[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT Wed JUL 13, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are 9:00 a.m. to 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:



An unseasonably strong upper-level trough dumped from one-tenth and one-third of an inch of rain, across the Willamette Valley, between Monday night and this morning, with the greatest totals near the Cascade foothills. The center of circulation was just off the southern Washington coast this morning with a continued moist southwesterly flow aloft over western Oregon.



Mid-morning visible satellite imagery showed clouds covering most of western Oregon again this morning, but a few breaks in the clouds indicated that the air mass is beginning to dry out.  Doppler radar showed a few light showers over the north coast range and possible sprinkles over the valley, especially near the Cascades foothills.  The surface map showed weak southerly pressure-gradients across the Willamette Valley.



The upper-level trough is forecast to weaken slightly today, as it slowly moves onshore across western Washington.  The air aloft is cool, so daytime heating will destabilize the air mass; likely increasing the shower activity across the region.  Surface and transport winds will veer to more westerly late this afternoon.  Temperatures will be 5-10 degrees below normal.



TODAY'S FORECAST:



Mostly cloudy with a few showers.  Areas of partial clearing, especially in the late-afternoon and evening.  Additional rainfall totals less than one-tenth (.10") of an inch.



Surface winds:  S 5-8 mph this morning; becoming WSW 6-10 mph this afternoon

Transport winds: SSW 10 mph this morning; becoming WSW 10 mph this afternoon

Salem's forecast high temperature:  74 degrees

Maximum mixing height:  5500 feet

Relative humidity:  Drops to 60% by noon and to near 40% by 5 p.m.

Salem sunset tonight: 8:57pm



THREE-HOURLY DATA:



                                 11am     2pm     5pm     8pm

     Temperature:                 66       71      73      69

     Relative Humidity:           63%      48%     43%     46%

     Surface Wind Direction:     190      210     260     280

     Surface Wind Speed:           6        6       8       8

     Transport Wind Direction:   210      230     270     280

     Transport Wind Speed:        10       10      10      10

     Estimated Mixing Height:   4000     5500    5500    3000

     Ventilation Index:           40       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.  That should bring generally dry weather and a few degrees of warming each day.  High afternoon mixing heights and southwesterly transport winds may provide favorable burning conditions.



Another upper-level trough is forecast to bring a weak cold front onshore Friday night and Saturday, with a renewed threat of light showers.  A moist southwesterly flow aloft will continue the chance of showers through at least early next week with temperatures likely remaining below normal.



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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