[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT THU OCT 4, 2012



BURN ADVISORY:



...State Fire Marshal Burn-Ban Conditions are likely 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 continuing the dry and stable northwesterly flow aloft over Oregon.  A surface thermal trough is building northward along the southern Oregon coast.  That is forcing very dry air, from eastern Oregon, through the Columbia Gorge and Cascades passes into the Willamette Valley.  Relative humidity values in the Willamette Valley have already dropped to near or below 30%, and wind speeds are approaching 15 mph.  State Fire Marshal Burn-Ban Conditions are likely this afternoon.



Satellite imagery shows clear skies across all of Oregon this morning, with the exception of low clouds and fog along the southern coast.  However, wildfire smoke is making for hazy conditions across most of the state, including the Willamette Valley.  The air aloft has warmed slightly since yesterday, so mixing heights should stay below 3000 feet this afternoon.



TODAY'S FORECAST:



Sunny, hazy, and breezy.



Salem's high temperature today will be near 75 degrees (normal is 70).

Relative humidity:  Dropping to near 20% this afternoon.

Surface winds:  NE 10-18 mph.

Transport winds: NE 20-25 mph.

Mixing height: Rising to near 2800 feet this afternoon.

Salem's sunset tonight: 6:46 p.m.



THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 73      73      61

     Relative Humidity:           20%     19%     30%

     Surface Wind Dir/Speed:    NE 15   NE 15   NE 12

     Transport Wind Dir/Speed:  NE 23   NE 25   NE 20

     Mixing Height:              2800    2800    1200

     Ventilation Index:           64      70      24



EXTENDED DISCUSSION:



Little change in the sunny and stable conditions is expected through most of next week.  Offshore flow should weaken by Monday, but there are no indications that this will create any burning opportunities.  The upper-level ridge is forecast to weaken by next Friday, with a mostly dry cold front possibly turning transport winds onshore and creating a burning opportunity.



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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