[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT THU SEP 13, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A broad upper-level ridge of high pressure remains over the Pacific Northwest this morning.  Its axis is slowly shifting east, to over Idaho, with increasing southwesterly flow aloft over Oregon. At the surface, a strong thermal trough extends northward along the Oregon Coast.  Low-level winds are still offshore across the interior of western Oregon and the north coast but have turned onshore along the central and south coast.



Satellite imagery shows mostly clear skies across the state, with marine low clouds advancing northward along the south coast.  The offshore flow has transported smoke, from wildfires in the Cascades, across NW Oregon. DEQ air quality monitors are showing elevated smoke levels across western Oregon, as far south as Roseburg.



The surface thermal trough is forecast to slowly drift from the coast into the Willamette Valley this afternoon.  Onshore flow will continue to spread northward up the coast, with winds in the valley becoming light and variable.  The combination of light winds and very warm air aloft will make for poor ventilation conditions this afternoon.



TODAY'S FORECAST:



Sunny, hazy, and very warm.



Salem's high temperature today will be near 92 degrees (normal is 78).

Relative humidity drops to below 20% this afternoon.

Surface winds:  N-NE near 5 mph.

Transport winds: NE 5 mph; becoming light and variable this afternoon.

Mixing height: Only rising to near 2500 feet this afternoon.

Salem's sunset tonight: 7:25 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 84      88      74

     Relative Humidity:           19%     16%     28%

     Surface Wind Dir/Speed:     NE 5     N 5     N 3

     Transport Wind Dir/Speed:  VAR 5   Var 5   Var 3

     Mixing Height:              2000    2500    1000

     Ventilation Index:           10      13       3



EXTENDED DISCUSSION:



The upper-level ridge is forecast to weaken on Friday, with the thermal trough shifting into eastern Oregon.  Increasing southwesterly flow aloft and onshore transport winds will likely create favorable burning conditions Friday afternoon and help to clear wildfire smoke from the valley.  Cooler onshore flow should cap high temperatures in the mid 80s.



A very weak and dry upper-level trough is forecast to move across Oregon on Saturday.  Skies should remain mostly sunny, but onshore flow may be just strong enough to bring some morning marine clouds into the valley.  Temperatures should cool back close to 80°F.  The flow aloft is forecast to turn more northerly on Sunday, with temperatures beginning to warm again.



A strong upper-level ridge is forecast to return next week; bringing another round of offshore flow and very warm weather.  Air quality is likely to deteriorate, as wildfire smoke gets transported back across northwestern Oregon.  The chances for burning opportunities next week look slim.



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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