[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT WED JUL 13, 2011



BURN ADVISORY:



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

Prep burning is allowed from 12:00 p.m. to 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from 12:00 p.m. to 5:00 p.m.



WEATHER DISCUSSION:



The center of an unseasonably strong upper-level trough was over northwestern Washington, late this morning, with a continued moist southwesterly flow aloft over Oregon.  Visible satellite imagery showed mostly cloudy skies over Western Oregon, where temperatures had only climbed into the low to mid 60s.  With cool air aloft, daytime heating is destabilizing the air mass; leading to vertical cloud development.  Late-morning Doppler radar showed an increase in both coverage and intensity of showers.



The upper-level trough is forecast to weaken slightly today, as it continues to slowly move across northwestern Washington.  However, the moist and increasingly unstable air mass will lead to scattered showers this afternoon, along with some sun-breaks.  Temperatures will be 5-10 degrees below normal.



TODAY'S FORECAST:



Mostly cloudy with a few showers.  Partial clearing.  Additional rainfall totals mostly less than one-tenth (.10") of an inch.



Surface winds:  S 5-8 mph; becoming SW 6-10 mph later this afternoon

Transport winds: SW 10 mph this afternoon

Salem's forecast high temperature:  73 degrees

Maximum mixing height:  5500 feet

Relative humidity:  Near 60% at noon; dropping to around 45% by 5 p.m.

Salem sunset tonight: 8:57pm



THREE-HOURLY DATA:



                                 2pm     5pm     8pm

     Temperature:                 69      72      69

     Relative Humidity:           53%     46%     46%

     Surface Wind Direction:     210     260     280

     Surface Wind Speed:           6       8       8

     Transport Wind Direction:   230     270     280

     Transport Wind Speed:        10      10      10

     Estimated Mixing Height:   5500    5500    3000

     Ventilation Index:           55      55      30



EXTENDED DISCUSSION:



The upper-level trough is forecast to weaken Thursday and Friday, as the main circulation center moves into southern British Columbia and then Alberta.  The threat of showers should greatly decrease by Thursday afternoon, and Friday looks to be a dry day.  Temperatures will remain below normal but warm a few degrees each day.  High afternoon mixing heights and southwesterly transport winds may provide favorable burning conditions.



Another upper-level trough is forecast set up camp just offshore and swing a weak cold front inland early Saturday.  That will bring a renewed threat of light showers and keep temperatures below normal.  The upper-level trough is forecast to strengthen and remain near the west coast throughout most of next week; rotating several disturbances across the Pacific Northwest.  That will maintain a treat of showers, at times, and keep temperatures moderate.



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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