[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 20, 2012



BURN ADVISORY:



Recommended burn times for agricultural burning are 11:00 a.m. to 7:00 p.m.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



The slow-moving upper-level trough that has been parked off the southern Oregon coast for most of this week is weakening and finally lifting northeastward.  Bands of showers and thundershowers are rotating around the center of circulation, which is no longer well-defined this morning and appears to be over northwestern Oregon. Most of the shower activity has pushed north, into Washington, but radar is still showing a few light showers over the Cascade foothills east of Salem.



The upper-level trough will push into eastern Washington this afternoon, with a drier west-southwesterly flow aloft bringing some clearing.  Cool air aloft and onshore transport winds will hold afternoon temperatures below average.



(Salem Airport's high temperature on Thursday: 84 degrees; Rainfall .00")



TODAY'S FORECAST:



Cloudy and muggy with a chance of light showers this morning.  There is still a risk of a thundershower.  Becoming partly sunny this afternoon.



Salem's high temperature today will be near 77 degrees (normal is 83).

Relative humidity drops to 50% around 3 p.m.

Surface winds:  SSW 5-10 mph; becoming W 5-10 mph late this afternoon.

Transport winds: SSW 8-12 mph; becoming WSW 8-12 mph late this afternoon.

Mixing height: Rising to near 3500 feet around 2 p.m. and 4500 feet by 5 p.m.

Salem's sunset tonight: 8:51 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 67       72      77      70

     Relative Humidity:           68%      55%     45%     57%

     Surface Wind Direction:     200      210     270     290

     Surface Wind Speed:           8        8       7       6

     Transport Wind Direction:   200      220     250     290

     Transport Wind Speed:        10       12      10      10

     Mixing Height:             2500     3500    4500    2300

     Ventilation Index:           25       42      45      23



EXTENDED DISCUSSION:



Another upper-level trough is forecast to drop southeastward, from the Gulf of Alaska to just off the Washington coast, over the weekend.  That will maintain the onshore flow with morning clouds and afternoon clearing.  Temperatures should be slightly below normal. The trough is forecast to move across Washington and extreme northern Oregon on Monday, introducing a slight chance of light showers and cooling temperatures to as much as 10 degrees below normal.



A return to a more typical summer-time pattern is forecast for Tuesday through Friday of next week.  Look for morning clouds and afternoon sunshine, with temperatures recovering to near average.



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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