Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 112226
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
226 PM PST Mon Dec 11 2017

.DISCUSSION...The weather will remain pretty much status quo through
at least Thursday. Therefore we can expect much of the same in terms
of the timing and location of low clouds and fog and freezing fog
will reform. The latest visible image shows low clouds slowly
eroding in the Umpqua Basin, Illinois Valley and Grants Pass area,
but like previous days, it probably won`t clear out at all or if it
does only for an hour or two.

As weak front offshore will dissipate as it moves closer to the
coast tonight and have little or no effect on our weather. The ridge
will re-establish along the West Coast Tuesday through Thursday
resulting in more of the status quo - morning fog/stratus in the
valleys west of the Cascades that will have difficulty breaking in
some areas during the afternoons. Overall, it will remain
cool/chilly in the valleys with milder conditions along the coast
and at elevations just above the valley floors. -Petrucelli

.Long Term...Friday, Dec 15th through Monday night, Dec 18th...
Model agreement is very good with the progression of a shortwave
trough through our forecast area for the first day of this long term
period, through 12Z on Saturday, Dec 16th. However, thereafter,
model agreement then quickly deteriorates regarding the amplitude of
a short wave ridge that will move in behind this trough, and with
details related to a subsequent trough after that early next week.
By next Wednesday the 20th, however, both models do agree on some
degree of troughing in the Pacific Northwest, albeit with sizable
significant weather differences. Technically speaking, the expected
changes in the pattern Friday through next Thursday (21st) are
generally consistent with phase 7 effects of the Madden Julian
Oscillation (MJO). This favors a period of precipitation in the
Pacific Northwest, though this round looks as if it will be light
(below daily averages) compared to climatological normals for mid-
December in our forecast area. Beyond the 21st, there are too many
competing variables to say what`s most likely to occur despite
current model leaning toward more high pressure ridging.

More specifically, the trough that`s expected to move in Friday is
likely to be enough to break and/ or significantly weaken the
persistent valley-mountain inversions over the forecast area. While
precipitation amounts look to be less than a tenth of an inch of
water with this trough, cooling aloft along with 30-50 knot 700MB
winds should provide enough mixing to freshen up the air for most,
if not all, areas.

For Saturday the 16th into Sunday the 17th, model guidance has
trended further north with the arrival of the next shortwave trough,
so the forecast has been trended in that direction. High pressure
ridging is likely to try to rebuild from the southwest during this
time period, but a faster flow aloft should result in better
vertical mixing across the forecast area- albeit with passing cloud
cover and some risk of precipitation, mainly over NW section of the
area.

The low presure trough expected early next week has had widely
varying details, in that there are several centers of competing low
pressure over the eastern Pacific as the trough arrives. If these
consolidate, the trough will be more significant than if they do
not. General indications are that this trough will be cold enough to
bring snowfall to pass levels or lower on or about Wednesday. How
much and for how long are details we`ll be attempting to grasp in
the coming days while this blocking high pressure ridge persists.
BTL


&&

.AVIATION...11/18Z TAF CYCLE...Going with a persistence forecast,
current LIFR conditions are expected to persist throughout 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 improve to VFR by late morning,
but confidence is low on exact timing and could persist into the
early afternoon. LIFR conditions are expected to return to the same
locations by late tonight.

VFR conditions will prevail elsewhere, including the coast and
coastal waters. /BR-y


&&

.MARINE...Updated 130 PM PST Monday, Dec 11th, 2017...Relatively
calm conditions are expected through much of the week. A dissipating
cold front will brush the outer portions of the coastal waters late
today, elevating seas late this afternoon through Tuesday afternoon.
Seas will peak around 9 feet at 13 seconds Tuesday morning through
afternoon, gradually subsiding overnight into Wednesday. North winds
will increase Wednesday as high pressure rebuilds, but will remain
below small craft advisory levels. Seas may briefly reach small
craft conditions late Thursday morning due to a mix of wind waves
and increased westerly swell, diminishing by Friday morning. The
weather will turn more active this weekend, bringing increased swell
and periods of increased southerly winds. /BR-y


&&

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

CA...None.

Pacific Coastal Waters...None.

$$


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