Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus66 kmfr 200523 

Area forecast discussion
National Weather Service Medford or
923 PM PST Fri Jan 19 2018

Discussion...the main forecast update this evening was to
increase precipitation probabilities tonight into Saturday morning
along and west of the Cascades, while also decreasing them south
and east of the Cascades. Expect a high probability of
precipitation from the Cascades west, especially north of the
Umpqua Divide and along the coast due to onshore flow, but with
relatively minor amounts (mostly a tenth of an inch or less).
Downslope flow will prevent most precipitation from getting much
past the mountains, but can't rule out isolated showers down to
around Mount Shasta and also the east side near Klamath Falls.
Snow levels will be pretty low, around 2000 feet west of the
Cascades, so if a heavier shower were to move through, some areas
down to around 1500 feet could see some wet snow late tonight or
Saturday morning. Coverage of showers gradually decreases Saturday
afternoon/evening to the immediate coast and mostly north of
Roseburg as a weak ridge aloft noses into the area.

The next front will approach the coast late Saturday night and
move onshore Sunday. We have added a high wind watch to the
immediate coast, mainly from Cape Arago southward during this time
frame as 925-850mb flow peaks at 65-70kt. -Spilde


Aviation...20/00z taf cycle...
a cold upper level trough will remain over the forecast area for the
next 24 hours. Therefore, rain and snow showers are expected to
continue through the forecast period, though the vast majority of
the shower activity will be from the Cascades westward. A mix of
MVFR and VFR conditions are generally expected through the forecast
period, though a period of valley IFR is possible, mainly west of
the Cascades, around sunrise Saturday morning. Snow levels will be
2500 to 3500 feet for most of the period, so east side airfields
will experience snow with showers that do make it there. Expect
partial terrain obscurations through the period, greatest along the
Coast Range, Umpqua Divide, and Oregon Cascades. Conditions are
expected to improve to VFR at most locations Saturday afternoon. Btl


Marine...updated 700 PM PST Friday 19 Jan 2018...westerly swell
will gradually subside but remain very high and very steep through
tonight. Swell dominated seas will stay high and steep but diminish
briefly to 11 to 13 feet late Saturday afternoon into Saturday
evening. Another strong front will move through late Saturday night
into Sunday. This front will bring another round of strong south
winds and very high seas. Current models are showing high end gales
for all of the waters, and possible storm force conditions north of
Cape Blanco. Will keep the storm watch in effect for now. Currently,
it looks like the best time window for possible storm force winds
will be early Sunday morning with winds diminishing Sunday

Seas will build again ahead of this front. Although not as high as
the previous event, very steep seas of 20 to 23 feet are expected by
early Sunday morning. High pressure will bring brief improvement
Monday, but seas will remain elevated and another strong front is
expected Tuesday. /Br-y/bs


Previous discussion... /issued 405 PM PST Fri Jan 19 2018/

Discussion...showers continue to diminish as the 700 mb flow
decreases this afternoon. An upper level ridge will continue to
nudge into the region and with decreasing onshore flow combining
with cooling surface temperatures scattered showers will slowly
become more isolated overnight. A warm front will move through the
area Saturday, continuing mostly light precipitation through the
day before moving north later Saturday afternoon. On Sunday the
real action moves in with winds increasing on the coast Sunday
morning. With a strong pressure gradient from Redding to Medford
warning strength southerly winds in the Shasta Valley are
possible, and with 700 mb winds nearing 70 kt east of the Cascades
warning strength winds are also possible. Winds at the ridge
level are more westerly than the usual south to southwest that
usually accompany these strong southerly wind systems, and snow
levels in this westerly flow will remain low compared to the
usual high snow levels with these systems. However, the southerly
flow at the surface will feed moisture that will ramp up moisture
feed into Siskiyou County from the SAC valley and precip rates
will increase and snow levels will decrease at times with the
increased precip rates. Onshore flow with cooler temperatures
aloft will keep showers active into Monday. The zonal flow pattern
will transition to a building weak upper level ridge later Monday
into Tuesday morning. Sven

Long term...Tuesday, Jan 23rd through Friday night, Jan 26th... the
next major shortwave trough in our ongoing storm series is expected
to arrive on Tuesday and persist through Friday. This trough of low
pressure is currently on track to be very significant in terms of
precipitation amounts, duration, and mountain snow, and will also
bring with it the possibility of lower elevation snowfall Wednesday
night through Friday. After this trough moves through model guidance
has generally been indicating high pressure ridging for next
Saturday the 27th into Tuesday the 30th, followed by a progressive
trough from the Gulf of Alaska clipping US as it focuses to our
north around the last day of the month into the first days of

Precipitation probabilities, amounts, and snow levels have been
adjusted for Tuesday through Wednesday to be mostly in line with
model consensus, but with a slight lean (60/40) toward the colder
and wetter 12z European model (ecmwf). While both models indicate significant
precipitation amounts, the European model (ecmwf) has been more consistent and seems
to hit the upslope areas more appropriately given the expected flow
directions. Snow levels have been raised some for the early part of
this weather system, up to about 5500 feet for most areas late
Tuesday. This is an indicator that the frontal system will be
capable of holding a significant amount of moisture. By Thursday
and Friday snow levels are expected to be teetering in the 1,000
to 2,500 foot range. Btl


Mfr watches/warnings/advisories...
or...high surf advisory until 4 am PST Saturday for orz021-022.
High wind watch from Sunday morning through Sunday evening for
High wind watch from late Saturday night through Sunday morning
for orz021-022.

California...high wind watch from late Saturday night through Sunday
afternoon for caz081.
Winter Storm Watch from Sunday morning through Sunday evening for

Pacific coastal waters...gale watch from Saturday evening through
Sunday afternoon for pzz350-356-370-376.
Storm watch from Saturday evening through Sunday afternoon for
Small Craft Advisory from 4 am to 10 PM PST Saturday for
Hazardous seas warning until 4 am PST Saturday for

National Weather Service Glossary of Abbreviations