[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT THU AUG 7, 2014



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is allowed from noon until 2 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



Only subtle changes to the weather pattern have occurred since yesterday.  The flow aloft has backed from westerly to southwesterly.  Otherwise, the morning sounding over Salem looked almost identical to 24 hours ago.  Weak onshore flow continues to keep wildfire smoke out of the region but is not strong enough to bring marine low clouds into the central Willamette Valley...leaving skies clear with excellent air quality.



The air aloft is cool, so daytime heating should yield high afternoon mixing heights.  Onshore pressure gradients should also be favorable to keep smoke elevated.  The limiting factor for burning, once again, today will be wind direction.  Low-level winds are northeasterly this morning and should turn northerly by early this afternoon.  It is possible that transport winds could turn enough north-northwesterly to allow for limited burning.



TODAY'S FORECAST:



Sunny and seasonably warm.



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

Relative humidity:  Dropping to near 50% by 11 a.m. and to near 30% by 5 p.m.

Surface winds: N 5-10 mph this morning; N-NNW 7-12 mph this afternoon.

Transport winds: NNE 10 mph this morning; N-NNW 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet around noon and to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:30 p.m.



(Salem Airport data for Wednesday, August 6th: High 85°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)



EXTENDED DISCUSSION:



Weak southwesterly flow aloft will continue on Friday.  Onshore may strengthen enough to bring some patchy brief morning marine clouds into the central valley.  Otherwise, skies should be sunny with seasonal temperatures.  Afternoon transport winds are predicted to remain generally northerly, so a burning opportunity is unlikely.



A weak upper-level trough is forecast to turn the upper-level winds southeasterly by Sunday, which should trigger thundershower activity over the Cascades...possibly drifting over the Willamette Valley Sunday night into Monday morning.



Increasing southwesterly flow aloft should push the thundershower threat east of the region on Tuesday, with developing onshore flow possibly creating a burning opportunity.  An upper-level ridge predicted to rebuild over Oregon during the second half of next week with mostly northerly transport winds.



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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