Storm Watch 2, NWS Medford, OR

Area Forecast Discussion
National Weather Service Medford OR
1012 PM PST Sun Dec 24 2023

.UPDATE...Updated aviation discussion for 06z.

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DISCUSSION...Rain is moving in and spreading to near the Cascades
with snow levels rising this evening. However, east of the
Cascades, most importantly along the I-5 corridor from Chiloquin
north, temperatures have already dropped into the upper 20s. The
radar is indicating returns east of the Cascades, but with a dry
layer above 700 MB the precipitation will take a while to reach
the ground until this moisture deficit is filled. Road
temperatures in this area are also below freezing and when the
precipitation does finally reach the ground the snow level will
continue to rise and the rain, albeit light, is expected to reach
the ground as freezing rain later tonight into Christmas morning.
A freezing fog advisory is in effect along the highway 97
corridor from 4 AM until 11 AM Christmas morning. -Sven

AVIATION...25/06Z TAFs...Showers will continue to increase in
coverage overnight and this will increase the likelihood that
periods of MVFR conditions become more widespread, but even so VFR
will remain the predominant condition for most places inland from
the coast. Increased terrain obscurations along and west of the
Cascades is also expected. Low level wind shear concerns will
peak during this TAF period and there is a very small chance it
could be experienced in the Umpqua Basin too, but we have left it
out of the TAF there for now.
-Miles

MARINE...Updated 230 PM Sunday December 24, 2023...Today is the
calmest that conditions are expected to be through at least the end
of the week. A series of fronts will pass through the region during
this time and this will bring multiple moderate to heavy swell
trains as well as periods of very high, wind driven seas due to
strong south winds. The front that moved in today brought another
round of increased southerly winds and a building northwest swell.
Small craft conditions began this morning and will continue through
Monday morning before conditions continue to deteriorate through the
week.

Starting Monday, a front looks to pass through the region roughly
every 24 hours, bringing a new swell train and very high wind driven
seas. Confidence has increased on the gale potential with the front
on Monday, so a gale warning has been issued for late Monday morning
through Tuesday afternoon. This front looks a bit stronger than
originally thought, and we're expecting high end gales across the
northern waters with gales reaching as far south as Brookings in the
outer waters. Isolated storm force gusts are possible Monday evening
across the northern waters as well. Meanwhile, a heavy west swell
(15 to 18 ft @ 15 to 16 seconds) will build in the waters following
this front by early Tuesday morning and very steep and hazardous
seas are expected to continue overnight Monday into Tuesday.

Another round of strong south winds and building seas are expected
Tuesday into Wednesday and this looks to be the period of time with
the worst conditions. This is when gales are likely across all the
waters with storm force winds having up to a 15% chance of occuring.
Seas look to peak around 20 to 25 ft (25 to 30 ft is not out of the
question) due to heavy swell and very high southerly wind driven
seas.

Very high and very steep seas will continue through the remainder of
the week. In fact, once seas come up above 17 ft on Monday
afternoon, they are likely to remain at least that high through end
of the week (peaking Tuesday night into Wednesday), with about a 36-
48 hour of seas at or above 20 ft Tuesday into Thursday. Conditions
may improve some late in the week, but guidance indicates active
weather continuing into the following weekend. -BR-y/Hermansen

BEACH HAZARDS...Updated 845 PM Sunday December 24, 2023...Large
breaking waves are expected Monday through at least Thursday, which
will create dangerous conditions along and within the surf zone, and
could inundate beaches and low lying shorelines.

Increasing the hazards of these breaking waves is the occurrence of
King tides December 24-27, resulting in high tides of between 1.0
and 1.3 feet above Mean High High Water (MHHW, the average height of
the highest tides) at Charleston. This means that the breaking waves
will be amplified and will increase the risk of beach erosion.
Normally we issue a coastal flood advisory if there is a heavy surf
and the tide is expected to reach 2.5 feet above MHHW, but because
we are under that threshold, we are withholding any further
highlights for now. Given locally heavy rains greater than 1" are
expected on Wednesday, Dec. 27 during a King tide, it is not
impossible the combination of waves, tide, and rain results in some
roads that are exposed to the ocean or that run along the local
sloughs to experience ponding from tidal wash. This is also the
timeframe breaking waves are expected to peak: Tuesday night (12/26)
into Wednesday afternoon (12/27). -Miles

PREV DISCUSSION... /Issued 511 PM PST Sun Dec 24 2023/

UPDATE...New Aviation section for 00z TAFs.

DISCUSSION...

SHORT TERM...

Scattered showers persist on radar today as a fairly dry cold
front is about to stall out and dissipate. Skies will remain
cloudy tonight ahead of the next warm front, which is anticipated
to push in later tonight. The chance of rain will remain high
along the coast and most locations west of the Cascades. Those
probabilities will decrease the farther east one moves with only a
20 to 30 percent chance of rain tonight around sections of Lake
County.

The main concern for tonight is the chance of freezing
rain/drizzle east of the Cascades as the warm front pushes inland.
Surface temperatures for many locations have a good chance of
being below freezing. The question is how warm temperatures will
become aloft while surface temperatures remain cold. Upper air
soundings look okay with many showing a warm layer, but it
doesn't appear that thick. Some only show temperatures pushing to
1C, which would suggest some melting will occur. In any case, the
threat of freezing drizzle exists along highway 97, which is known
to have ice problems. Given the lack of travel Christmas morning
and the uncertain nature of freezing drizzle, we opted not to
issue a winter weather advisory. However, the risk of freezing
drizzle is about 20 to 30 percent. A 95th percentile event
according to the NBM could produce up to 0.03 inches of ice on
roads, which would be concerning.

Warm moist flow will persist into Christmas with snow levels
pushing up to 8000 feet according to the latest run of the NBM.
This shows how warm the airmass is with surface temperatures
reaching the low 50's for highs west of the Cascades and upper
40's east of the Cascades. Crater Lake is also anticipated to push
above freezing on Christmas. Overall, the threat for travel
is pretty low in the evening.

We'll remain under this warmer moist flow for Tuesday.
Temperatures are anticipated to remain about 10 degrees above
normal for this time of year with some places near the coast
coming close to 60 degrees. Tuesday is also expected to be very
dry, although the coast as as slight chance of rain under the
warmer southerly flow.

-Smith

LONG TERM...Tuesday 12/26 through Saturday 12/30... The low
pressure system on Tuesday night into Wednesday has trended stronger
over the last few guidance packages. That trend is continuing with
the newest set of guidance. This low pressure system is expected to
be strong, but remain offshore. Overall, the main impacts will be
mainly from the winds. The Medford to Redding Gradients between all
the operational models are showing between 8 and 10 mb. Mid level
guidance (850 and 700 hPa) is also showing a southerly wind spread
of 50 to 80 kts which has spread farther inland on Wednesday. Still
the overall pattern is that the low will weaken as it approaches
Canada. The National Blend of Models is still suggesting a 70 to 90
percent chance for advisory levels winds for the portions of the
Shasta Valley, East Side (especially the higher terrain like the
Winter Rim), and even for the southern half of the Rogue Valley.
Strong winds are likely at the coast as well. Damaging winds in
excess of 60 mph will be possible, but ensemble guidance has lower
probabilities of this with the continued nature of the pattern in
conflict (specifically the strong gradients versus the weakening
low).

This system will also act like a weak atmospheric river with
moderately high snow levels of around 5000 to 6000 feet. Rainfall
will largely be moderate and focused at the coast, the Coastal
Mountains of Curry County, and western Siskiyou County. While
rainfall amounts have also increased, expect the fast moving nature
of this system to keep impacts due to rain being too much. Elsewhere,
expect the precipitation to mostly evaporate before it reaches the
ground due to the wind blowing down the mountains causing the air to
warm. (This is called downsloping.) Now, some precipitation reaching
the ground will be possible; but with most of it evaporating,
precipitation amounts will be light. This will also lead to mild
temperatures this week.

Snow, meanwhile, will largely be confined to the higher passes.
During the peak of the precipitation, snow levels will be between
5000 and 6000 feet. This means that even in the case of enhanced
precipitation rates on the south side of Mt. Shasta (Black Butte
Pass and Snowmans' Summit Pass), chances are that the precipitation
will remain rain for most of it. That being said, there is a chance
that heavy precipitation could entrain cold air with it lowering
snow levels during the periods of heavy precipitation, but it is
doubtful that the snow levels would be lowered by that much. Worst
case scenario (10% chance) would be 1 inch of snow on snowmans'
summit and maybe a half an inch (5% chance) on I-5 at Black Butte
Summit during the bulk of precipitation on Wednesday.

Snow levels will drop throughout the week to around 4000 feet by the
time the precipitation becomes more showery and tapers off Thursday
night. Overall snowfall amounts for the main passes look to be
meager, up to an inch for Siskiyou Summit, Black Butte Summit,
Snowmans Summit, and Highway 140 near Lake of the Woods.

Another low pressure system then looks to arrive Friday into
Saturday with additional wind, rain and snow impacts. This system,
however, appears to be weaker than the one coming mid-week. Still it
is worth paying attention to the forecast in the coming days as
changes may drastically change the potential impacts. -Schaaf

AVIATION...24/18Z TAFs...MVFR conditions exist along the coast and
likely in portions of western Siskiyou and Josephine County under
heavy showers. For all other areas, including the Medford, Roseburg,
and K-Falls terminal VFR conditions remain.  showers will continue
to increase in coverage overnight and this will increase the
likelihood that periods of MVFR conditions become more widespread,
but even so VFR will remain the predominant condition for most
places inland from the coast. Increased terrain obscurations along
and west of the Cascades is also expected. Low level wind shear
concerns will increase along the coast towards the end of the TAF
period and there is a very small chance it could be experienced in
the Umpqua Basin too, but we have left it out of the TAF there for
now.
-Miles

MARINE...Updated 230 PM Sunday December 24, 2023...Today is the
calmest that conditions are expected to be through at least the end
of the week. A series of fronts will pass through the region during
this time and this will bring multiple moderate to heavy swell
trains as well as periods of very high, wind driven seas due to
strong south winds. The front that moved in today brought another
round of increased southerly winds and a building northwest swell.
Small craft conditions began this morning and will continue through
Monday morning before conditions continue to deteriorate through the
week.

Starting Monday, a front looks to pass through the region roughly
every 24 hours, bringing a new swell train and very high wind driven
seas. Confidence has increased on the gale potential with the front
on Monday, so a gale warning has been issued for late Monday morning
through Tuesday afternoon. This front looks a bit stronger than
originally thought, and we're expecting high end gales across the
northern waters with gales reaching as far south as Brookings in the
outer waters. Isolated storm force gusts are possible Monday evening
across the northern waters as well. Meanwhile, a heavy west swell
(15 to 18 ft @ 15 to 16 seconds) will build in the waters following
this front by early Tuesday morning and very steep and hazardous
seas are expected to continue overnight Monday into Tuesday.

Another round of strong south winds and building seas are expected
Tuesday into Wednesday and this looks to be the period of time with
the worst conditions. This is when gales are likely across all the
waters with storm force winds having up to a 15% chance of occuring.
Seas look to peak around 20 to 25 ft (25 to 30 ft is not out of the
question) due to heavy swell and very high southerly wind driven
seas.

Very high and very steep seas will continue through the remainder of
the week. In fact, once seas come up above 17 ft on Monday
afternoon, they are likely to remain at least that high through end
of the week (peaking Tuesday night into Wednesday), with about a 36-
48 hour of seas at or above 20 ft Tuesday into Thursday. Conditions
may improve some late in the week, but guidance indicates active
weather continuing into the following weekend. -BR-y/Hermansen

BEACH HAZARDS...Updated 230 PM Sunday December 24, 2023...Large
breaking waves are expected Monday through at least Thursday, which
will create dangerous conditions along and within the surf zone, and
could inundate beaches and low lying shorelines.

Increasing the hazards of these breaking waves is the occurrence of
King tides December 24-27, resulting in high tides of between 1.0
and 1.3 feet above Mean High High Water (MHHW, the average height of
the highest tides) at Charleston. This means that the breaking waves
will be amplified and will increase the risk of beach erosion.
Normally we issue a coastal flood advisory if there is a heavy surf
and the tide is expected to reach 2.5 feet above MHHW, but because
we are under that threshold, we are withholding any further
highlights for now. Given locally heavy rains greater than 1" are
expected on Wednesday, Dec. 27 during a King tide, it is not
impossible for some roads that are exposed to the ocean or that run
along the local sloughs to experience ponding from tidal wash. This
is also the timeframe breaking waves are expected to peak Tuesday
night (12/26) into Wednesday afternoon (12/27). -Miles

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.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory from 10 PM Monday to 10 PM PST Thursday for
     ORZ021-022.

     Winter Weather Advisory from 4 AM to 11 AM PST Monday for ORZ027-
     029-030.

CA...High Wind Watch from late Tuesday night through Wednesday
     afternoon for CAZ081.

PACIFIC COASTAL WATERS...High Surf Advisory from 10 PM Monday to 10
     PM PST Thursday for PZZ350-356.

     Small Craft Advisory until 4 PM PST Tuesday for PZZ350-356-370-
     376.

     Gale Warning from 10 AM Monday to 4 PM PST Tuesday for PZZ350-
     356-370-376.

&&