[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 23, 2015



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is allowed between 11 a.m. to 1 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



An upper-level ridge is offshore Oregon, and will be weakened & pushed out by an approaching trough over the Canadian coastline. Westerly onshore flow aloft will prevail throughout the day as a result. At the surface, a ridge has weakened and moved south towards the Oregon/California border. Light gradients persisted overnight, with reduced cloud coverage across the Willamette Valley, and cooler air aloft has kept mixing conditions optimal. Daytime heating should be greater today, allowing onshore gradients to improve rapidly and may provide a larger window for open burning opportunities this afternoon. Departing clouds should allow warmer temperatures, generating a mild afternoon sea breeze that will move inland before sunset as the Canadian trough crosses the U.S. border. This indicates that open burning could extend into the early evening hours today.



TODAY'S FORECAST:



Cloudy becoming Partly Cloudy skies this afternoon.



Salem's high temperature today will be near 83°F (average is 84°F).

Relative humidity:  Drops near 45% by 11 a.m. and near 30% by 2 p.m.

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

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

Mixing height: 5000 feet by 11 a.m. and 5000 feet or greater through the remainder of the day.

Salem's sunset tonight: 8:49 p.m.



(Salem Airport data for Wednesday, July 22nd: High 77°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)



EXTENDED DISCUSSION:



An upper-level trough slides east-southeast into Washington on Friday, producing westerly onshore flow. A surface trough will form east of the Cascades and cause light onshore gradients with westerly winds to set up early, signaling another day of potential open burning opportunities. Mixing conditions will be excellent, and an evening sea breeze may provide an extended day of activity in the Silverton Hills. A strong marine push after sunset will bring cool air and coastal clouds inland to provide more seasonal temperatures through the weekend. No rainfall is expected to occur.



An upper-level ridge will rebuild itself Sunday evening and throughout Monday, turning onshore flow aloft more northwesterly. A surface ridge offshore Oregon will reform as well, turning winds more northwesterly as gradients increase steadily throughout the day. A lack of significant clouds should allow enough heating to generate an afternoon sea breeze, suggesting an auspicious beginning to the upcoming week for open burning opportunities.



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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