[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 MON AUG 26, 2013



BURN ADVISORY:



Agricultural Burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



An autumn-like upper-level trough remains anchored in the eastern Gulf of Alaska, with brisk southwesterly flow aloft over Oregon. The region is between weather systems, at midday.  Mostly sunny skies and increasing south winds are aiding in the drying of damp fields across the Willamette Valley.  However, some fields may need to be fluffed to be dry enough for burning.



The combination of slight warming aloft and sunshine will help temperatures recover into the upper 70s this afternoon.  However, the air aloft will remain cool enough for mixing heights to exceed 5000 feet this afternoon.  Brisk southwesterly transport winds should be favorable for burning but may be too strong to allow for good vertical smoke plume development.  PIBALS will begin at noon to monitor transport wind directions and speeds.



A slow-moving weather disturbance, currently just off the Oregon coast, will likely spread rain onto the north coast later this afternoon.  Some light rain may progress into the northern Willamette Valley tonight, as the weather system weakens and stalls over NW Oregon.



TODAY'S FORECAST:



Mostly sunny and breezy through early this afternoon.  Increasing clouds later this afternoon with a chance of light rain tonight.



Salem's high temperature today will be near 77 degrees (normal is 81).

Relative humidity:  Drops below 50% by 1 p.m. and to near 40% by 5 p.m.

Surface winds:  S 10-15; veering to SSW 10-15 mph this afternoon.

Transport winds: SSW 15-20 mph; veering to SW 15-20 mph this afternoon.

Mixing height: Rising to 5000 feet by 2 p.m. and to near 6000 feet by 5 p.m.

Salem's sunset tonight: 7:59 p.m.



(Salem Airport data for Sunday, Aug. 25th: High 71°F; Rainfall .07")

(Maximum Ventilation Index expected today: 120)



EXTENDED DISCUSSION:



The flow aloft will turn more southerly by Tuesday, with a decaying frontal system keeping skies mostly cloudy over NW Oregon.  Southwesterly transport winds will likely remain favorable for burning, but continued warming aloft will begin to suppress mixing.  Dampness of fields may also limit burning.



A mostly dry day is forecast for Wednesday, ahead of another weather system slated to come onshore Thursday afternoon.  Some burning opportunities are possible each day, but that will depend of dampness of fields and the timing of incoming weather systems.



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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