Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Medford or
838 am PDT Thursday Apr 17 2014

Discussion...the cold front is knocking on our door step this
morning. The bulk of the precipitation is still offshore and north
of the forecast area, but that will change late this morning along
the north coast and into Douglas County as the front progresses
southeast. The visible image shows mostly clear skies in northern
cal, much of the Eastside and most of the Rogue Valley, but that
will change and clouds will increase and move overhead. The models
still hint at some instability over parts of Modoc and southeast
Lake County, so we'll leave in the slight chance of thunder. The
front will move through tonight and an upper trough will swing
through. Showers are expected to continue into this evening, but
will be decreasing with most ending by daybreak Friday.


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 Umpqua
basin and Cascades late this morning through the afternoon. 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 and Rogue Valley...VFR conditions will prevail this morning.
Local MVFR ceilings are possible 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


Previous discussion... /issued 443 am PDT Thursday Apr 17 2014/

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


Mfr watches/warnings/advisories...


Pacific coastal waters...none.