[willamette-fcst] Silverton Hills Midday Forecast.doc

Thu Aug 23 11:40:06 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT THU AUG 23, 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 allowed from now until 5:00 p.m.

WEATHER DISCUSSION:

A strong upper-level trough will move inland today, across southern British Columbia and northern Washington, strengthening the dry and stable westerly flow aloft over Oregon.  Water vapor imagery indicates the presence of an upper-level cold front extending from north-central Washington to the northwestern tip of Oregon.  There is also a band of low clouds associated with this system that has been producing some sprinkles on the north coast.

As the weak cold front slowly sags southeastward, ventilation conditions should become more conducive for open burning this afternoon.  Mixing heights are forecast to climb to more than 5000 feet. Minor gradient-stacking is improving and may balance out by mid-afternoon.  Pibals are scheduled to begin at 1 p.m., to monitor for a possible shift in transport winds from northerly to a more favorable northwesterly direction.

TODAY'S FORECAST:

Partly cloudy and cooler.

Salem's high temperature today will be near 74 degrees (normal is 82).

Relative humidity drops to near 40% this afternoon.

Surface winds:  N 5-10 mph; becoming NW 7-13 mph this afternoon.

Transport winds: N 10-15 mph; becoming NW 12-18 mph this afternoon.

Mixing height: Rising to 5000 feet by 2 p.m. and to near 6000 feet by 5 p.m.

Salem's sunset tonight: 8:04 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 71      74      65

     Relative Humidity:           44%     37%     50%

     Surface Wind Dir/Speed:     N  8   NW  9   NW 12

     Transport Wind Dir/Speed:   N 13   NW 15   NW 18

     Mixing Height:              5000    6000    2500

     Ventilation Index:           65      90      45

EXTENDED DISCUSSION:

On Friday, the upper-level trough will push east of the region.  Drying northerly winds will make for unfavorable open burning conditions.  Clearing skies and warming aloft will lift valley temperatures into the upper 70s.

A fairly strong upper-level trough is forecast to dig offshore Saturday.  Southwesterly flow aloft will warm valley highs back into the 80s with mostly sunny skies.  Computer models are still inconsistent on the strength and speed of this system, as it progresses eastward and eventually onshore.  If it moves inland quickly, then it will likely have enough strength to bring some showers to the Willamette Valley late Sunday.  If it stays offshore until Monday or Tuesday, then it may just bring some clouds and cooler temperatures back to the region, along with possible burning opportunities.

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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