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Area forecast discussion 
National Weather Service Medford or
905 am PDT Thursday Sep 18 2014

Discussion...current visible image shows most of the cloud cover
along and west of the Cascades while skies are clear to partly
cloudy east of the Cascades. The highest rainfall totals in the
past 24 hours were confined to the Coast Range. The big Winner was
Red Mound where 0.82 of an inch fell, next was Quail prairie with
0.68 and Flynn prairie with 0.63. Lesser amounts were reported in
western Siskiyou County with anywhere between 0.10-0.16 of an
inch.

Made a few adjustments to the forecast this morning. Adjusted probability of precipitation
higher for the band of precipitation orientated central Curry
County northeast to northern Klamath County. Also trimmed back the
isolated thunderstorm coverage west of the Cascades. The models
(in particular the nam) shows weak instability this afternoon in
western Siskiyou County this afternoon. The GFS is not as unstable
and would not be surprised if we don't get any storms there due in
part that we'll have some warning aloft as the cool pool aloft moves
east of the Cascades. East of the Cascades stand the best chance
of getting a few isolated storms. -Petrucelli




&&

Aviation...based on the 18/12z taf cycle...VFR conditions will
prevail through this evening...except for local MVFR ceilings near
the coast and MVFR visibilities in smoke near wildfires. Areas of higher
terrain obscured. Areas of MVFR ceilings will develop Thursday evening
near the coast and in the Umpqua basin...persisting into Friday
morning.



&&

Marine...updated 315 am PDT Thursday 18 Sep 2014...weak low pressure
just south of the coastal waters will dissipate today. Showers and
isolated thunderstorms are possible this afternoon south of Cape
Blanco. 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.



&&

Previous discussion... /issued 322 am PDT Thursday Sep 18 2014/

Discussion...a quick look at the local surface observation
network shows light rainfall amounts so far with this system,
ranging from a trace or a few hundredths of an inch to 0.20
inches, with the larger amounts falling over the mountains of
southwest Oregon, western Siskiyou County, and the Cascades. This
rainfall should be enough to help with wildfire fighting efforts
in the area, but will likely not be enough to put an end to fire
activity. More precipitation is expected through this afternoon
and evening, followed by another warm and dry period through the
weekend. Models have made a dramatic shift in the long term, but
more on that later.

Today, the main trough axis will swing through the area. Other
factors to include in consideration will be well above normal
amounts of moisture inflow, which will result in precipitable waters
of roughly 1 to 1.5 inches, and the influences of the jet stream,
which will act to produce lift over the region this afternoon. Also,
the cooler air aloft associated with the trough will result in
increased lapse rates and instability. Therefore, have kept chance
to likely probabilities of precipitation in the forecast for today,
and bumped up precipitation amounts that can be expected. Have also
added a slight chance of thunderstorms in areas that model solutions
are producing instability, namely the southwestern coast of Oregon
into western Siskiyou County, and the Cascades and all areas east.
The trough will exit the area later this evening, coinciding with
the end of daytime heating. This will end precipitation over the
area rather quickly, most likely before midnight.

When the main portion of the trough exits to the east Friday, the
southern half will split off, cut off, and take up residence over
Southern California for most of the weekend. Meanwhile, ridging will
build in over the forecast area, and the thermal trough will return.
Expect warmer temperatures and dry conditions to persist through the
weekend, and despite any rainfall received form the current system,
much of the area will dry out very quickly under offshore flow. The
cut off low to our south will then begin to lift northeast by Sunday
morning, and although it will push moisture north along with it, the
bulk of the moisture will stay to our south and east. Some model
solutions suggest a chance for convection over Modoc and lake
counties, but given the scenario, do not have confidence in
thunderstorm development there Sunday afternoon/evening.

The movement of the low to the northeast will be due to a very broad
and deep trough, or perhaps series of troughs, approaching the
region from the eastern Pacific. Temperatures will gradually cool as
the system approaches through the first half of the week, but a much
more drastic cooldown is possible Wednesday through the remainder of
the term. Also, the system will have a strong moisture inflow, and
if current trends hold out, this could be a very wet event. Another
interesting aspect is that snow levels are showing signs of dropping
to below the elevation of the area's highest mountaintops, which
could result in the first High Peak snowfall of the season. It
should be noted that uncertainty remains high, as this drastic
pattern change is only just now starting to resolve itself.
However, there is remarkable similarity with the latest batch of
models. Given the, for lack of a better term, squirrelly GFS
solution this time yesterday, this latest run is much more
believable, and it coincides well with the much more consistent
European model (ecmwf) solution. Taking all of this into account, have kept
precipitation chances in the forecast beginning Tuesday, and
continued those through at least Thursday night. Have also dropped
temperatures down considerably beginning Wednesday.

While the middle to late week system should not be characterized as a
"fire season ending" event just yet, the chance is there for at
least a portion of the area. However, it must be said again that
uncertainty remains, as there is a lot of time between now and then,
and the situation could change as we get closer to next week. The
next several forecast shifts will be watching the event with great
interest and updating as necessary. -Bpn



&&

Mfr watches/warnings/advisories...
or...none.

California...none.

Pacific coastal waters...Small Craft Advisory for winds from 2 PM Friday
to 11 am PDT Saturday for pzz350-356-370-376.
Small Craft Advisory for hazardous seas from 2 PM Friday to 11
am PDT Saturday for pzz350-356-370-376.

$$

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