Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service Medford or
533 am PDT Thursday Apr 24 2014

Discussion...an upper trough offshore will push east and send a
cold front into the northwest part of the region this morning, then
push inland during the day. Snow levels start out near 6000 feet,
but will lower to around 5000-5500 feet by late afternoon with the
coolest air mass still offshore by then. Gusty winds are expected
east of the Cascades with parts of the higher terrain and around
Summer Lake approaching advisory criteria. Most of the precipitation
is expected to be along the Coast Range and Cascades where we could
pick up 0.30-0.50 of an inch.

A broad upper trough will dig offshore tonight with a strong
shortwave rotating around the base of the trough into central cal by
late Friday morning. At the same time cold air aloft with 500mb
temperatures between -30 and -32 c will move into western Oregon. The
models also suggest middle level lapse rates between 7-8 degrees
c/km...lifted indice's in the negative territory over most of the area and cape
values near 400 j/kg along the Coast Range. For the reasons noted
above, would not be surprised if there are isolated thunderstorms
in the above mentioned areas, so have expanded the slight chance
coverage.

Snow levels will lower between 3500-4500 feet tonight and
rise to 4000-5000 Friday, but could temporally lower in heavier
showers. The highest chance for accumulating snow on roads will be
near crater and Diamond Lake and Highway 140 where 3-6 inches are
possible, most between 11 PM PDT Thursday and 11 am PDT Friday.
Siskiyou Summit could pick up an inch or two if heavier showers move
overhead by Friday morning, otherwise expect mostly roadside
accumulations.

In a nutshell, Friday will be much like Tuesday with showers over
much of the area, a few sun breaks and cooler temperatures.

The upper trough will continue to dig southeast and weak ridging
will build in Saturday morning, but it will quickly flatten out as
another frontal boundary approaches. The models continue to remain
in fairly good agreement with the front arriving Saturday night.
Suspect the GFS is too bullish with the eastward extent of the quantitative precipitation forecast
and will be following the ec/NAM solution and adjusting probability of precipitation down
considerably, especially further inland.

The front will move inland Saturday night, but with nearly a zonal
flow aloft, expect most of the precipitation to be along and west of
the Cascades while east of the Cascades will be dry most of the
time, but windy. A dirty ridge sets up Sunday, but a moist west to
northwest flow remains and thus expect precipitation to continue
most of the day and again mainly west of the Cascade crest.

A transition in the pattern is expected Sunday night as riding
starts to build into the area and precipitation pushes north of the
forecast area. Most of next week looks dry and warmer as the upper
ridge continues to strengthen and build along the Pacific northwest.
-Petrucelli



&&

Aviation...based on 24/12z taf cycle...conditions will remain
unsettled over the next 24 hours as several impulses pass over the
area...resulting in widespread showers. Expect MVFR/IFR ceilings and
visibilities to persist through early morning along the coast and much of
the west side...with the exception of the Rogue Valley...where VFR
should prevail...but MVFR ceilings are possible. On the east
side...expect VFR with periods of MVFR in heavier precipitation
early morning. Gusty winds currently impacting the coast and the
east side will continue through much of the day today. A front will
finally push through the area this evening...bringing another round
of heavier precipitation and reduced flight conditions. Terrain
obscurations will be common throughout the taf period...as well as
mountain wave turbulence downwind of the Cascades and other high
terrain. -Sven

&&

Marine...updated 530 am PDT. South winds will turn more westerly
and decrease by sunrise dropping below small craft level by late
morning. However...west swell dominated seas will remain hazardous
to small craft through Friday night. Another front will bring strong
south winds and steep seas to the region Saturday...possibly
reaching gale strength. -Sven




&&

Mfr watches/warnings/advisories...
or...Wind Advisory until 3 PM PDT this afternoon for orz030-031.

California...none.

Pacific coastal waters...Small Craft Advisory for winds until 8 am
PDT
this morning for pzz350-356-370-376.
Small Craft Advisory for hazardous seas until 3 am PDT Saturday
for pzz350-356.
Hazardous seas warning until 11 am PDT this morning for
pzz350-356-370-376.
Small Craft Advisory for hazardous seas from 11 am this morning
to 3 am PDT Saturday for pzz370-376.

$$