[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 AUG 24, 2012



BURN ADVISORY:



...State Fire Marshal Conditions may be met late this afternoon due to the combination of low humidity (30% or less) and wind (15 mph or greater)...



Agricultural burning:

(From Salem North):  Recommend burning conducted only from 11 a.m. to 6 p.m.

(South of Salem):  Not recommended.



Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A transitory upper-level ridge will move over Oregon today, with a continued dry and stable westerly flow aloft.  North to northeasterly winds will make for unfavorable open burning conditions.  After a chilly start, sunny skies and warming aloft will help afternoon temperatures climb into the upper 70s.



(Salem Airport on Thursday: High 76°F; Rainfall .00")



TODAY'S FORECAST:



Sunny skies with breezy north winds.



Salem's high temperature today will be near 77 degrees (normal is 82).

Relative humidity drops to 50% about 11 a.m. and to near 30% by 5 p.m.

Surface winds:  NE 5-10 mph; becoming N 10-15 mph this afternoon.

Transport winds: NE 15 mph this morning; NE 15-20 mph this afternoon.

Mixing height: Rising to 3000 feet by noon and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 8:02 p.m.



THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 63       70      77      68

     Relative Humidity:           52%      39%     31%     44%

     Surface Wind Dir/Speed:    NE 10    NE 12   N  14   N  14

     Transport Wind Dir/Speed:  NE 15    NE 15   NE 20   NE 20

     Mixing Height:              3000     3500    4000    1500

     Ventilation Index:           45       53      80      30



EXTENDED DISCUSSION:



A fairly strong upper-level trough is forecast to dig offshore Saturday, turning the flow aloft southwesterly.  Sunny skies and continued warming aloft should lift high temperatures back into the 80s.  Computer models are still inconsistent on the strength and speed of this system, as it progresses eastward and eventually onshore.  If it moves inland quickly, then it will likely have enough strength to bring some showers to the Willamette Valley late Sunday.  If it stays offshore until Monday or Tuesday, then it may just bring some clouds and cooler temperatures back to the region, along with possible burning opportunities.



The National Weather Service's digital forecast is available at:

http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text



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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