[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT FRI OCT 5, 2012



BURN ADVISORY:



...State Fire Marshal Burn-Ban Conditions may be reached this afternoon due to the combination of low relative humidity (≤ 30%) and wind (≥ 15 mph).



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A strong upper-level ridge of high pressure, centered in the Gulf of Alaska, is producing a dry and stable northerly flow aloft over Oregon.  A surface thermal trough is building northward along the Oregon coast.  That is continuing to draw very dry air into the Willamette Valley through the Columbia Gorge and the Cascades passes.  Pressure gradients have relaxed somewhat, but State Fire Marshal Burn-Ban Conditions are still possible this afternoon, as wind speeds approach 15 mph.



Satellite imagery shows clear skies across the interior of Oregon.  However, low clouds and fog have moved northward along the coast to about Tillamook. Wildfire smoke is continuing to make for hazy conditions across most of the state, including the Willamette Valley.  Slightly cooler air aloft will allow mixing heights to climb to about 4000 feet this afternoon.



(Salem Airport data for Thursday, Oct. 4th: High 76°F; Rainfall .00")



TODAY'S FORECAST:



Sunny but a little hazy.  Not as breezy.



Salem's high temperature today will be near 75 degrees (normal is 69).

Relative humidity:  Dropping below 30% by 11 a.m. and below 20% by 2 p.m.

Surface winds:  NE 5-10 mph; increasing to 10-15 mph this afternoon.

Transport winds: NE 10 mph; increasing to 12-20 mph this afternoon.

Mixing height: Rising to near 4000 feet this afternoon.

Salem's sunset tonight: 6:44 p.m.



THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 63       73      73      60

     Relative Humidity:           27%      19%     17%     29%

    Surface Wind Dir/Speed:    NE  6    NE 12   NE 12   NE  7

     Transport Wind Dir/Speed:  NE 10    NE 16   NE 18   NE 10

     Mixing Height:              1200     4000    4000    1200

     Ventilation Index:           12       64      72      12



EXTENDED DISCUSSION:



Sunny and stable conditions are expected to persist through Wednesday of next week, with lighter, mostly northeasterly, winds.  Some smoke dispersion models are showing a significant decrease in air quality for the Willamette Valley over the weekend, but that will depend largely on the amount of smoke produced by area wildfires.



The upper-level ridge is forecast to finally push east of the state next Thursday, with the flow aloft becoming southwesterly.  A cold front may create a burning opportunity Thursday afternoon or Friday.  The timing of the system is crucial, because it may bring enough rain to dampen fields.



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 times

         the transport wind speed 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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