[willamette-fcst] Silverton Hills Forecast

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT THU JUL 7, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are 10:00 a.m. to 7:00 p.m.



WEATHER DISCUSSION:



A "marine push" underway this morning, with pressure gradients turning onshore, in response to an upper-level trough approaching the Washington and northern Oregon coast.  Mid-morning visible satellite imagery showed coastal low clouds penetrating into the northern and central coast range gaps and up the Columbia River into the northern Willamette Valley.  Some low clouds had made it inland as far south as northern Linn County.



The early onset of the onshore push has created some gradient-stacking this morning, which may persist into the afternoon.  Rapid cooling aloft will yield high mixing heights this afternoon, if the marine clouds break up enough to allow for some surface heating.  In any case, temperatures will cool to at least slightly below normal. Look for mixing heights to quickly drop below 3000 feet this evening, as more cool low-level marine air pours into the valley.



TODAY'S FORECAST:



Morning clouds giving way to partly sunny skies and much cooler.  Slight chance of a light shower this afternoon, especially near the Cascade foothills.  After a high temperature Wednesday of 90 degrees, today's high will only climb into the mid 70s.  With any clearing, mixing heights should climb to around 3000 feet by midday and to more than 5000 feet from 2 p.m. through 5 p.m. Surface winds will be northwesterly at 5-10 mph.  Transport winds will also turn northwesterly and increase to 10-15 mph this afternoon. Relative humidity levels will drop below 60% by midday and down to near 45% this afternoon.



Salem sunset tonight: 9:00pm



THREE-HOURLY DATA:



                                 11am     2pm     5pm     8pm

     Temperature:                 69       75      77      66

     Relative Humidity:           57%      47%     42%     58%

     Surface Wind Direction:     330      330     320     320

     Surface Wind Speed:           6        7       9      12

     Transport Wind Direction:   340      320     320     330

     Transport Wind Speed:         7       10      15      18

     Estimated Mixing Height:   3000     5500    5500    2500

     Ventilation Index:           21       55      83      45



EXTENDED DISCUSSION:



The upper-level trough is forecast to move across Washington and northern Oregon on Friday.  Strong onshore flow will further cool the air mass and bring considerable marine clouds inland.  There is a slight chance of morning drizzle or a light shower.  Even with some afternoon clearing, high temperatures will only be in the 70-75 degree range.  Temperatures will warm to near normal over the weekend, with morning clouds and afternoon sunshine.  Another upper-level trough is forecast to move onshore early next week, bringing cooler-than-normal temperatures and a threat of light showers.



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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