FXUS66 KMFR 270548

Area Forecast Discussion
National Weather Service Medford OR
1048 PM PDT Sun Mar 26 2017

.Updated aviation discussion.

.DISCUSSION...A cold front is moving through the forecast area
tonight, and showers are affecting most locations in Southwest
Oregon and Northern California. Models and satellite imagery suggest
the heavier band of showers will move across the Cascades this
evening and through Klamath, Lake, and Modoc Counties tonight. After
this band, there is yet another upper trough that will enhance
showers Monday morning. Snow levels will decrease through the
evening, dropping to around 3500 feet overnight, and a Winter
Weather Advisory through Monday morning highlights expected snowfall
in the Cascades and Siskiyous. Given cooling road surfaces tonight,
the next upper trough early tomorrow morning will likely cause the
greatest impact to the mountain passes. Those traveling should at
least stay aware of road conditions and (if not wanting to encounter
snow covered roads) consider traveling over passes with elevations
of 4000 feet or above later in the morning when showers diminish and
road surfaces warm.

Conditions dry out Monday afternoon, and warming is expected
Tuesday. Please see the previous discussion below for details on
expected conditions later this week and into the weekend.


.AVIATION...27/06Z TAF CYCLE...A cold front is moving east of the
Cascades tonight with another disturbance following through Monday
morning. This will continue widespread rain and snow showers with
mountain obscuration and freezing levels falling from around 5000
feet this evening down to 4000 feet MSL on Monday morning.
There will be local IFR (mainly near the coast) and local VFR
(mainly on the east side) ceilings but MVFR and mountain obscuration
for a majority of the area through Monday morning. Conditions will
begin to improve Monday afternoon with diminishing showers and
mostly VFR conditions expected. SK/FB


.MARINE...Updated 800 PM PDT Sunday 26 March 2017...Winds
have turned west and are decreasing quickly below advisory
criteria. However west winds will increase behind the front late
tonight and the combination of steep wind wave and higher west
swell will lead to small craft conditions for seas Monday beyond
10 nm. Westerly swells will build into the 10 to 15 foot range
Monday leading to high and steep seas in all areas. Beyond 30 nm
seas could be very steep around 15 feet. Steep seas could linger
through at least Thursday morning. /FB


.PREV DISCUSSION... /Issued 538 PM PDT Sun Mar 26 2017/


The northern hemispheric projection shows a wave number of four
around the globe and occasionally hints at a five over the next
several days. This argues for a moderately progressive pattern.
That is indeed what we will get over the next several days.
Overall the pattern looks quite spring like...with weaker storms
driven by low amplitude upper level troughs. The primary storm
track will remain to the north of the area...but there will be
some action down here.

A weak long wave ridge is now moving out to the east. An upper
level trough is approaching the coast...and it is ejecting short
waves short waves out ahead of it. The first of these short waves
is supporting a surface front which has now moved onshore. Light
precipitation continues to be reported over the most of the west
side and is spreading farther east on the east side. Winds have
been gusty along the coast...over the ridges...in the Shasta
Valley...and east of the Cascades...but nothing meeting advisory
criteria so far. With the cold core aloft...isolated thunderstorms
are also possible this afternoon and evening over northern Klamath
and Lake counties. Snow levels are running in the 5000-6000 foot
range now and will fall to around 3500-5000 feet tonight.

Subsequent short waves will support plenty of shower activity
tonight into Monday. The last of the short waves and the long
wave trough will move through the area Monday. Showers will
diminish in the wake of the trough. Snow levels will be in the
3500-4500 foot range Monday.

After that, there will be a longer break in the action as a long
wave ridge moves toward the west coast. This will bring dry and
warmer weather to the area Monday night into Tuesday. The ridge
axis will break to the east of the area Tuesday night and a strong
short wave riding up the back side of the ridge will move onshore
to the north of the area Wednesday. A warm front will move onshore
Tuesday night...followed by a trailing cold front Wednesday
afternoon. With most of the upper level support remaining to the
north...both fronts will be relatively weak this far south. Even
so...most if not all of the Medford CWA will get some light
precipitation Tuesday night into Wednesday. A stronger short wave
with a more southerly trajectory will move onshore Wednesday
night with the long wave trough. This will support post-frontal
showers lasting into Thursday.

LONG TERM...Thursday through Sunday.

There remains model uncertainty regarding the strength of an upper
trough and whether it will have more of a southeast or east
trajectory as it moves across the Pacific Northwest on Thursday.
The 12Z ECMWF has been more consistent with a colder/wetter
southeast track than the 12Z GFS while a slim majority of the GFS
ensemble members shade toward the operational GFS solution. A blend
of models was utilized with showers/snow showers in the chance to
likely category with the highest probability on and near northwest
facing mountain slopes. Snow levels are forecast to be around 4000
to 4500 feet and snow amounts likely to be around 1 to 4 inches.

A cold northerly flow is expected Thursday night into Friday morning
with diminishing cloud cover and perhaps a few lingering evening
showers. Low temperatures still look to be a couple to several
degrees below normal for most of our area with lows mainly in the
30s on the west side and in the 20s on the east side.

A high pressure ridge is likely to bring a warming and drying trend
Friday into Saturday with the storm track pushed northward into
Canada. Although there is no support from the GFS ensemble, a slight
chance of showers was included for the Coos and Douglas County
coast. The ECMWF solution is stronger and farther south with the
track of an occluded front. In this 'worst case' scenario the bulk
of light precipitation would still remain north of our area.

Model uncertainty is very high for day 7/Sunday. The forecast high
temperatures for Sunday were kept near Saturday's expected readings.
But, these will likely be adjusted as the ECMWF indicates a building
ridge while the GFS shows another deep trough moving from the Gulf
of Alaska into the Pacific Northwest with an increasing
probability of precipitation Sunday night into Monday. -DW


OR...Winter Weather Advisory until 11 AM PDT Monday for ORZ027-028.

CA...Wind Advisory until 8 PM PDT this evening for CAZ081.

Pacific Coastal Waters...Small Craft Advisory for hazardous seas
until 11 PM PDT this evening for PZZ350-356. Small Craft
Advisory for hazardous seas from 11 AM Monday to 11 AM PDT
Thursday for PZZ350-356. Hazardous Seas Warning from 11 AM to
11 PM PDT Monday for PZZ370-376. Small Craft Advisory for
hazardous seas until 11 AM PDT Thursday for PZZ370-376.


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