Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Medford or
837 PM PDT Wednesday Aug 20 2014
Discussion...temperatures are another 5 degrees cooler this
evening compared to yesterday at this time...continuing the
cooling trend. A drier air mass is also being observed compared to
yesterday...wtih dew points several degrees lower this evening. I
have adjusted the sky grids a bit to reflect current conditions...and
interestingly the coastal waters are void of any marine stratus at
the moment. The expectation is that the stratus will fill in this
evening...affecting the coast and coastal valleys late tonight.
Onshore flow is significantly less than 24 hours ago so stratus
won't make it as far inland as last night...and Roseburg should
stay mostly clear tonight.
Below normal temperatures are expected across the forecast area
tomorrow...by a few degrees...and afternoon gusty northwest to west
winds are expected again.
Offshore flow will gain momentum Saturday into Sunday with drying
conditions due to a thermal trough pattern near the coast...and that
will likely invigorate existing fire behavior. Please see the
previous discussion below for more details on an otherwise relatively
quiet weather pattern expected through the weekend.
Aviation...stratus with IFR ceilings is expected to return to the
Coos coast (including koth) and coastal valleys overnight and
then burn off to VFR by middle morning Thursday. Elsewhere, VFR
conditions will persist for the next 24 hours. -Wright
Marine...updated 220 PM PDT Wednesday 20 Aug 2014...with offshore high
pressure and a thermal trough inland, light to moderate north winds
and steep seas will persist through this evening over much of the
area. An area of north gales and very steep seas offshore south of
Cape Ferrelo will also persist through the evening. Winds and seas
will gradually diminish later tonight and remain low through
Thursday. Moderate north winds will likely re-develop south of Cape
Blanco later Friday and continue into the weekend. -Wright
Previous discussion... /issued 304 PM PDT Wednesday Aug 20 2014/
Short term...tonight through Saturday night...mid-level cloud
cover developed today with an increase of moisture from a weak
shortwave in the northwest flow aloft on the back side of a
longwave trough. Drier air will filter in tonight with decreasing
clouds...except for the expected return of coastal stratus north
of Cape Blanco. Thursday will be similar to today with breezy
afternoon and evening winds, and high temperatures around normal.
But, skies that were mostly sunny today will be sunny with less
extensive coverage of west side morning stratus and the lack of
The next upper level trough will move southeast across British
Columbia on Thursday then usher in another push of cooler marine
air. The increase in moisture will be limited to the lower levels,
below around 5000 feet msl, with late Thursday night into Friday
morning low clouds expected at the coast and into the Umpqua
valley. I introduced a 10% probability of very light showers at
the Coos and Douglas County coast on Friday morning.
Friday will be cooler across the area as the trough continues
southeastward into eastern Oregon. The track of the trough will
keep our region on the drier and more stable back side of the trough.
The weekend will begin with the trough tracking east to The
Rockies with the West Coast under a northerly flow and shortwave
ridging while a stronger ridge lurks far offshore with its axis
around 145w. This pattern will produce a strong thermal trough
that extends from northern California to the southern Oregon coast
with offshore winds on the west side Friday night through
Saturday night that will produce the fire weather concern of poor
to moderate overnight humidity recoveries at middle-slopes and
ridges. It will also result in several degrees of warming and
clear skies...except for late night and early morning low clouds
early Saturday at the immediate coast north of Cape Blanco.
Long term...Sunday through Wednesday...
The weather on Sunday will resemble what occurs on Saturday but
with a few additional degrees of heating as the offshore ridge
moves slowly east and thermal trough moves inland.
For Monday through Wednesday, the 12z GFS and European model (ecmwf) models
continue to differ on the strength of features but are in good
agreement on the big picture of a dry cold front with scattered
high clouds and a couple to a few degrees of cooling in our area
on Monday then the ridge moving to the coast with well above
normal temperatures and a stable air mass into mid-week. The European model (ecmwf)
remains slightly stronger/warmer than the GFS, but the day 6 and 7
temperatures have been raised by a few degrees as the GFS has
trended toward the European model (ecmwf) solution.
Pacific coastal waters...Small Craft Advisory for winds until 11 PM PDT this evening for
Small Craft Advisory for hazardous seas until 11 PM PDT Thursday
Hazardous seas warning until 11 PM PDT this evening for pzz356.
Small Craft Advisory for hazardous seas until 11 PM PDT this
evening for pzz350-370.
Small Craft Advisory for winds until 5 PM PDT Thursday for
Gale Warning until 11 PM PDT this evening for pzz376.
Hazardous seas warning until 5 am PDT Thursday for pzz376.