[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 FRI JUL 15, 2011



BURN ADVISORY:



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

Prep burning is allowed from 10:00 a.m. to 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from 10:00 a.m. to 5:00 p.m.



WEATHER DISCUSSION:



The upper-level trough that brought cool and showery weather to the region for the past couple of days has temporarily weakened.  The Salem sounding this morning showed minor warming and drying of the air mass from the surface up to 10,000 feet.  Onshore surface pressure gradients have relaxed, so most of the Willamette Valley was already seeing partly to mostly sunny skies at mid-morning.  More sunshine will help high temperatures climb to near normal this afternoon.



The next in a series of weather disturbances is rapidly approaching the coastline and shows up well on infrared satellite imagery about 200 miles off the Washington and northern Oregon coast.  High clouds from this system are already reaching the northern Oregon coast.  Visible satellite imagery shows low clouds covering the entire coastal strip, but they only extend inland to about the crest of the coast range.  Some light rain was falling on the extreme north coast at Astoria.  look for increasing clouds this afternoon with transport winds veering to more westerly.  The combined lift from daytime heating and the approaching weather system will make for good smoke dispersal conditions today.



Clouds will lower and thicken this evening with light rain likely moving into the Willamette Valley by the early morning hours on Saturday.



TODAY'S FORECAST:



Mostly sunny early with increasing clouds in the afternoon.



Salem's forecast high temperature:  80 degrees

Surface winds:  S 3-7 mph this morning; becoming WSW 6-10 mph this afternoon

Transport winds: SSW 10 mph this morning; becoming WSW 12 mph this afternoon

Maximum mixing height:  5500 feet

Relative humidity:  Drops to 60% by 10 a.m. and to near 38% by 5 p.m.

Salem sunset tonight: 8:56pm



THREE-HOURLY DATA:



                                 11am     2pm     5pm     8pm

     Temperature:                 69       76      79      72

     Relative Humidity:           55%      44%     38%     53%

     Surface Wind Direction:     190      210     250     270

     Surface Wind Speed:           5        6       7       8

     Transport Wind Direction:   200      220     250     270

     Transport Wind Speed:        10       10      12      14

     Estimated Mixing Height:   3500     4500    5500    2500

     Ventilation Index:           35       45      66      35



EXTENDED DISCUSSION:



An upper-level trough is forecast set up camp just offshore this weekend.  A weak front will bring light rain to the region on Saturday, with another weather disturbance maintaining at least a chance of showers Sunday through Tuesday.  Yet another system is forecast for late next week, so it appears as if the cool damp weather pattern will continue through next week.



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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