Scientific Forecaster Discussion

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Area forecast discussion...updated 
National Weather Service Medford or
443 am PDT Thursday Apr 17 2014

Updated aviation section

Discussion...a cold front is approaching 130w this morning and
will be making its way through our forecast area today. Clouds are
overspreading the west side right now in advance of the system,
but the coldest cloud tops which represent the precipitating
portion of the front are still a ways offshore. Models show rain
arriving at the coast around dawn and then spreading across the
west side through noon. There isn't a lot of wind with this front,
but there is enough downslope to hold off the rain around Medford
for a while. It's always tricky to time rain in Medford since it
depends so much on downsloping, but we'll probably see showers
develop late morning followed by a few hours of light rain in
Medford as the actual front passes around middle afternoon. The front
jumps the Cascades late this afternoon into the evening and that's
when we'll see the best chance for rain on the east side along
with gusty winds.

This system isn't very wet and probably will probably produce less
than a tenth of an inch altogether for most locations, but we
should get a quarter to a half inch total from the Coos coast
across the Umpqua into the Cascades. Snow levels will start out
above 8000 feet and then drop as low as 5000 feet overnight
tonight, but precipitation will be largely over by then, thus
snow will not be an impact. Models still indicate some instability
ahead of and with the front over lake and Modoc counties this
afternoon, and while it is less than previously shown, we've kept
a slight chance for thunder in those locations.

Showers will linger into Friday morning over the mountains,
otherwise Friday will be dry and warmer as a weak, transient ridge
moves through. Another weak front will bring a chance of rain on
Saturday, but with this focused from the Coos coast into the
Cascades, most of US won't see much if anything.

Didn't change much in the extended period. Sunday looks dry with
southwest flow aloft ahead of a large developing trough over the
northeast Pacific. This trough will move toward the pacnw early
next week. The best chance of rain will be over the west side on
Monday but then as the trough approaches, precipitation will be
possible in all areas. Snow levels are expected to be high until
the trough axis moves in middle week at which point snow may start to
be a concern at pass level. This is a ways out but it something
Worth keeping an eye on. -Wright


Aviation...based on 17/12z taf cycle...from the Cascades west...
conditions will deteriorate this morning. MVFR and local IFR
ceilings/visibilities in rain at the coast will spread inland to the Cascades
during the day. Higher terrain will become obscured. These lower
conditions will continue into this evening with only slight
improvement as precipitation becomes more showery and ends
overnight. East of the Cascades...VFR conditions will prevail this
morning. Local MVFR ceilings will develop this afternoon with areas of
higher terrain becoming obscured in showers. An isolated
thunderstorm cannot be ruled out over the far east side in portions
of Modoc/lake counties. Lower conditions will improve overnight.


Marine...light to moderate southerly winds can be expected today
ahead of a cold front. Rain will accompany the frontal passage, then
precipitation will become more showery as the front moves onshore
later this afternoon and evening with a wind shift to northerly.
High pressure will briefly build Friday into Friday night. A
stronger front will move through Saturday with the potential for
Small Craft Advisory level winds and seas.

Confidence is increasing in a heavy west to northwest swell event
Saturday night into Sunday. Swan and wave watch 3 models are
indicating a rapidly rising heavy westerly swell of 15 to 20 feet
at 16 seconds late Saturday night into Sunday. A marine weather
statement (mwsmfr) has been issued to highlight this potential
hazard. Seas should subside Sunday night but will remain high
through Monday night. Another front will move through the area
late Sunday night or early Monday. Spilde


Mfr watches/warnings/advisories...


Pacific coastal waters...none.