Scientific Forecaster Discussion
fxus66 kmfr 211618
Area forecast discussion
National Weather Service Medford or
918 am PDT Thu Sep 21 2017
Discussion...the axis of the upper trough has shifted east of
our area and it will be much more quiet day compared to yesterday
in terms of shower activity. A few isolated showers are expected,
but most will be over the higher terrain and northwest facing
slopes due to upslope flow. Even then precipitation amounts will
be very light in these areas.
Some clearing took place and this allowed temperatures to cool
down. Also patchy fog developed early this morning in portions of
the rogue and Illinois Valley. Road snow accumulations were
confined to the higher elevations near and at Crater Lake rim and
Crater Lake entrance. A trace of slush was at Diamond Lake and
dead Indian Memorial Summit, but roads were wet in these areas.
Another cold night is expected east ofthe Cascades with
temperatures likely dropping near or below freezing. Some Westside
locations will have chilly temperatures and some of the typical
cool spots could dip into the upper 30s late tonight into Friday
morning. Probably not cold enough to warrant a frost advisory, but
we'll take a closer look at this. -Petrucelli
Aviation...21/12z taf cycle...
areas of MVFR with localized IFR/LIFR exist across the valleys this
morning due to mostly ceiling and some visibility obstructions,
while partial terrain obscurations exist across the higher terrain.
MVFR is generally expected at the primary airfields this morning,
improving to VFR late morning through early afternoon. Light rain
showers and higher mountain snow showers exist this morning and are
expected, at times through the taf period. A mix of IFR and MVFR
ceiling/visible conditions are expected tonight through Friday morning
across most valley areas of the west side. Btl
Marine...updated 330 am PDT Thursday, 21 September 2017...
steep northwest swell hazardous to small craft will briefly continue
this morning with seas hovering near 10 feet, mainly beyond 10
nautical miles of the coast. Seas will then briefly subside below
Small Craft Advisory level to around 8 feet this afternoon before
returning this evening for areas well offshore. Winds will be light
today as weak low pressure drifts southward through the coastal
waters bringing with it more showers. Friday afternoon and evening
north winds and seas will begin to build with Small Craft Advisory
level conditions expected for much of the area south of Cape Blanco
by evening. This will begin a prolonged period of northerlies that
will maximize in strength each afternoon and evening as thermal
troughing returns and persists. Btl
Previous discussion... /issued 635 am PDT Thu Sep 21 2017/
Discussion...an upper level trough centered over the region will
continue to bring much cooler than normal temperatures across the
area today. Additionally, a weak low, just off the coast, is
moving down the backside of the trough and will bring increased
showers to the area this morning into this afternoon. Showers will
decrease in coverage tonight. On Friday, some lingering light
showers are possible as the upper trough remains over the area,
gradually moving eastward. These lingering showers are expected
mainly in the southern Oregon Cascades and in the mountains east
of the Cascades on Friday. An upper ridge to the west will then
move over the area beginning Saturday. This will bring dry and
warmer weather on Saturday.
Mainly scattered to isolated showers are expected today with snow
levels around 4500 to 5000 feet this morning, rising to 5500 to
6000 feet during the afternoon. Snow showers are expected to be
light and bring little or no additional snow accumulations today.
Shower activity will decrease tonight as the upper trough axis
gradually shifts east of the area. With the upper trough still in
the vicinity, lingering light showers are possible through Friday,
mainly in the southern Oregon Cascades and across portions of the
east side. Otherwise, expect warming temperatures on Friday with
afternoon high expected in the upper 60s to low 70s for western
valleys. However, due to cold air lingering east of the Cascades,
expect high temperatures to only reach the upper 50s for eastern
valleys on Friday.
With the cold air mass remaining in place through at least Friday,
freezing or near freezing temperatures will occur during the late
night and early morning hours over areas east of the Cascades in
Klamath, lake, eastern Siskiyou and Modoc counties. Freeze
warnings and frost advisories are in effect this morning and again
late tonight and Friday morning for these areas. Please see the
npwmfr for details.
Beginning Saturday, an upper level ridge will begin to nudge
inland into the region. However, expect another cold start to the
day, especially east of the Cascades, with low temperatures near
or below freezing once again in eastern locations. Dry weather is
expected across the area on Saturday and afternoon temperatures
are expected to be closer to normal for this time of year in
valleys west of the Cascades with highs in the mid 70s. Continued
cool temperatures are expected east of the Cascades though on
Saturday with high temperatures only warming into the mid 60s for
east side valleys.
The ridge will strengthen and remain in place through Thursday. As
a result expect temperatures to rise to near normal or slightly
above normal Sunday into next week with no precipitation. By
next Friday, models indicate that the pattern may change with
another trough moving towards the Pacific northwest.
Fire weather...updated 630 am PDT Thu, 21 September 2017....
an unusually cold upper level trough will remain over the forecast
area today through Friday with a general north wind flow across the
area. Isolated to scattered showers along with substantial cloud
cover is expected, mainly in northwest flow upslope areas during the
afternoon and evening hours. Altogether, the general northerly flow
is a fairly dry one for our forecast area, so precipitation amounts
today are expected to be pretty light, mainly less than a tenth of
an inch. Guidance is also showing the mid-levels to be dry enough
today to keep precipitation amounts light.
High pressure will begin to build in more assertively from the west
tonight through Friday, resulting in a drying and warming trend. A
thermal trough will develop along and near the coast Friday,
persisting through early next week. This will result in east winds
each night and morning, and warm temperatures all the way to the
coast each afternoon. Temperatures are expected to be 5 to 10
degrees above normal next week. Afternoon humidity will likely lower
into the 20s and 30s across the forecast area. With the recent
rainfall, models may be somewhat overdoing the amount of drying, at
least initially, so, for this weekend we've leaned toward the
consensus guidance rather than seeking the drier solutions (as we
were doing prior to the significant rainfall).
Cpc's 8-14 day outlook as well as weeks 2-4 on the cfsv2 continue to
indicate enhanced probabilities for above normal temperatures and
below normal precipitation. So, a steady and long-duration warm and
dry period is expected, albeit tempered by the declining sun angle
associated with the changing of the seasons. Btl/spilde
or...frost advisory from 2 am to 9 am PDT Friday for orz029>031.
Freeze warning from 2 am to 9 am PDT Friday for orz029>031.
California...frost advisory from 2 am to 9 am PDT Friday for caz084-085.
Freeze warning from 2 am to 9 am PDT Friday for caz084-085.