[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT FRI JUL 8, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are 10:00 a.m. to 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 and turning surface gradients from onshore to northerly.  As a result, low clouds have cleared from the central and southern coast and are patchy in the central and southern Willamette Valley.  Low clouds still blanket the north coast and the northern Willamette Valley, from northern Marion County to the Columbia River.



The Salem sounding this morning showed northeasterly winds from 1-3000 feet above the surface.  Offshore transport winds will continue to dry the air mass today and allow rapid clearing of the morning low clouds.  Even with plenty of sunshine, high temperatures will be about 5 degrees below normal.



TODAY'S FORECAST:



Partly cloudy this morning.  Sunny and a little breezy this afternoon.



Surface winds:  N 5-10 this morning; N 10-15 mph this afternoon

Transport winds: NNE 10-15 mph this morning; NNE 15-20 mph this afternoon

Salem's forecast high temperature:  75 degrees

Maximum mixing height:  5000 feet

Relative humidity:  Drops to 60% by 10 a.m. and to near 30% by 5 p.m.

Salem sunset tonight: 9:00pm



THREE-HOURLY DATA:



                                 11am     2pm     5pm     8pm

     Temperature:                 64       70      75      70

     Relative Humidity:           47%      36%     31%     39%

     Surface Wind Direction:     360      360     360     360

     Surface Wind Speed:           7       10      12      12

     Transport Wind Direction:   020      020     020     020

     Transport Wind Speed:        12       17      17      18

     Estimated Mixing Height:   4000     4500    5000    3500

     Ventilation Index:           48       77      85      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, with transport winds turning northwesterly.



A fairly impressive upper-level trough is forecast to move down the British Columbia coastline on Monday and park itself just off the Oregon Coast Tuesday and Wednesday, before coming onshore Thursday.  That would bring mostly cloudy skies, cooler-than-normal temperatures, and a threat of showers to the region for most of the coming 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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