Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
250 PM PDT Thursday Sep 18 2014
Discussion...as expected, the upper trough is splitting as it moves
inland as evidenced on the water vapor image. The visible image
shows plenty of cumulus clouds, but they are noticeably absent in
the rogue, Illinois, Scott and Shasta valleys. So far little or no
shower activity has been observed in the last several hours and my
gut feeling is we probably wont see much through this evening.
However there is still a few more hours of heating and with the cool
pool aloft moving east of the Cascades, I would expect most of the
activity to be confined there. We'll keep in a slight chance of
thunderstorms for parts of lake, northern Klamath and southwest
Modoc County, but they will be isolated, if they happen at all.
Over the next 24 hours a cutoff low will develop in central cal and
an upper ridge will build into the area. This will bring a return to
dry and milder conditions starting Friday, then heating up over the
weekend as a thermal trough develops along the coast Friday
afternoon, then moves inland Saturday. The thermal trough will bring
increasing offshore flow with gusty east to northeast winds over the
ridges in western Siskiyou County and southwest Oregon.
The thermal trough is expected to expand north Saturday and this
will allow marine stratus to push northward into southern Oregon.
The main question is how far north. Current thinking is that it
should remain south of Cape Blanco Saturday afternoon, then surge
northward Saturday night. The thermal trough will move inland over
the Westside. Skies will be clear and temperatures will heat up into
the middle 90s in the rogue, Illinois and parts of the Umpqua basin.
The models are in good agreement showing another large scale upper
trough deepening near 140w Saturday night and slowly move east
Sunday. This will act as a kicker pushing the cutoff low in
California northeast into northern Nevada, but it will have little
or no impact on our weather. Any threat of thunderstorms will remain
south and east of the forecast area.
The above mentioned upper trough will continue to push east Monday
and send a weak front through the area bringing cooler temperatures
and increasing onshore flow, but remaining dry. An upper low will
develop near 140w Tuesday with a southwest flow developing ahead of
it. Another, but slightly stronger front is expected to reach the
coast Tuesday afternoon or evening and bring measurable rain to the
coast and Coast Range.
The ec and GFS have remained in remarkable good agreement showing a
much more active pattern starting next Wednesday. The last several
runs of the ec and GFS have continue to highlight this and it's no
surprise the rmop is now show showing a higher probability (21
percent) of a trough moving into the area. This does not seem like a
lot, but it's high given the forecast period is a 168 hours out
(based on 0z run from last night). Given the above mentioned
reasoning, have continued the upward increase in probability of precipitation from next
Wednesday through Friday. Confidence is fairly high we'll have a
significant change in the pattern and one that will lead to much
cooler temperatures and measurable precipitation. In fact could not
rule out moderate to heavy precipitation along the coast Wednesday
and Thursday as stronger fronts move through. Additionally, winds
will increase over the marine waters and coastline.
The ec and GFS diverge after Thursday with the progression of the
upper trough. The ec is more progressive and splits the trough with
ridging nudging in from the west. The GFS keep the trough overhead
with a continued cool and unsettled conditions. However both models
show strong ridging building in the eastern U.S. So it's possible
we'll continue to have troughing in the Pacific northwest. -Petrucelli
Aviation...based on the 18/18z taf cycle...
scattered showers will occur across much of the area this afternoon
and evening, primarily across inland areas. An isolated
thunderstorms are possible, primarily over Modoc and lake counties.
Some partial mountain obscurations will continue into this evening.
Most of the showers will be light, though some MVFR conditions are
expected, and isolated IFR is possible in isolated thunderstorms.
Tonight patchy fog and low clouds will occur in areas that got
appreciable rain, primarily on the west side along and near the
Coast Range and Umpqua basin- though some fog is likely in the upper
Klamath basin. Expect this to be a mix of MVFR and IFR with local
LIFR. Smoke production from area wildfires will be greatly
diminished through the period due to the recent rains and higher
Marine...updated 900am PDT Thursday 18 Sep 2014...
low pressure centered southwest of the coastal waters will move
southeastward along the California coast today. Showers will
continue today, especially in the southern waters. A developing
thermal trough will bring increased north winds with steep wind
waves Friday afternoon through Friday night. The thermal trough
will weaken Saturday and seas will transition back to being
dominated by a moderate swell during the weekend. The weaker
pressure pattern will persist into early next week. Btl
Pacific coastal waters...Small Craft Advisory for winds from 2 PM to
11 PM PDT Friday for pzz350-356.
Small Craft Advisory for hazardous seas from 2 PM Friday to 11
am PDT Saturday for pzz350-356-370-376.
Small Craft Advisory for winds from 2 PM Friday to 5 am PDT
Saturday for pzz370.
Small Craft Advisory for winds from 2 PM Friday to 11 am PDT
Saturday for pzz376.
Hazardous seas warning from 5 PM to 11 PM PDT Friday for pzz376.