Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 150458
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
858 PM PST Thu Dec 14 2017

.DISCUSSION...There is no reason to update the forecast this
evening. 00z guidance coming in right now fits very nicely with
the current forecast. If anything, models are just a touch slower
than their previous runs with most of the precipitation associated
with Friday`s front waiting until after 18z to arrive. This is
good news since it should allow the west side valleys, some of
which are currently near freezing...or will be around dawn...to
rise above freezing by the time precip moves in. Even so, there
could still be some slippery spots Friday morning due to areas of
fog/frost, so be on the lookout for that.

Most areas will get a little light rain with the front Friday.
There may be some downslope effects just to the lee of the
Cascades (Highway 97 corridor), but frontal forcing appears
sufficient enough that we`ll see light precipitation reach well
east of the Cascades. Overall, liquid amounts will range from only
a trace to a hundredth or two across the far southwest part of
the CWA (SW Siskiyou County) to near 0.25 of an inch in the
eastern Douglas County foothills. Snow levels will drop quickly as
the front moves through due to precipitation falling into
relatively drier air (wet-bulb cooling effect), so any rain from
the Cascades eastward should quickly change to snow above 4500
feet. There won`t be that much moisture left though, so, overall,
amounts will be mostly an inch or less. We`ll finally be able to
end the air stagnation advisory, at least temporarily, since the
front should provide enough vertical mixing and scour out the
inversion.

Please refer to the previous discussion below for details on the
forecast for this weekend and beyond. -Spilde

&&

.AVIATION...15/00Z TAF CYCLE...Going with a persistence forecast
until the front moves into the coast in the morning, current LIFR
conditions are expected to continue through most of the TAF period
for portions of the Umpqua Basin, including KRBG, and the lower
portions of the Rogue Valley around Grants Pass. LIFR conditions at
KMFR are expected to return around sunrise.

VFR conditions will prevail elsewhere, including the East Side,
coast, and coastal waters. Similar fog coverage and duration is
expected tonight into Friday morning with a similar large-scale
pattern persisting. Fog coverage and duration is expected to be less
Friday night and Saturday morning as a weak cold front moves
through the area. Sven

&&

.MARINE...Updated 230 PM PST Thursday 14 Dec 2017...Moderate north
winds and west swell will persist through tonight with small craft
conditions expected south of Cape Arago through Friday morning. A weak
front will move onshore Friday and west swell will subside. This front
will bring another round of moderate northerly winds, but will remain
below small craft conditions. High pressure will build offshore again
Saturday and a thermal trough will develop. As a result seas will
become wind driven and mixed with a light northwest swell and this will
result in small craft advisory conditions through early Sunday morning
south of Cape Blanco. North winds should subside Sunday into Monday
ahead of a stronger cold front due to arrive late Tuesday. Confidence,
however, is much lower for this part of the forecast. BR-y/SK

&&

.PREV DISCUSSION... /Issued 328 PM PST Thu Dec 14 2017/

SHORT TERM...This afternoon through Sunday, Dec 17th...
High clouds are streaming in ahead of a frontal system that will
be the first to bring precipitation to most of the area since
December 4th. With a strong inversion still in place, there is
some slight concern regarding brief freezing rain late Friday
morning due to cold air trapped in the valleys as this frontal
system arrives. This concern is primarily for areas near Camas
Mountain on Highway 42 and in portions of the Illinois Valley. The
going forecast covers the most likely scenario, however, which is
that temperatures will warm above freezing, even at these
locations, before precipitation begins. Of course, we`ll be
keeping a very close eye on this situation and alerting ODOT
should our expectations change.

For tonight temperatures should be a bit less cold in the valleys
than last night due to the influence of partially transparent high
cloud cover moving through. We`re still expecting some fog in the
same areas where we`ve been seeing it the last two nights/mornings,
as dewpoints have risen a bit under the inversion. Meanwhile, the
inversion will begin to weaken overnight, so very warm
temperatures at higher elevations will start to come down
overnight, returning to more normal values across the area as
Friday progresses.

As the front pushes through tomorrow the inversion will
disintegrate and valley air quality will improve. With most of the
precipitation behind the front in the colder air snow levels may
initially begin between 6000 and 7500 feet, but will rapidly fall
to the 4000 to 6500 foot range. Late Friday evening and overnight
snow levels will fall more, so we could see some light skiffs of
snow at lower elevations between 2500 and 4000 feet where
precipitation hangs on a little longer under a NW to N flow. Given
the recent warm weather, however, mild road temperatures should
melt the snow off easily over the passes.

This weekend valleys will experience better air quality as wind
and better vertical mixing continue with a fresh air mass in the
valleys. However, by Sunday there could be some stagnation that
will likely persist through Tuesday, so we`ll look into this more
in the next 24 hours to see if we`ll need another air stagnation
advisory. BTL

LONG TERM...Mon (12/18) through Thu (12/21)...A broad upper
level ridge will remain over the PacNW on Monday. While it will be
much weaker than the ridging we`ve seen lately, it will keep
moderate to strong inversions trapping cold air in the valleys
with milder weather above. Temperature forecasts continue to show
this pattern but have been trended a little less extreme due to
the ridge being weaker. Inversions will likely continue into
Tuesday morning, but should be weaker as a frontal system
approaches. Models are trending faster and wetter with this front,
and thus pops and precipitation amounts have been increased later
Tuesday and Tuesday night, especially from the Cascade crest
westward. Precipitation will diminish Wednesday and Wednesday
night, but we`ll keep chance pops due to residual mainly
orographic precipitation. Snow levels will drop quite a bit behind
this front
- down to around 2000 feet by Wednesday morning - so any of this
left over precipitation could produce travel problems even over
the lower passes along I-5 north of Grants Pass.

Ensemble and deterministic model runs are in good agreement today
and all show the upper ridge rebuilding offshore late next week,
which opens the door to colder intrusions into the Great Basin. This
looks like a dry pattern as none of the 12Z deterministic models
showed any precipitation-producing shortwaves for our area. As a
result, the most likely scenario is just cold, dry northerly flow.
Forecasts continue to be trended that way with the focus of the
colder air being east of the Cascades. -Wright

AVIATION...14/18Z TAF CYCLE...Going with a persistence forecast,
current LIFR conditions are expected to continue through most of the
TAF period for portions of the Umpqua Basin, including KRBG, and the
lower portions of the Rogue Valley around Grants Pass. LIFR
conditions at KMFR are expected to clear within the next few
hours.

VFR conditions will prevail elsewhere, including the East Side,
coast, and coastal waters. Similar fog coverage and duration is
expected tonight into Friday morning with a similar large-scale
pattern persisting. Fog coverage and duration is expected to be less
Friday night and Saturday morning as a weak cold front moves
through the area. Sven/SK

MARINE...Updated 230 PM PST Thursday 14 Dec 2017...Moderate north
winds and west swell will persist through tonight with small craft
conditions expected south of Cape Arago through Friday morning. A weak
front will move onshore Friday and west swell will subside. This front
will bring another round of moderate northerly winds, but will remain
below small craft conditions. High pressure will build offshore again
Saturday and a thermal trough will develop. As a result seas will
become wind driven and mixed with a light northwest swell and this will
result in small craft advisory conditions through early Sunday morning
south of Cape Blanco. North winds should subside Sunday into Monday
ahead of a stronger cold front due to arrive late Tuesday. Confidence,
however, is much lower for this part of the forecast. BR-y/SK

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Air Stagnation Advisory until 10 AM PST Friday for
     ORZ023-024-026-029>031.

CA...None.

Pacific Coastal Waters...Small Craft Advisory from 4 PM Friday to
     4 AM PST Sunday for PZZ356-376.
     Small Craft Advisory for hazardous seas until 4 AM PST Friday
     for PZZ370-376.

$$

MAS/BTL/TRW/SBN



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