Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 070108
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
250 PM PST Sat Mar 6 2021

.SHORT TERM...Through Monday Night...Weak radar returns are still
occurring near the coastline and across Douglas County. Some of
these returns have been associated with light precipitation
amounts in the coast ranges and higher terrain of Douglas County.
Have accordingly retained a slight chance to chance across these
areas through the afternoon.

Another front will follow through tonight. It has been slow to
advance towards the coast, but as the low builds off the B.C.
coast the jet level winds will increase in the base of the trough
and it will begin to accelerate towards the coast. This front
will lose much of its momentum after interacting with the coast
and precipitation is still not expected east of the Cascades or
south of the Rogue Valley, except in western Siskiyou County. As
the front moves onshore slightly colder air will fill in behind
it. This will bring the snow level down from 3000 feet to around
2000 feet. Cold air trapped in valleys could cause the snow level
to go a few hundred feet below this, like in the Camas Valley and
near Hayes hill. The impact should be minimal though given the
lack of precipitable moisture with this front. Also, roadways
have a had a chance to rewarm, many of which did not even reach
freezing last night, and so the effect tonight will be much like
last night. Minimal if any accumulation should occur on roads,
with most of the snow accumulating on better insulated surfaces.
The only place where a more substantial accumulation of snow is
expected is in the southern Coast range and western Josephine
County where at higher terrain between 3" and 5" of snow could
accumulate. Other than the chance for some 40 mph gusts at the
headlands, winds should not be as strong as the last front and
pose no to minimal impact.

Sunday night temperatures along portions of the entire coast
still remain conducive to a possible frost. Wind speeds have also
lightened overnight, increasing the possibility that frost forms.
Chance of frost conducive temperatures per the NBM is between 15
and 30% through Monday morning. Given that temperatures have not
trended warmer, it seems likely that a frost advisory will be
issued at some time tomorrow unless a total reversal occurs from
the guidance. Also retained a mention of patchy fog overnight
ahead of yet another front. This is mostly due to the buildup of a
stratus type layer around valley rims and wanting to indicate
that these areas could have some visibility reducing conditions.
The exception could be in the Umpqua Basin where statistical
guidance is split on whether the cloud layer might lower nearer to
the valley floor.

The low then cuts off from the upper-level flow and meanders
along the PNW coastline through Monday morning. During this time
more energy will build and move through the base of the trough.
This will bring another front onshore. This front has fairly
favorable conditions for some lightning production and graupel-to-
small hail along the coast, especially south of Cape Blanco.
Areas south of Cape Blanco look to be favored because of the
better connection to the moisture moving north and shearing
component of the 700-500 mb winds. Given the proximity of the low,
this second front on Monday should have more energy behind it to
drive precipitation east of the Cascades. However, anticipate much
of it being terrain locked with portions of the Klamath basin and
Christmas Valley areas remaining dry through Monday night.

Snow levels with this second front will mostly flux with diurnal
temperatures, as the air advected in being of relatively similar
temperatures. Snow levels will begin around 2000 feet Monday
morning, rise to between 2700 and 3000 feet in the afternoon, then
lower to between 1500-2000 feet. By the time snow levels reach
1500 to 2000 feet the environment should be showery and terrain
locked, limiting the impact of any precipitation during this
time. Again, larger snow amounts will be contained to the south
coast range into western Josephine County and the mountains of
Siskiyou County. Amounts will be very light in all other areas.
Winds may be stronger with this front, but only along slopes and
ridges. -Miles

.LONG TERM...Mar 9th through Mar 13th. Operational models remain in
remarkably good agreement throughout all but the tail end of the
extended period. Tuesday starts with a closed upper low offshore
of the southern Oregon coast, spinning short waves around it and
wrapping showers into the region. A cold air mass is above the
region with 500 MB temperatures below -30C. With this unstable,
showery pattern have added slight chance of thunderstorms to the
coastal waters and close to the coast. Slight instability is also
indicated inland in the afternoon on Tuesday, but it is more
marginal and will leave it out for now and evaluate after further
runs.

Models remain in good agreement with the upper low meandering down
to just offshore the central California coast, with the wrap around
showers diminishing. The upper low continues to move down the
California coast, and by Thursday morning is an the southern
california coast with an upper level ridge nudging into the Pacific
northwest. The vast majority of the GEFS and ECMWF meteogram members
are also showing very little to no precipitation for this time
period as well.

Models remain in agreement for the overall pattern as we head into
the end of next week, but with timing differences. The overall
patter is the upper low weakening and moving to the southeast ahead
an upper trough that brings successive systems through the region
through next weekend. Sven

&&

.AVIATION...For the 07/00Z TAF Cycle...Fair weather cumulus covers
southern Oregon and northern California this evening.  There is a
chance of showers along the coast tonight as the next system moves
in.  However, ceilings are expected to remain at MVFR through
tonight.  These lower MVFR ceilings will spread inland Sunday
morning as the weather system moves onshore. Eventually, relatively
drier air and VFR ceilings will filter into the region Sunday.

-Smith

&&

.MARINE...Updated 200 PM PST Saturday 06 Mar 2021...The main
update was to upgrade the Hazardous Seas Watch Sunday night to a
Hazardous Seas Warning.

First, a cold front will bring increasing south winds tonight with
gusts approaching gale force north of Cape Blanco. This will result
in seas becoming wind driven, and very steep and hazardous.
Strongest winds are expected late this evening, and steepest seas
will be roughly north of Gold Beach.

Conditions briefly improve Sunday morning, but swell will then build
with a very high and very steep long period west-northwest swell
peaking late evening into early Monday morning at 16 to 18 ft . A
series of disturbances will continue through Tuesday with high and
steep seas likely. This includes a low (and associated cold front)
on Monday that has about an equal probability to either remain just
west of the coastal waters or pass through the waters.

Winds are likely to become northerly Tuesday night with seas
diminishing but becoming dominated by northerly wind seas as high
pressure builds toward the area. North winds Wednesday into Thursday
will be strongest south of Cape Blanco. The pattern may turn active
again at the end of the week. -DW

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...High Surf Advisory from 1 PM Sunday to 7 AM PST Monday for
     ORZ021-022.

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 4 PM PST Sunday for PZZ356-376.
     Hazardous Seas Warning from 7 PM this evening to 4 AM PST Sunday
     for PZZ350-356-370-376.
     Hazardous Seas Warning from 4 PM Sunday to 4 AM PST Monday for
     PZZ350-356-370-376.
     Small Craft Advisory until 7 PM PST this evening for PZZ350-370.
     Small Craft Advisory from 4 AM to 4 PM PST Sunday for
     PZZ350-370.

$$

MB/SBN/SBN


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