[willamette-fcst] Silverton Hills AM Forecast_Tom

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT THU JUL 16, 2015



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is allowed from 12 p.m. to 2 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



Onshore flow aloft increases as a series of upper-level troughs move in from the Pacific Ocean. A surface ridge offshore Oregon will provide northerly flow throughout the Willamette Valley until it weakens mid-afternoon, allowing a mild sea breeze front to approach from the northwest. Mixing conditions are pronounced within a deep marine layer, and will be maintained through sunset. Favorable conditions may arise after noon, when remnant clouds move into the Cascades and the marine layer heats quickly, reducing humidity. Gradients are currently stacked unfavorably, with +1.0mb onshore from Newport to Salem and +3.3mb onshore from Salem to Redmond. Several hours of persistent heating will be required to overcome these conditions. This points towards a late day of burning opportunities in the Silverton Hills, if all of these wrinkles can be ironed out.



TODAY'S FORECAST:



Mostly Cloudy skies becoming Partly Cloudy this afternoon.



Salem's high temperature today will be near 77°F (average is 83°F).

Relative humidity:  Drops to near 60% by 11:30 a.m. and to near 40% by 5 p.m. this afternoon.

Surface winds: N 5-10 mph this morning; N-NW 10-15 mph this afternoon.

Transport winds: N 5-10 mph this morning; NW 15-20 mph this afternoon.

Mixing height: Rising to 5000 feet by 12 p.m.

Salem's sunset tonight: 8:55 p.m.



(Salem Airport data for Wednesday, July 15th: High 85°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 100)



EXTENDED DISCUSSION:



Mild weather continues into Friday, as the upper-level ridge over the Pacific rebuilds, forcing an upper-level low over Canada further east. A surface ridge will shift into the Oregon coast, producing northerly flow across the Willamette Valley while weakening onshore gradients. This points towards a lower probability of open burning opportunities, but limited prep burning may be possible. The atmosphere will begin a period of warming as it dries out over the weekend. There will be no rainfall for this time as temperatures creep slightly above seasonal averages during the afternoon hours.



Late Sunday night, the upper-level ridge will weaken considerably as a trough from Alaska and remnants from Hurricane Dolores, near southern California, force it back out to sea. The surface ridge will weaken as a result, opening Oregon to westerly flow at all levels. Persistent onshore flow appears to provide multiple days of favorable burning conditions next week.



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 15 mph.



     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 (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.



To add/remove your email address from this list, please go to:



http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst



Tom Jenkins

ODF Meteorologist
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