Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
1139 am PDT Friday Sep 4 2015
Discussion...the upper low is now centered over western
Washington and will continue to move south over the area late
this afternoon. A line of showers and isolated thunderstorms broke
out in advance of an shortwave trough rotating around the upper
low which is now in northwest cal. Showers and isolated
thunderstorms will increase over time as the upper low and cold
air aloft (-25c at 500mb) moves overhead increasing instability.
The latest hrrr model run shows most of the showers and isolated
thunderstorms along and east of the Cascades, but suspect it may
be underestimating the convective activity west of the Cascades.
Therefore we'll continue to keep probability of precipitation higher west of the Cascades
this afternoon. In addition there's a chance some thunderstorms
could produce gusty winds and small hail.
The 12z NAM and GFS show the trough axis moving east of the
Cascades late tonight at which point the bulk of the precipitation
will shift into lake, Modoc and eastern Klamath County. Saturday
is still a transition day with any showers confined to Eastern
Lake County in the morning and dry conditions elsewhere.
Aviation...for the 04/18z taf cycle... numerous showers and
isolated thunderstorms will occur today from the Cascades eastward,
while showers will be more scattered east of the Coast Range and
west of the Cascades. Partial terrain obscurations and isolated MVFR
conditions are expected in more significant west side showers this
afternoon into this evening. From the Cascades east expect higher
terrain to be mostly obscured with MVFR to isolated IFR in heavier
showers and thunderstorms. The freezing level will be at 8000 feet
today and then will fall overnight to near the surface on the east
side. Overnight areas of MVFR and local IFR is expected in low
clouds, primarily on the north and northwest slopes and valley
edges, and in the Umpqua basin. Btl
Marine...updated 845 am PDT Friday 4 September 2015... north winds
will continue through the forecast period. A period of Small Craft
Advisory level winds is expected this afternoon and evening in an
area south of Cape Blanco beyond 10 nautical miles of the coast.
Winds then strengthen again Sunday as a thermal trough forms over
northern California. Gales will be possible over the southern waters
Monday and Tuesday. Seas will be primarily wind driven through early
next week. Btl/Wright
Previous discussion... /issued 639 am PDT Friday Sep 4 2015/
Update...put out a quick update just now to increase probability of precipitation on the
east side and allow for thunderstorms over all areas this morning.
Air mass is already destabilizing quickly and we've seen six
lightning strikes along the Siskiyou/Modoc border, and this is
with the majority of the cold pool and instability yet to come
today. It also wouldn't be out of the question to see a few
snowflakes over lake and Modoc counties this morning as surface
temperatures are near or below freezing. Temperature/dewpoint spread are
fairly high, though, so it may be just virga out there. -Wright
Previous discussion... /issued 325 am PDT Friday Sep 4 2015/
Discussion...temperatures this morning have been quite cold
relative to normal for this time of year, with temperatures even
dropping to the middle 20s in the general region near Chemult. With
several more hours of cooling to go, and clouds not quite in the
area yet, the current frost advisory seems to be on track for the
east side. Will therefore let this ride through the morning.
Temperatures will remain much cooler than normal today, as ample
clouds and rain should keep solar heating to a minimum.
An upper level low, currently located over the Seattle area, will
drop south today, and take position over Oregon by this afternoon.
As it drops south, a very potent shortwave orbiting the low will
pass overhead, and with the lowering temperatures aloft, enough
instability and dynamics should be present to produce showers and
isolated thunderstorms over the bulk of the forecast area late
this morning into this evening. While most rainfall amounts will
be light, there is a good chance that many locations could see
wetting rains. This is particularly true east of the Cascades,
where forcing will be strongest during the peak of daytime
heating. Overnight, the cold air mass will begin to settle in.
However, there should be ample clouds remaining in the area, so
nighttime lows may not drop as low as would be expected. Have
added patchy frost to the forecast, but given the cloud cover, do
not think that this would be a widespread event, so will not issue
any headlines at this time.
The low will begin moving east Saturday morning, and gradually,
showers and clouds will move out of the forecast area as well. Expect
that the skies will be mostly clear by Saturday night, aided in part
to developing offshore flow drying out the middle levels. With the cold
air still at the surface, and no clouds to act as a blanket, suspect
that this will be the coolest night of the week. In fact, widespread
frost and freeze is likely for the east side.
These cold temperatures will not last, however, as upper level
ridging will build into the area, and the thermal trough will return
to our south. Offshore flow should keep the area clear of clouds,
and temperatures will rise to values around 5 degrees above normal
for this time of year by the middle of next week. Several waves will
approach the West Coast during this time, but the ridge should keep
the track of any precipitation making systems well north of the
forecast area. Therefore, for the remainder of the forecast term,
expect gradually warming temperatures and dry conditions. -Bpn
Pacific coastal waters...
- Small Craft Advisory until 11 PM PDT this evening for pzz376.