[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT MON JUL 20, 2015



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



Increasing onshore flow is bringing cooler marine air into the Willamette Valley today, with temperatures running 5-10 degrees lower than 24 hours ago.  At the surface, there was some minor pressure gradient-stacking late this morning, with 1.6mb onshore from Newport to Salem and 2.6mb onshore from Salem to Redmond.  An 11 a.m. pibal showed SW surface winds veering to the west near the mixing height, so the limiting factors for beginning field-burning this afternoon will be the mixing height and pressure gradients.  Sunny skies should warm the valley quickly, lifting mixing heights above 3000 feet by noon and creating more favorable gradients early in the afternoon.  Another pibal is scheduled for noon.



A dry upper-level disturbance will move across the region today, increasing the onshore flow and continuing the cooling trend.  It will also help to maintain general lifting of the atmosphere and limit down-mixing of smoke. WSW transport winds should slowly veer to the NW as the day progresses.  An impressive sea breeze is expected this evening, bringing another surge of marine air into the Willamette valley overnight.



TODAY'S FORECAST:



Sunny but cooler.



Salem's high temperature today will be near 87°F (average is 83°F).

Relative humidity:  Dropping to near 35% by 5 p.m.

Surface winds: SW 5 mph; WNW 5-10 mph later this afternoon.

Transport winds: WSW 5-10 mph; becoming WNW 10 mph later this afternoon.

Mixing height: 3000 feet at noon; rising to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:52 p.m.



(Salem Airport data for Sunday, July 19th: High 96°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 50)



EXTENDED DISCUSSION:



A mostly dry upper-level trough will remain over the region Tuesday through Friday, maintaining onshore flow and near to below average temperatures.  Expect varying degrees of morning clouds and afternoon sunshine.  No significant rainfall is expected, but areas of morning drizzle or very light showers can't be ruled out.  Transport winds will be generally northwesterly, with possible afternoon burning opportunities each day.



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



Pete Parsons

ODF Meteorologist
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