[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:54 AM PDT FRI JUL 22, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are from now until 7:00 p.m.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A west-northwesterly flow aloft is slowly drying and warming the air mass over Oregon.  Late-morning visible satellite imagery showed partly cloudy skies over the western valleys, with mostly clear skies over the coast and Cascades.  Valley temperatures were mostly in the mid 60s with north winds up to 10 mph.



With high pressure continuing to push into northwestern Oregon, and a surface thermal trough building into extreme southwestern Oregon, northerly surface and transport winds will likely turn slightly offshore by late this afternoon.  That will promote clearing skies and help valley temperatures climb into the middle and upper 70s (still 5-8 degrees below normal).



TODAY'S FORECAST:



Clearing skies and slightly warmer.  Increasing north to northeasterly winds.



Salem's high temperature today will be near 78 degrees (normal is 83).

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

Surface winds:  N 10-15 mph this afternoon.

Transport winds: NNE 12-15 mph this afternoon.

Mixing height: Rising to 5500 feet by 5 p.m.

Salem's sunset tonight: 8:50 p.m.





THREE-HOURLY DATA:

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

     Temperature:                 74      77      73

     Relative Humidity:           37%     31%     41%

     Surface Wind Direction:     360     005     010

     Surface Wind Speed:          10      12      12

     Transport Wind Direction:   010     015     020

     Transport Wind Speed:        12      13      15

     Mixing Height:             5000    5500    3000

     Ventilation Index:           60      72      45



EXTENDED DISCUSSION:



A flat ridge will move over Oregon Saturday for continued warming and drying of the air mass.  High temperatures will likely climb into the mid 80s.  The flow aloft will further warm and turn southwesterly on Sunday.  Highs will likely approach 90 degrees.  An upper-level trough is forecast to come onshore Monday.  The resulting marine push will bring increasing clouds and cool temperatures back to normal.



There is a slight chance of drizzle or very light showers Monday night and early Tuesday, as far south as the Silverton Hills, but this system appears as if it will be mainly dry.  High temperatures on Tuesday will drop back into the middle 70s.  A broad upper-lever trough will maintain mostly dry weather for the remainder of next week.  Gradual warming aloft will help temperatures recover to near or slightly above normal by Friday.



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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