Area Forecast Discussion
Issued by NWS Medford, OR

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432
FXUS66 KMFR 220515
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
915 PM PST Tue Nov 21 2017

.UPDATE...Rain continues over southwestern Oregon, though has
generally skirted north of the Rogue Valley. Added slight chance
of precipitation to the Rogue Valley for tonight; based on current
obs and hires guidance, couldn`t rule it out. Also brought
probability of precipitation up for tomorrow... a weak disturbance
coming through may be enough to squeeze a few drops out. Cloud
cover tomorrow should keep temperatures from getting as warm as
today... but still expect above normal highs. Otherwise, current
forecast is on track.

&&

.AVIATION...22/00Z TAF CYCLE...VFR conditions will prevail
initially, but areas of MVFR cigs and local IFR cigs/vsbys in low
clouds and fog will develop late tonight through mid-morning
Wednesday, mostly over the west side and coastal valleys. Low level
wind shear will persist through the night along the coast, over the
Coast Range, and in the Umpqua Basin. All areas will clear to VFR by
late Wednesday morning and VFR conditions will prevail into
Wednesday evening.

&&

.MARINE...Updated 830 PM PST Tuesday, 21 November 2017...Southerly
winds will diminish to below gale force tonight but seas are
likely to remain very steep overnight through Wednesday morning.
High northwest swells will move into the coastal waters Saturday
night and remain through the middle of next week. Calmer
conditions are expected late in the week.

&&

.PREV DISCUSSION... /Issued 232 PM PST Tue Nov 21 2017/

DISCUSSION...Our area is sandwiched between a large upper low near
50n/150w and a ridge in southern California. A moist southwest flow
is over our area and there is general agreement impulses of energy
(weak surface lows along a frontal boundary) will bring moderate to
occasionally heavy precipitation along and just off the coast. The
exact location of the heaviest rainfall varies between the
operational models and this will be the biggest forecast challenge
this evening through Wednesday. Much will depend on the locations of
each individual low pressure waves. Confidence is high the areas
mentioned above will get measurable rain, but low to moderate on the
amounts. Right now the thinking is the heaviest rainfall will be
just off the coast, but the south coast could experience rain at
varying rates. In other words it will vary from light to moderate to
heavy then back to light. Further inland west of the Cascades, rain
for the most part will be light and intermittent. Meanwhile it could
be dry most of the time east of the Cascades. Plenty of cloud cover
will remain tonight, so overnight temperatures will be mild once
again (similar to what we had this morning).

Not much will change Wednesday, although the NAM suggest the bulk of
the precipitation will shift north during the day. In contrast the
EC and GFS show moisture extended further south over our area. This
solution is possible in that the ridge axis will shift slightly to
the east allowing the moisture plume to also shift southward. The
forecast for Wednesday will lean towards the EC/GFS solution, but
confidence remains low.

The front will push inland on Thanksgiving Day with most of the
rainfall focused along the coast, coastal mountains and Cascades.
Even then rain amounts are not expected to be significant.

Snow levels between now and Thanksgiving will be well above all the
higher passes, therefore road snow will not be an issue for those
traveling between now and Thanksgiving Day. They will lower Thursday
afternoon and evening as the front moves through, but they should
still remain remain all of the passes. A "dirty ridge" remains over
the area Thursday night. This type of pattern can result in light
intermittent light rain or drizzle. For now kept in a slight chance
to chance pops, but most places may not get anything measurable.

Friday and Friday night look dry and mild for most, if not the
entire area, then more moisture will move up from the southwest
with precipitation returning early Saturday morning. Snow levels
will be above all the passes, therefore were not expecting any
road snow.

Gusty winds are still expected, especially in the Shasta Valley and
higher elevations east of the Cascades. Wind advisories remain in
effect for these areas into this evening. Please see NPWMFR for more
details. Gusty winds will still continue in the Shasta Valley near
Weed tonight into Wednesday, but they should remain below advisory
criteria. -Petrucelli

EXTENDED DISCUSSION...Sat (11/25) through Tue (11/28)...The
21/12Z EC and GFS start off the extended in good agreement.  They
both show a long wave trough along 142W and a ridge over the
intermountain states, with the Pacific Northwest under moderate
southwest flow aloft.  Usually such troughs eject short waves toward
the coast, and these waves support surface fronts that move onshore
every 1-3 days.  In this case, a warm front will move north through
the Medford CWA Saturday, followed by a cold front Sunday.  Expect
light precipitation with the warm front, then increasing winds as it
moves to the north and the trailing cold front moves in. The winds
will be strongest along the coast, over the ridges, and east of the
Cascades. The Shasta Valley will be windy as well.

There won`t be much of a break behind the Sunday front, as the
pattern will remain progressive. The models do show some minor
timing differences, all of them move the long wave trough onshore
Sunday night. The associated cold front, which will be quite wet and
windy, will also move onshore Sunday night.

A long wave ridge will move onshore in the wake of the trough, and
the ridge axis will break to the east of the area sometime Tuesday.
Post-frontal showers will diminish Monday into Monday night, and
Tuesday looks to be dry, or at least mostly dry.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Wind Advisory until 10 PM PST this evening for ORZ030-031.

CA...Wind Advisory until 10 PM PST this evening for CAZ085.

Pacific Coastal Waters...Small Craft Advisory until 10 PM PST this evening for
     PZZ350-356-376.
     Small Craft Advisory for hazardous seas until 10 AM PST Thursday
     for PZZ350-356-376.
     Gale Warning until 10 PM PST this evening for
     PZZ350-356-370-376.
     Hazardous Seas Warning until 10 AM PST Thursday for
     PZZ350-356-370-376.

$$

MSC/MAP/FJB



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