[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT THU JUL 7, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are from now until 7:00 p.m.



WEATHER DISCUSSION:



A "marine push" began early this morning and was continuing at midday.  Pressure gradients have turned strongly onshore, in response to an upper-level trough moving onto the Washington and northern Oregon coast.  Much cooler air is pouring into the valleys of western Washington and northwestern Oregon with Willamette Valley temperatures mostly in the mid to upper 60s late this morning (about 5-10 degrees cooler than 24 hours ago).



Late-morning visible satellite imagery showed coastal low clouds covering all of western Washington and extending as far south as interior Lane County in western Oregon.  The marine clouds deck will only partially break up this afternoon, with the added lift from the approaching weak weather system also bringing a slight chance of sprinkles.  Rainfall so far today has been limited to just few hundredths of an inch across the extreme north coast.



The early morning onset of the "marine push" has created unfavorable gradient-stacking conditions, which may persist well into the afternoon.  Cooling aloft will help maximum mixing heights climb to near 6000 feet by late this afternoon, as temperatures reach the low to mid 70s.  Mixing heights should quickly drop below 3000 feet this evening, in response to more marine air pouring into the valley.



TODAY'S FORECAST:



Mostly cloudy and much cooler.  Partial afternoon clearing. Slight chance of sprinkles, especially near the Cascade foothills.



Surface winds:  NW 5-12 mph

Transport winds: NW increasing to 10-15 mph

Salem's forecast high temperature:  75 degrees

Relative humidity:  Drops to 50% by around 2 p.m.

Salem sunset tonight: 9:00pm



THREE-HOURLY DATA:



                                 2pm     5pm     8pm

     Temperature:                 72      75      69

     Relative Humidity:           50%     44%     49%

     Surface Wind Direction:     330     330     310

     Surface Wind Speed:           8      10      12

     Transport Wind Direction:   320     320     320

     Transport Wind Speed:        12      14      18

     Estimated Mixing Height:   5000    6000    2500

     Ventilation Index:           60      84      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 by Tuesday and Wednesday.



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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