[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 THU JUL 12, 2012



BURN ADVISORY:



Recommended burn times for agricultural burning are from noon to 7:00 p.m.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A large upper-level ridge remains centered over eastern Nevada with a dry and warm southwesterly flow aloft over Oregon.  A weak upper-level trough, just off the Washington coast this morning, has very limited moisture but is increasing the southwesterly flow aloft.  Wildfire smoke that has been producing hazy skies the past few days is getting forced east of the region.



Increasing onshore flow has brought some marine clouds inland this morning, into the extreme northern and southern sections of the Willamette Valley.  A few morning low clouds could form in the Silverton Hills region, but otherwise just look for some high clouds at times today.  Surface and transport winds, which have been northerly the past couple of days, will turn northwesterly today and cool temperatures a few degrees. The air mass is still quite warm, but some minor cooling aloft should help afternoon mixing heights climb to near 3500 feet.

TODAY'S FORECAST:



Sunny.  A little cooler.



(Salem's high temperature on Wednesday was 89 degrees)



Salem's high temperature today will be near 85 degrees (normal is 82).

Relative humidity drops to 50% by noon and to near 35% by 5 p.m.

Surface winds:  NW 5-10 mph this morning; NW 7-12 mph this afternoon.

Transport winds: NW 5-8 mph this morning; NW 10-14 mph this afternoon.

Mixing height: Rising to near 3500 feet this afternoon.

Salem's sunset tonight: 8:57 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 72       80      84      77

     Relative Humidity:           53%      41%     36%     45%

     Surface Wind Direction:     340      320     320     300

     Surface Wind Speed:           5        7      10       8

     Transport Wind Direction:   320      320     310     300

     Transport Wind Speed:         5       10      12      12

     Mixing Height:             2000     3200    3500    1500

     Ventilation Index:           10       32      42      18



EXTENDED DISCUSSION:



A very weak upper-level trough is forecast to stall over Oregon Friday and Saturday.  That should maintain enough onshore flow to bring some morning marine clouds into the Willamette Valley, but expect plenty of sunshine in the afternoons with seasonably warm temperatures.  A little stronger upper-level trough is forecast to drop into the region late Sunday and Monday.  That will strengthen the onshore flow enough to cool temperatures below normal, by Monday, and introduce a slight chance of light showers.



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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