Area Forecast Discussion
Issued by NWS Medford, OR

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000
FXUS66 KMFR 231230
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
530 AM PDT Sun Apr 23 2017

The Aviation section has been updated...

.SHORT TERM...Rainfall from yesterday`s front peaked at around a
half inch along the coast and in the coastal ranges. Amounts
decreased quickly inland, and little to nothing fell at observing
sites farther east. Showers have largely ended this morning,
although I can`t really rule out some light precipitation flying
under the radar (literally) in coastal mountains. Moist, onshore
flow will continue today, so showers remain possible through the
day in many areas.

A strong jet stream and associated very rich moisture plume
extends across much of the northeast Pacific this morning, and
this will push into southern Oregon and northern California
tonight. Low pressure will form just north of the jet tonight and
move into northwest Oregon on Monday. The associated frontal
system will bring very heavy precipitation to much of our area.
Rainfall from this first system should amount to 2-3 inches along
the Curry coast and adjacent mountains with lesser but still
significant amounts inland.

Curiously, statistical and ensemble guidance is much wetter with
this system than deterministic guidance. For example, for Medford,
SREF plumes show a mean of around a half inch with outliers
ranging up to three-quarters of an inch. MOS guidance across the
board shows similar QPF for Medford. By contrast, the wettest
deterministic guidance is the EC with a half inch, but most
guidance is much drier with the low being the NAM with just a
tenth of an inch. I think the models are overdoing the amount of
downsloping we will get in this pattern. That and the fact that
moisture parameters are expected to be above the 90th percentile
for this time of year in our area, rainfall forecasts have been
skewed toward the wettest guidance.

All this precipitation will fall as snow in the higher mountains,
roughly above 5500 feet. Snow will be especially heavy along the
west slopes, and this pattern really favors Crater lake for heavy
snow. A winter storm watch has been issued for the park for
tonight into Monday (see WSWMFR for details).

This system will be very dynamic, but the orientation of surface
gradients and the fact that winds aloft are almost all westerly
really only favors the east side for strong winds. Even there,
winds may only reach advisory criteria Monday afternoon, so no
watches were issued.

There will be a brief break in precipitation Monday night, but the
next low pressure system arrives Tuesday. This system takes a more
northerly route as the jet lifts slightly to our north. As a
result, winds and precipitation are likely to be less with the
second system, but we can still expect significant rainfall along
the coast, snow in the mountains, and breezy winds in the wind-
prone areas on Tuesday. -Wright

.LONG TERM...Wednesday through Saturday...The strong Pacific jet
will become oriented more/less northwest to southeast across the
area Wednesday and Thursday. This will keep SW Oregon and northern
California in a moist, onshore flow with plenty of showers, focused
along the coast, adjacent coast ranges and into
the Cascades/Siskiyous. While this pattern favors showers in those
areas, scattered showers are also expected in the valleys and
south/east of the mountains. Temperatures will likely remain below
average with highs in the upper 50s to low 60s over the west side
and in the low to mid 50s over the east side.

By Friday, the best forcing associated with an upper trough will
shift to the east and showers will diminish/end. Temperatures should
edge back closer to normal.

Models then indicate high pressure offshore building into the area
Friday night into Saturday. The ECMWF builds the ridge strongly,
which, if correct, temperatures could take a run at 80F here in
Medford. The GFS is more subdued and flatter with the ridge. Either
way though, temperatures should make it back above normal. -Spilde

&&

.AVIATION...23/12Z TAF CYCLE...
MVFR conditions with local IFR exist, from the Coastal Mountains
westward into the coastal waters this morning, as well as just west
of the Oregon Cascades, and across much of the East Side northeast
of Mount Shasta as scattered showers develop. Partial terrain
obscurations also exist across the area. During the day today
conditions are expected to remain MVFR with local IFR along and near
the coast. Elsewhere, VFR conditions are mostly expected, but MVFR
will occur in isolated to scattered showers. This evening and
overnight conditions will deteriorate as an unseasonably wet weather
system moves in. Expect MVFR to develop overnight at most locations
with areas of IFR along and near the coast. Partial terrain
obscurations will become near total this evening into Monday
morning. Light icing in showers today will increase tonight as
steady rainfall with convective elements overspreads the area. BTL

&&

.MARINE...Updated 300 AM PDT Sunday, 23 April 2017...
Showers and steep seas are expected today as a weak front lingers
over the area. A stronger frontal system will arrive this evening
with increasing south winds and steepening seas. A brief period of
gales is possible north of Cape Blanco early Monday morning. Winds
and seas will peak early Monday morning as the cold front moves
through. Another frontal system is expected to move through Tuesday
into Wednesday. Thursday into the weekend high pressure is likely to
build in from the southwest bringing northerly winds.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Winter Storm Watch from this evening through Monday evening for
     ORZ027.

CA...None.

Pacific Coastal Waters...Small Craft Advisory until 5 AM PDT Tuesday
     for PZZ350-356-370-376.
     Hazardous Seas Warning from 2 AM to 11 AM PDT Monday for
     PZZ350-370.

$$



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