[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 WED SEP 26, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



An upper-level ridge of high pressure is bringing dry and rather stagnant weather to the state today.  At the surface, a thermal trough is beginning to push northward along the southern Oregon coast, with northerly pressure gradients beginning to increase across western Oregon.  Visible satellite imagery shows a sliver low clouds persisting over the northern and central Willamette Valley at midday.  Skies are mostly clear over the remainder of the state, with the exception of areas of wildfire smoke, mainly from the Cascades eastward.



Skies should begin clearing shortly, but warm air aloft will suppress mixing today.  North to northeast transport winds will be unfavorable for burning.



TODAY'S FORECAST:



Cloudy skies soon giving way to sunshine.



Salem's high temperature today will be near 75 degrees (normal is 74).

Relative humidity drops to near 50% around 2 p.m. and to near 35% by 5 p.m.

Surface winds:  N to NE 5-10 mph.

Transport winds: N to NE around 10 mph.

Mixing height: Rising to only about 2800 feet by 5 p.m.

Salem's sunset tonight: 7:01 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 67      75      62

     Relative Humidity:           50%     30%     50%

     Surface Wind Dir/Speed:      N 6     N 8     N 6

     Transport Wind Dir/Speed:  NE 10    N 12    N 10

     Mixing Height:              2200    2800    1500

     Ventilation Index:           22      34      15



EXTENDED DISCUSSION:



Little change in the weather pattern is forecast for Thursday, with a broad upper-level ridge of high pressure remaining parked over the Pacific Northwest.  Expect sunny skies after areas of morning fog.  Temperatures should be about 5 degrees warmer than today, with valley highs near 80°F.  Continued northerly transport winds will be unfavorable for burning.



On Friday, the upper-level ridge is forecast to shift eastward, to over the Rockies.  Skies should remain mostly sunny, but increasing westerly flow aloft will force the thermal trough into eastern Oregon.  Northwesterly transport winds may become strong enough to allow for the burning of remaining fields.  A weak cold front will pass mainly north of the state Friday night, with no precipitation expected across western Oregon.  High pressure is forecast to quickly rebuild across the region over the weekend.  No significant change to the mostly dry and stagnant weather pattern is forecast by computer models for the next two weeks.



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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