasional high and mid level
clouds over the area. Meanwhile an upper low is taking shape
around 42N and 148W. This will move towards our area over the next
24-36 hours. More on this to follow later in the discussion.

The latest satellite image shows low clouds and patchy to areas of
fog over most of the same areas as yesterday. Low clouds and fog
should burn off late in the morning or early afternoon. However like
yesterday, it may not break out at all in the Umpqua Basin and
portions of the Illinois Valley and Grants Pass area. Even when the
fog burns off, in it's place will be varying amounts of high and mid
level clouds.

There's good agreement Thursday will end up dry for the whole day.
The operational models are in good agreement keeping the upper and
surface low and associated front well offshore through the day. Even
several of the individual ECMWF ensemble members show little or
nothing during the day Thursday for many locations, including the
coast. However, upslope flow could help to contribute to some
precipitation in the Mount Shasta region and kept in a slight chance
of rain and higher elevation snow. Speaking of which snow levels
Thursday are expected to be around 6000 feet, so that will keep it
above the I-5 corridor and highway 89.

The front will inch closer to the coast Thursday night. Mid level
flow will also increase and we can expect moderate to occasionally
strong winds over the marine waters and along the coast,
specifically south of Cape Blanco. Right now, were not anticipating
any high winds for the south coast, but it's something that will
need to be monitored.

Meanwhile 700 mb winds and favorable upper flow from the southeast
late Thursday afternoon through Friday could contribute to moderate
to strong winds in the Shasta Valley and south end of the Rogue
Valley from about Medford down to Ashland. Wind advisory criteria
could be met in both areas. However pressure gradients are not
anything too impressive to warrant high wind concerns in these
areas. Also the east side will be windy, but not until Friday when
stronger 700mb winds are expected to move overhead.

The front will continue to move inland on Friday with moderate to
heavy precipitation likely for the coast, coastal mountains and
south through southeast Siskiyou County. However it's not expected
to be for a prolonged period of time which means precipitation
impacts are likely to be fairly short lived. Snow levels for this
event are honing in on elevations around 5500 feet, however they
could lower down to around 5000 feet in areas of heavy
precipitation, especially for the Mount Shasta area. Even then the
bulk of the snow impacts should remain above the major passes. If
nothing else, this will help to add to the snowpack.

The upper level trough will make its way overhead Saturday
afternoon, then weaken and push off to the southeast Saturday night
through Sunday. Moisture-rich onshore flow with this system will
produce plenty of precipitation in the area, especially for the
usual areas, like the coast, coastal mountains and western through
southern Siskiyou County. However the trajectory will be more
southerly, so inland areas, especially the East Side, will be
sheltered from the heaviest precipitation. We aren't concerned about
hydrological hazards Friday night through Saturday night, because
mainstem rivers and streams flows are very low. There will be some
rises on area streams, creeks and rivers though, especially across
southern and western portions of the CWA.

Snow levels should hover right around 5500 feet Saturday, dropping
off to around 4500-5000 feet Saturday night in to Sunday as
precipitation tapers off and colder air aloft moves overhead. Expect
significant snowfall, in the higher terrain of Siskiyou County. Snow
is also expected for the Cascades, Warners, and the high terrain of
the East Side, but amounts will be less. The forecast for Sunday and
beyond has been retained in the previous long term discussion below.
-Petrucelli

LONG TERM...Sunday through Tuesday Night...Showers will decrease
in coverage and intensity Sunday, then ridging and high pressure
return by Monday. Along with clearing skies, temperatures are
expected to warm slightly for Monday. The exception to this will
be valley fog, typical for this time of year, which will keep some
areas cool, and could easily stick around throughout the day.

The next system is slated to arrive Tuesday, and although models are
in surprising overall agreement, solutions have been trending a bit
slower with the arrival. Details are still uncertain regarding
strength, but given the pattern and the model trends, the event does
not appear at this time to be a major rain or weather producer.
-BPN

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...None.

CA...None.

Pacific Coastal Waters...None.