[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 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 the southwestern United States, with a dry and warm southwesterly flow aloft over Oregon.  A weak upper-level trough is just off the Washington coast.  It has very limited moisture but is increasing the southwesterly flow aloft and onshore transport flow across western Oregon.  That is improving air quality and pushing residual wildfire smoke east of the region.



Increasing onshore flow brought some marine clouds into the extreme northern and southern sections of the Willamette Valley this morning.  Low clouds and fog continue to blanket the coastline late this morning, with considerable high clouds stretching across the interior of western Oregon. Onshore Pressure gradients, from Newport to Redmond, have increased to more than 6 millibars late this morning.  Corvallis has westerly winds in excess of 10 mph...indicating that a marine push is initiating into the Willamette Valley.



Transport winds, which have been northerly the past couple of days, will turn northwesterly today; capping temperatures in the mid 80s (a few degrees cooler than yesterday). The air mass is still quite warm, but minor cooling aloft, combined with daytime heating, should help afternoon mixing heights climb to near 3500 feet.

TODAY'S FORECAST:



Considerable high clouds with a developing sea breeze this afternoon.



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

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

Surface winds:  NW 7-12 mph.

Transport winds: NW 10-15 mph.

Mixing height: Rising to near 3500 feet this afternoon.

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



THREE-HOURLY DATA:

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

     Temperature:                 80      84      77

     Relative Humidity:           41%     36%     45%

     Surface Wind Direction:     320     320     300

     Surface Wind Speed:           8      10       8

     Transport Wind Direction:   320     310     300

     Transport Wind Speed:        10      12      12

     Mixing Height:             3200    3500    1500

     Ventilation Index:           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 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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