[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 TUE JUL 12, 2011



BURN ADVISORY:



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



WEATHER DISCUSSION:



An unseasonably strong upper-level trough was centered about 150 miles west of the mouth of the Columbia River late this morning. An associated cold front moved across western Oregon early this morning, dropping generally from one-tenth to one-quarter of an inch of rain across Marion County and extreme northern Linn County.  The highest amounts were near the Cascades foothills.



The cold front has pushed into eastern Oregon, and rain has tapered off across the Willamette Valley.  A composite of area Doppler radars shows that rainfall has tapered off, with most of the remaining activity near and over the northern Cascades.  Satellite imagery continues show cloudy skies covering virtually all of western Oregon.  Skies were partly cloudy east of the Cascades.



Clouds were holding late-morning temperatures in the low 60s, across the Willamette valley, with surface reports confirming a few continuing areas of light rain.  An embedded weather disturbance is forecast to rotate around the parent upper-level trough and onshore this afternoon.  As it moves over northwestern Oregon, additional lifting of the moist air mass over the region should generate scattered showers.  Most valley locations will likely pick up from a trace to a couple hundredths of an inch, but stronger showers could produce around one-tenth of an inch.  High temperatures will be about 15 degrees below normal.



TODAY'S FORECAST:



Cloudy and unseasonably cool with scattered showers.



Surface winds:  SSW 5-8 mph this afternoon

Transport winds: SW 10 mph this afternoon

Salem's forecast high temperature:  66 degrees

Maximum mixing height:  4500 feet

Relative humidity:  Remains above 65% all day

Salem sunset tonight: 8:58pm



THREE-HOURLY DATA:



                                 2pm     5pm     8pm

     Temperature:                 64      66      62

     Relative Humidity:           76%     68%     78%

     Surface Wind Direction:     200     210     220

     Surface Wind Speed:           5       6       7

     Transport Wind Direction:   210     220     230

     Transport Wind Speed:         8      10      10

     Estimated Mixing Height:   4000    4500    2500

     Ventilation Index:           32      45      25



EXTENDED DISCUSSION:



The upper-level trough is forecast to move inland on Wednesday, weaken Thursday, and then lift northeastward out of the region on Friday.  Look for showers to continue on Wednesday with slow drying and minor warming on Thursday.  By Friday, temperatures should recover to within about 5 degrees of normal with some afternoon sunshine.  Another cool trough is forecast to bring back a chance of showers over the weekend.



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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