[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT FRI AUG 5, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from noon until 5:00 p.m.



WEATHER DISCUSSION:



A weak upper-level trough moved inland and across Oregon on Thursday.  That turned that flow aloft westerly and initiated a surge of cooler marine air into the Willamette Valley Thursday evening.  Visible satellite imagery showed low clouds blanketing all but the extreme southwestern corner of western Oregon this morning.  However, some breaks in the clouds were evident over the coastal range.



The Salem sounding showed several degrees of cooling, below 6000 feet, compared to 24 hours ago, but the low-level air mass is not saturated through that entire layer.  Daytime heating should be sufficient to break up the marine clouds by midday with plenty of afternoon sunshine.



The air aloft is still fairly cool, so surface heating should result in high mixing heights again this afternoon.  The surface map shows only moderate gradient-stacking this morning, which will likely be overcome this afternoon.  The limiting factor for open burning today will most likely be the direction of the transport winds, which are forecast stay mostly northerly.



TODAY'S FORECAST:



Morning clouds, then mostly sunny and a little cooler.



Salem's high temperature today will be near 80 degrees (normal is 84).

Relative humidity drops below 60% by noon and to near 50% by 2 p.m.

Surface winds:  NW 3-8 mph this morning; NNW 5-12 mph this afternoon.

Transport winds: NNW 8 mph this morning; NNW 10-15 mph this afternoon.

Mixing height: Rises to 3000 feet about noon and to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:33 p.m.





THREE-HOURLY DATA:

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

     Temperature:                 68       75      79      72

     Relative Humidity:           63%      50%     42%     55%

     Surface Wind Direction:     330      340     340     340

     Surface Wind Speed:           5        6       8      10

     Transport Wind Direction:   340      340     340     340

     Transport Wind Speed:         8       10      15      15

     Mixing Height:             2800     4200    5500    2800

     Ventilation Index:           22       42      83      42



EXTENDED DISCUSSION:



Little change in the weather pattern is forecast through early next week.  Expect varying degrees of morning clouds with afternoon sunshine and moderate temperatures.  Long-range computer models begin to diverge late next week, but that average of their solutions basically maintains the same general weather pattern.  Some burning opportunities are possible.



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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