[willamette-fcst] Silverton Hills AM Forecast.doc

Thu Sep 22 08:56:24 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT THU SEP 22, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from 2:00 p.m. until 5:00 p.m.

WEATHER DISCUSSION:

A persistent upper-level ridge of high pressure, centered over the Rockies, is beginning to weaken with increasing southwesterly flow aloft over Oregon

Satellite imagery shows a broad band of clouds extending from southern British Columbia across western Washington and northwestern Oregon.  Low clouds have also moved onshore along the entire length of the Oregon coast.  Radar and surface observations show areas of light rain across much of western Washington, but Oregon has remained dry. Skies are still mostly sunny from the Cascades eastward and over the interior of southwestern Oregon.

The Salem sounding this morning had a very similar temperature profile to 24 hours ago.  The biggest difference was an increase in southwesterly winds aloft.  At the surface, pressure-gradients are still weakly offshore across western Oregon with gradients turning southerly across the Willamette Valley.

The frontal system to our north and west is forecast to stall and weaken today.  Some clouds will continue to stream across the Willamette Valley, but rain should spread no further south than the northern Oregon coast.  The air mass aloft is still very warm, which will suppress afternoon mixing again today.  Valley temperatures should be about 5 degrees cooler today due to more clouds and weak onshore flow expected this afternoon.

South-southwesterly transport winds are forecast for this afternoon, which could produce favorable burning conditions, if pressure-gradients turn strongly enough onshore.

TODAY'S FORECAST:

Partly cloudy and warm.

Salem's high temperature today will be near 83 degrees (normal is 75).

Relative humidity drops below 50% by 11 a.m. and to near 35% by 5 p.m.

Surface winds:  S 3-6 mph this morning; becoming SSW 4-8 mph this afternoon.

Transport winds: SSW 10 mph this morning; SW 10-15 mph this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 7:10 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 69       78      82      70

     Relative Humidity:           47%      41%     36%     55%

     Surface Wind Direction:     180      200     200     230

     Surface Wind Speed:           4        5       5       4

     Transport Wind Direction:   200      220     220     230

     Transport Wind Speed:        10       12      14      10

     Mixing Height:             1700     3000    4000    1000

     Ventilation Index:           17       36      56      10

EXTENDED DISCUSSION:

Little change in the weather pattern is forecast for Friday, so another afternoon burning opportunity is possible.  The flow aloft, and at transport level, is forecast to become more southerly, on Saturday, as an upper-level trough digs offshore.  Computer models are showing rain-making weather systems moving onshore Sunday through Tuesday of next week with a possible break from the rain next Wednesday and Thursday.

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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