[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT MON JUL 11, 2011



BURN ADVISORY:



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



WEATHER DISCUSSION:



Upper level trough has moved in over the state this morning with the center of the upper low over northern Vancouver Island.  This low and trough will gradually move southeastward toward Oregon throughout the day.  The result will be for the region to remain under the marine influence with below normal temperatures and mainly broken to overcast cloud cover.  Transport wind flow will be mainly light southwesterly.  Gradient stacking is strongest over the Cascades with 4.0 mb onshore flow from Salem to Redmond while only 1.1 mb onshore flow from Newport to Salem.



Morning satellite picture shows a combination of marine low clouds over western Oregon as well as increasing higher clouds due to a disturbance moving in associated with the upper level trough.  Some rain showers are showing up on radar over the eastern Pacific.  However, expect the best chance of rain to occur tomorrow and Wednesday.



TODAY'S FORECAST:



Mostly cloudy with light SW winds through this afternoon.



Surface winds:  S-WSW 5-8 mph

Transport winds: SW 6-10 mph

Salem's forecast high temperature:  71 degrees

Maximum mixing height:  4500 feet

Relative humidity:  Drops to near 50% by 4 p.m.

Salem sunset tonight: 8:58pm



THREE-HOURLY DATA:



                                    2pm     5pm     8pm

     Temperature:                    68      71      66

     Relative Humidity:              55%     49%     58%

     Surface Wind Direction:        220     260     270

     Surface Wind Speed:              6       6       7

     Transport Wind Direction:      220     230     260

     Transport Wind Speed:            7       7      10

     Estimated Mixing Height:      4200    4500    3800

     Ventilation Index:              29      32      38



EXTENDED DISCUSSION:



Upper level low and trough move in closer to the state tomorrow bringing more clouds and the chance of some light showers.  Temperatures will likely cool another 5-7 degrees from today.  Wind flow will remain onshore, mainly from the SSW to W.



Upper level low moves over the state on Wednesday continuing the chance of showers.  Gradually, the low weakens but remains over the region through the remainder of the week.  Any shower activity should decrease by Thursday but clouds and cooler than normal temperatures will remain.  Wind flow will remain onshore with SSW to W winds through Thursday, turning more NW'erly by Friday.



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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