[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT FRI SEP 16, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are from 8:00am to 5:30pm.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



Upper level trough will move through the region today keeping the air mass cool and cloudy.  Morning satellite picture shows a mass of clouds moving through the westerly flow aloft.  These may produce some sprinkles or light showers but little more.  It appears the main bulk of these clouds will move through this afternoon to early evening with some clearing later today.



Upper level sounding shows a stable layer up to 2000 ft with mostly unstable and very moist conditions above that.  With slow heating this layer will mix out and provide good to excellent mixing by early afternoon.



Morning surface map shows weak high pressure over the region with little pressure gradient.  Current gradient stacking shows .4 mb onshore from Newport to Salem with 0 mb between Salem to Redmond.  It does not appear gradients will increase much today and will keep winds generally light but mostly W to NW.  This factor may limit open burning today.



TODAY'S FORECAST:



Mostly cloudy and cool.



Salem's high temperature today will be near 68 degrees.

Relative humidity drops below 50% by 2 p.m. and to near 40% by 5 p.m.

Surface winds:  Light and variable this morning; W to NW at 4-7 mph this afternoon.

Transport winds: Light and variable this morning; W to NW at 5-8 mph this afternoon.

Mixing height: Brief morning stability, then mixing height rising to 5000 ft by 1 p.m.



Salem's sunset tonight: 7:34 p.m.



THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm

     Temperature:                 60       66      68      60

     Relative Humidity:           69%      49%     40%     55%

     Surface Wind Direction:     360      290     300     280

     Surface Wind Speed:           2        4       7       6

     Transport Wind Direction:   350      300     290     290

     Transport Wind Speed:         2        6       7       7

     Estimated Mixing Height:   3500     5000   5000+    2500

     Ventilation Index:            7       30      35      18









EXTENDED DISCUSSION:



Very minor upper level ridge moves into the region tomorrow behind the exit of today's trough.  This feature will be short-lived as the next trough and minor front move into the region Sunday.  However, each succeeding run of the models show this front weakening with now only a chance of light showers in the northern Willamette Valley.



Weak trough moves through Monday then an upper level ridge builds in later Monday and Tuesday.  The ridge will move on to the east later Tuesday and Wednesday.  Models continue to show mostly SW'erly flow through much of the extended period which should provide continued opportunities to burn even if they're somewhat limited.



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.



Nick Yonker

ODF Meteorologist



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