Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion...updated 
National Weather Service Medford or
954 PM PDT Friday Aug 29 2014

Update...a second update to the forecast was issued to adjust the
coverage of smoke and haze through Tuesday night based on the 700 mb
steering winds indicated by the 00z GFS and NAM models.


Discussion... /issued 722 PM PDT Friday Aug 29 2014/

A change to cooler temperatures for the Holiday weekend is
underway. The forecast was updated this evening to shorten the
length of the Small Craft Advisory for the outer waters south of
Cape Blanco. It will now expire at 8 PM with winds and seas
diminishing over the waters into Saturday morning. Also, the sky
cover forecast for tonight into Saturday was adjusted slightly
with a modest increase made to the northern tier of our
area...especially for north of the Umpqua Divide on Saturday. In
that same vein, the portion of Coos County and Douglas County with
a slight chance of light showers Saturday afternoon was increased.
The probability will be highest on northwest facing slopes of the
foothills and mountains. The 18z GFS is just a couple hours slower
than the 18z NAM with the surface trough. If the GFS were to
verify, it would support a continued mention of a slight chance of
showers in Douglas County into early Saturday evening. The
aforementioned trough will also bring another afternoon and
evening of breezy to windy westerly winds that will be strongest
east of the Cascades. Wind speeds on Sunday and Monday will
weaker, back to around or slightly below normal.


Aviation...based on the 30/00z taf cycle...marine stratus is
already in place with IFR ceilings at Newport and expect this to push
south and/or develop at koth around 03z to 04z then south of Cape
Blanco a couple of hours later. The marine push will be strong enough
to push the stratus into at least the western portion of the
Umpqua basin bringing a chance for MVFR ceilings at krbg. Latest met
guidance from the NAM model shows this, but the lamp does not. For
now went with borderline MVFR ceilings at krbg towards 13z, but later
shifts will want to re- evaluate. Elsewhere VFR conditions will
prevail except for areas of wildfire smoke producing MVFR/IFR
visibilities in Siskiyou County. The plume from those fires will
also produce areas of haze and reduced visible downwind, particularly
across the rest of northern California and into the Klamath basin,
where occasional MVFR visible will continue for most of tonight.
Models show upper winds becoming more westerly, possibly northwest
which could push the smoke out with improved visibility at klmt by
12z Saturday.


Marine...updated 220 PM PDT Friday 29 Aug 2014...current observations
at buoy 15 shows a north winds near 10 kts and the latest ascat pass
shows winds near 10 kts in the southeast part of pzz376. The latest
models have backed off on the magnitude of the winds in the southern
waters and inner waters. The buoys are also showing low sea heights,
therefore scaled back the coverage area of the small craft for seas
to the southern outer waters. Even then it will barely reach
criteria and it's possible it could be canceled later this evening
if we do not see an increase in sea height. Guidance shows
increasing northwest swell Saturday afternoon and night, but only to
around 6 feet with a period between 10-11 seconds, so do not
anticipate any headlines. Models are in fairly good agreement
showing north winds increasing Monday afternoon in the southern
waters with steep wind driven seas. -Petrucelli


Previous discussion... /issued 258 PM PDT Friday Aug 29 2014/

A change in the weather is underway as a broad upper trough moves
across the Pacific northwest through the weekend...a mainly dry
change however. The main affects of this trough will be locally
increasing and shifting winds over the area this evening and
Saturday and cooling temperatures. Rain chances are minimal and
confined to the coast and Umpqua where some light drizzle looks
possible tonight and just a wee chance of a light rain shower
Saturday. Temperatures across most areas Saturday will be roughly
10 degrees cooler than today...and up to a few degrees below
normal. Marine clouds will likely make it well inland and may be
seen surrounding the Rogue Valley in the morning. Sunday looks
drier and not as breezy. Low temperatures will be much cooler as
well with lower 50s across the west and 40s east. The air
conditioners can take a break.

A warming trend follows Monday-Tuesday as high pressure rebuilds
slightly over the region. Only subtle differences are shown in the
models thereafter with another digging trough advertised through
late week with a possible closed low positioning somewhere near
northern California toward next weekend. Fairly low confidence at this
point how that will turn out...but this could mean a chance for
thunderstorms (mainly northern ca) perhaps as soon as Thursday
afternoon. In addition, offshore flow could kick up for a short
spell. Stavish

Fire amounted to another day of plume driven fire
behavior, especially over the Happy Camp complex in western Siskiyou
County. Elsewhere, dry humidities and breezy winds will precede a
dry cold frontal passage expected later today. While the cold front
will produce some gusty winds, it will also bring cooler
temperatures, slightly higher humidities, and some improvement to
overnight humidity recoveries. Unfortunately, those humidity
improvements will only be slight across the most active fire areas
in northern California, and will only provide marginal relief to
efforts there, at best.

Tomorrow, the dominant flow will switch to the northwest behind the
front, and another round of breezy winds is expected, especially over
ridge lines, and along any valleys or drainages oriented from the
northwest to southeast. After a brief period of warmer and dry
conditions late this weekend into early next week, another dry front
is expected midweek, once again providing gusty winds and some
temperature and humidity relief. There is no indication of anything
more than a very slight chance of precipitation for at least the
next week. -Bpn


Mfr watches/warnings/advisories...


Pacific coastal waters...none.



National Weather Service Glossary of Abbreviations