[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT WED AUG 28, 2013



BURN ADVISORY:



Agricultural Burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



The weather pattern today is very similar to what we saw on Tuesday, so another significant burning opportunity is likely this afternoon.  The most probable timeframe is from 2:00 p.m. until 6:00 p.m.  PIBALS are scheduled to begin at 1:00 p.m.  Due to spotty light rain in the northern Willamette Valley overnight, some fields may be damp enough to require fluffing.



An autumn-like upper-level trough remains anchored in the eastern Gulf of Alaska, with a warm and moist south-southwesterly flow aloft over Oregon.  Surges of moisture continue to circulate around the trough, with the next batch of rain not slated to come onshore until tonight.



Midday satellite imagery shows an impressive cloud-shield, associated with a nearly stationary frontal system, stretching from just off the southern British Columbia coast southwestward to off the northern California coast.  Cloudy skies extend inland across much of western Oregon, especially from the coast range westward, but no rain is being reported.  The Silverton Hills are on the eastern edge of the clouds, with sunshine from the Cascades eastward.



At the surface, a thermal trough extends from central Washington to over the northern Oregon Cascades.  Pressure gradients are weakly onshore from Newport to Salem and flat from Salem to Redmond.  Like yesterday, daytime heating should shift the thermal trough into central Oregon this afternoon, which will increase the onshore gradients; making for favorable burning conditions.



Transport wind directions are likely to be southwesterly again this afternoon.  Light low-level winds should also turn southwesterly, and eventually westerly; similar to what occurred yesterday.  The air aloft is still cool enough to support mixing heights rising above 5000 feet.



TODAY'S FORECAST:



Partly sunny and warm.



Salem's high temperature today will be near 84 degrees (normal is 81).

Relative humidity:  Drops to 50% by 2 p.m. and to near 40% by 5 p.m.

Surface winds: Becoming SW 3-7 mph early this afternoon and W 5-10 mph late this afternoon.

Transport winds: Becoming SW 15 mph this afternoon.

Mixing height: Rising to near, or above, 5000 feet after 2 p.m.

Salem's sunset tonight: 7:56 p.m.



(Salem Airport data for Tuesday, Aug. 27th: High 85°F; Rainfall Trace)

(Maximum Ventilation Index expected today: 90)



EXTENDED DISCUSSION:



Computer models are consistent in showing the offshore upper-level trough moving closer to the coast on Thursday, with another surge of moisture likely spreading mostly light rain inland across NW Oregon, beginning Thursday morning.  The latest guidance suggests about one-tenth to one-quarter of an inch of rain in the Silverton Hills.  Temperatures will drop back into the mid 70s.



Things should dry out Friday, with more sunshine helping temperatures recover to near normal.  Increasing sunshine will boost temperatures back into the 80s Saturday and Sunday.  Another upper-level trough is forecast to spread showers and cooler temperatures back across the region Monday (Labor Day) and/or Tuesday of next week, but this system is showing up as "weaker" with each update of the computer models.



The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text



Notes:



     1.  Mixing height, as used here, is the lowest height at which the

         potential temperature exceeds the potential temperature at the

         surface.  As a practical matter it is the approximate height to

         which a smoke plume will rise assuming good ignition, dry fuels, and

         winds less than about 15mph.



     2.  Transport winds are a layer average through the mixing height,

         weighted slightly toward the winds at the top of the layer.



     3.  Ventilation Index is the height of the mixing layer (ft) times

         the transport wind speed (mph) divided by 1000.



     4.  Surface wind direction is the general expected wind direction.

         At a specific point surface winds are highly dependent on local

         terrain conditions.



This forecast is provided under an agreement between the Oregon Department of

Agriculture and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.



Pete Parsons

ODF Meteorologist
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