[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT FRI JUL 8, 2011



BURN ADVISORY:



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



WEATHER DISCUSSION:



An upper-level trough will move across southern British Columbia and northern Washington today with a cool but dry westerly flow aloft over Oregon.  High pressure is building into northwestern Oregon; increasing the northwesterly gradients across the northern Willamette Valley and the northerly gradients across the central and southern valley.



Late this morning, visible satellite imagery showed low clouds persisting over the northwestern corner of Oregon, extending about as far south and east as the Silverton Hills region.  Skies were generally clear across the remainder of the state.



Northwesterly surface winds should turn more northerly, with clearing skies, by early this afternoon. Above the surface, northeasterly transport winds will continue to dry the air mass today and allow for plenty of afternoon sunshine.  However, yesterday's "marine push" forced a cool air mass into the region, so high temperatures will be about 5 degrees below normal.



TODAY'S FORECAST:



Sunny and a little breezy this afternoon.



Surface winds:  becoming N 10-15 mph this afternoon

Transport winds: NE 15-20 mph this afternoon

Salem's forecast high temperature:  75 degrees

Maximum mixing height:  5000 feet

Relative humidity:  Drops to near 30% by 5 p.m.

Salem sunset tonight: 9:00pm



THREE-HOURLY DATA:



                                 2pm     5pm     8pm

     Temperature:                 70      75      70

     Relative Humidity:           36%     31%     39%

     Surface Wind Direction:     360     360     360

     Surface Wind Speed:          12      13      12

     Transport Wind Direction:   030     030     030

     Transport Wind Speed:        17      18      18

     Estimated Mixing Height:   4500    5000    3500

     Ventilation Index:           77      90      63



EXTENDED DISCUSSION:



The flow aloft will turn southwesterly and warm slightly on Saturday. Brief morning clouds will give way to sunshine with northerly transport winds and high temperatures returning to normal (near 80).  Sunday looks to be another pleasant day, with near-normal temperatures.  However, an upper-level trough will likely initiate a "marine push" Sunday afternoon.  Transport winds will turn northwesterly; initiating a general cool-down across western Oregon.



A fairly impressive upper-level trough is forecast to move down the British Columbia coastline, on Monday, to just off the southern Oregon Coast by Wednesday.  It is forecast to move inland, most likely across southern Oregon and northern California, Thursday and Friday.  That would bring mostly cloudy skies, cooler-than-normal temperatures, and at least the threat of showers during much of next week.



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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