[willamette-fcst] Silverton Hills Midday Forecast.doc

Fri Aug 31 11:56:48 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT FRI AUG 31, 2012

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from noon until 2:00 p.m. with a 50 acre limit.

Propane flaming is not allowed.

WEATHER DISCUSSION:

A dry upper-level trough, slowly drifting eastward across southern British Columbia, pushed a band of high clouds across northwestern Oregon this morning.  Water vapor imagery shows the trough as a band of high-level moisture extending southwestward across most of Washington and western Oregon. The back edge of that moisture band had moved onshore as was approaching Marion County late this morning.

One effect of the trough moving into the region this morning was to increase the pressure-gradient-stacking across western Oregon.  That is not favorable for the prospects of open burning this afternoon.  As of late this morning, there was 1.1mb onshore gradient from Newport to Salem and 3.9mb onshore gradient from Newport to Redmond.  Some down-mixing of smoke is likely, unless those pressure differences equalize this afternoon.  It is also questionable whether or not the trough will be strong enough to turn the transport winds from northerly to northwesterly this afternoon.  Pibals are scheduled to begin at 1:00 p.m.

TODAY'S FORECAST:

Mostly sunny.

Salem's high temperature today will be near 79 degrees (normal is 81).

Relative humidity drops to near 30% by 5 p.m.

Surface winds:  N 5-10 mph.

Transport winds: N 5-10 mph; becoming N-NW 10-15 mph late this afternoon.

Mixing height: Rising to near 4500 feet by 5 p.m.

Salem's sunset tonight: 7:48 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 74      79      68

     Relative Humidity:           36%     30%     53%

     Surface Wind Dir/Speed:     N  7   N   7   NW  8

     Transport Wind Dir/Speed:   N 10   NW 10   NW 14

     Mixing Height:              4000    4500    1500

     Ventilation Index:           40      45      21

EXTENDED DISCUSSION:

An upper-level trough will progress from southern British Columbia into Alberta, Canada on Saturday.  The flow aloft will further dry out and turn more westerly over Oregon.  Another weak trough will maintain the dry westerly flow aloft Sunday and Monday.  Patchy morning clouds should give way to mostly sunny skies with near-normal temperatures.

Long-range computer models are showing the development of an upper-level ridge over the region much of next week.  That would produce above normal temperatures and offshore flow, which is not conducive to open burning.

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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