Scientific Forecaster Discussion
fxus66 kmfr 122244
Area forecast discussion
National Weather Service Medford or
244 PM PST Tue Dec 12 2017
Persistence is the key yet again looking ahead until Friday. Went
ahead and issued a freezing fog advisory and dense fog advisory
for the exact same areas as last night. The only difference with
tonight vs last night is some upper level energy is moving through
and might destabilize the atmosphere a bit, but the inversion is
looking like it will be quite strong yet again with lows in the
lower 20's in the valleys. Would like it to be noted that driving
can be hazardous with freezing fog. Co-workers reported a crash on
McAndrews Road in Medford this morning, which may have been due
to freezing fog and associated slippery conditions.
Also not a huge impact, but added haze to the weather forecast
since it's looking rather hazy outside. This is probably a
combination of the pollution in addition to the smoke from
wildfires and perhaps some other Burns around the region.
The temperature forecast should be completely persistent. The only
other possible change is slightly cooler for daytime highs given
the increase in haze and the possibility of some high clouds
getting into the area Thursday. Relative humidity around 300 mb is showing some
brief periods around 60-70%, so high clouds are not out of the
question on Thursday.
Finally, a trough moves through Friday and models have been pretty
consistent bringing it through, the only question now is the
moisture. Thinking the latest dry trend seems reasonable and the
NAM is, yet again, the wettest solution. So went with a slight
chance for rain with only a few hundreths of accumulation around
terrain. The other thing we wanted to note is that the mixing
height on Friday evening will reach 3500 feet, which will meet
criteria to cancel air stagnation.
Long term...Sat (12/16) through Tue (12/19)...the models are
generally in poor agreement in the extended period (both run to
run and model to model) , and this results in lower than normal
confidence, at least in the later part of the extended forecast.
Looking back over the last four runs of the European center (ec)
model, it has uncharacteristically shown the worst run to run
consistency, especially regarding the potential formation of a
cutoff low over or offshore of the pacnw early to mid next week.
Over these runs, the ec has gone from a progressive trough moving
quickly into The Rockies early next week to a cutoff low over the
Great Basin to a huge cutoff low over California and finally, at
12z this morning, a huge low way off California. For this reason,
I have little confidence in the ec.
The GFS had an offshore cutoff low solution earlier as well, but the
last three runs have been largely consistent in showing a trough
coming ashore and then cutting off over the pacnw by Tuesday. It's
issues are largely timing rather than strength or placement. It
should also be noted that while I haven't looked at previous runs of
the Canadian, it doesn't look much like either the GFS or ec. As a
result, I haven't changed the forecast in the extended much, but
where I have, it's been toward the GFS. That said, it wouldn't be
surprising (or unprecedented) to have the models do a complete 180
on the next run and render all this moot.
Despite the differences, there is at least agreement that our area
will be dry this weekend into Monday with the exception of far
northern areas that may catch some light rain at times from a warm
front well to our north. Of course, we will still have night time
fog issues over the valleys. Regarding Tuesday and beyond: there is
a chance for some cooler and wetter weather, especially if the
currently more consistent GFS ends up being right, but again,
confidence in this is quite low.
Aviation...12/18z taf cycle...areas of IFR/LIFR in fog and freezing
fog over the Umpqua, Illinois Valley will continue through at least
early this afternoon. The taf at Roseburg shows VFR cigs developing
around 21z, but this could be delayed by an hour or two. Meanwhile
low clouds and fog could linger until later this afternoon in the
Illinois Valley and Grants Pass area.
Meanwhile, freezing fog is shallow in the Rogue Valley, including
Medford and should clear out around 19z.
Fog and freezing fog will return to the same areas this evening and
persist into Wednesday morning. VFR conditions will prevail
elsewhere, including the east side, coast, and coastal waters.
Marine...updated 200 PM PST Tuesday 12 Dec 2017...relatively calm
will persist through the rest of the week. North winds will persist
and will be strongest over the southern waters, but they are
expected to remain below small craft. Moderate west swell will build
late Wednesday night through Thursday and subside Friday, but the
general consensus is seas will remain below 10 feet. A weak front
will move onshore Friday, followed by offshore high pressure
Saturday. North winds will increase Saturday and could be strong
enough to reach small craft in the southern waters.
or...air stagnation advisory until 10 am PST Friday for
Dense fog advisory from midnight tonight to 9 am PST Wednesday
Freezing fog advisory from midnight tonight to 9 am PST
Wednesday for orz024-026.