Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 212219

Area Forecast Discussion
National Weather Service Medford OR
219 PM PST Tue Feb 21 2017

.SHORT through Friday...Showers are plentiful this
afternoon across the west side and into the Cascades in the wake
of low pressure that is now moving off to our north and east.
Deep, moist, onshore flow is fueling the convection, which is
being enhanced orographically. We`ve seen some 40-50 dbz echoes on
radar, which isn`t terribly surprising given the amount of
instability present. We haven`t seen any lightning yet, but it
certainly wouldn`t be surprising to see a few strikes this
afternoon. Gusty winds continue east of the Cascades, but the
strong upper jet has moved off and speeds have diminished enough
to lower the wind headlines. The exception is in Lake County where
they are closer to the low and winds continue to gust to 50 mph
in a few locations. I continued wind advisories there into the
evening, but ended the high wind warning.

Guidance is consistent in showing showers diminishing this
evening. However, there will be a secondary front that moves in
from the north overnight and into Wednesday morning. There is a
noticeable increase in frontogenesis as the front encounters the
terrain of the Umpqua Divide late tonight, and models are
focusing precipitation in that area. At the same time, snow levels
will be falling, and with 850 mb temperatures of around -4C, snow
will probably fall all the way down to I-5 by late tonight. I`ve
already issued a winter weather advisory for all the passes
between Grants Pass and Roseburg and much of the higher terrain of
Jackson, Josephine, and Douglas counties. Lower elevation road
temperatures are warm, but Metro forecasts for Canyon Creek Pass
show surfaces dropping to near freezing by tomorrow morning. If
precipitation comes down heavily enough, it should be able to
overcome any surface warmth and accumulate. While I confined snow
accumulations to 1-3 inches along I-5, this will have to be
watched closely, because the 18Z guidance came in with higher
precipitation amounts than earlier guidance. If the front stalls
in the area, which is a distinct possibility, there could be much
more snow. Temperatures will warm tomorrow, and high sun angles
should melt anything that falls by late morning.

Strong cold advection will continue over the entire area tomorrow
and into Thursday. At the same time, an upper low currently
spinning its way down the southeast Alaska coast will move through
here. As the cold air associated with this system moved out over
the Gulf of Alaska, buoy observations showed air temperatures in
the upper 20s to lower 30s, which is an indication of how cold the
airmass is. As this trough moves through our area, the threat for
low elevation snow continues. The airmass will be plenty cold
(850 mb temperatures of -6C to -8C) for snow over almost all of
our forecast area inland from the coast (and maybe even at the
coast) Wednesday evening through Thursday. The question will be:
how much precipitation is left. The trough itself will be very
dynamic and it will certainly produce showers, so I would expect
more impacts along I-5 north of Grants Pass, as well as some snow
in the snow-prone valleys like the Illinois, Evans, and possibly
even in Grants Pass and Roseburg. Precipitation looks much more
spotty around Medford, but this will also bear watching, as any
change in the path of the upper low will change precipitation

This cold, unsettled pattern continues all the way through Friday,
so we could easily see snowflakes in the air at times through the
remainder of the week. -Wright

.LONG TERM...Saturday 25 Feb through Tuesday 28 Feb 2017.

The medium range models are in rough agreement at the beginning of
the extended period with a long wave trough oriented from northeast
to southwest across the area. They differ a bit on the location of
the closed low at the base of the trough, but all have it offshore
to the west of the Medford CWA.  The cold air aloft will support a
generally unstable air mass with variable winds aloft in the col
region east of the upper level low. Expect slow moving scattered
shower activity Saturday.

The closed low will dive south Saturday night into Sunday...but
short waves will continue to move into the trough...causing it to
dig along the west coast.  Cold air aloft will remain in place with
showers continuing.

The trough will move slowly to the east from Sunday night on...and
the flow aloft over the area will become northwesterly as a ridge
edges in from the west. This will stabilize the air mass. A drying
and warming trend will kick in at this juncture...but the showers
will not end least not through Tuesday. -JRS


.AVIATION...21/18z TAF CYCLE...VFR conditions with local MVFR cigs
in showers will persist into this evening with areas of higher
terrain obscured.  MVFR cigs/vsbys will increase tonight as showers
persist and gradually change to snow showers over the inland areas.
Freezing levels across the area will drop to 2000 feet elevation
tonight and to 1000 to 2000 feet tomorrow morning. The lower
conditions will persist through mid-morning Wednesday. -JRS

.MARINE...Updated 200 PM PST Tuesday 21 February 2017...Gusty west
to northwest winds will continue through the rest of the afternoon,
diminishing through the evening and overnight hours. Seas will
subside to around 9 feet for most of the area with the period
increasing as the evening progresses. This will keep seas just below
small craft advisory level through Wednesday. Moderate to locally
heavy showers will persist through the evening with a chance of
thunderstorms for all of the waters through Wednesday morning. Winds
could be quite gusty with any of these showers and/or thunderstorms.
Increasing northwest winds and seas are expected Wednesday night
into Thursday, with seas building to at least small craft advisory
levels during this time. This active wet weather pattern will
continue through the week. /BR-y


OR...Winter Weather Advisory from 10 PM this evening to 10 AM PST
     Wednesday for ORZ023-024-026.
     Wind Advisory until 7 PM PST this evening for ORZ031.
     Winter Weather Advisory until 10 AM PST Wednesday for
     Winter Weather Advisory until 10 PM PST this evening ABOVE 4000
     FEET for for ORZ028.


Pacific Coastal Waters...Small Craft Advisory until 4 PM PST this
     afternoon for PZZ350-356-370-376.

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