[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 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 be limited from now until 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.  Brisk north to northeasterly winds will make for unfavorable open burning conditions.  Sunny skies and warming aloft will help afternoon temperatures climb into the upper 70s.



TODAY'S FORECAST:



Sunny, cool, and breezy.



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

Relative humidity drops to near 30% by 5 p.m.

Surface winds:  N 12-17 mph this afternoon.

Transport winds: NE 17-22 mph this afternoon.

Mixing height: 3-4000 feet this afternoon.

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



THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 71      76      68

     Relative Humidity:           38%     32%     44%

     Surface Wind Dir/Speed:    NE 15   N  15   N  15

     Transport Wind Dir/Speed:  NE 17   NE 20   NE 20

     Mixing Height:              3500    4000    1500

     Ventilation Index:           60      80      30



EXTENDED DISCUSSION:



A fairly strong upper-level trough is forecast to dig southward, from the Gulf of Alaska, to about 600 miles off the Washington coast on Saturday.  A warmer southwesterly flow aloft will combine with ample sunshine to lift valley high temperatures back into the 80s.  The trough is forecast to slowly move closer to the coast on Sunday, initiating a cooling trend.



Longer-range computer models eventually bring the weakening trough onshore early next week.  Although moisture is limited, there is a treat of light showers beginning Sunday night.  Increasing onshore flow will continue the cooling trend through Wednesday, 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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