[willamette-fcst] Silverton Hills Midday Forecast.doc

Wed Aug 31 11:47:21 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:46 AM PDT WED AUG 31, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from now until 2 p.m. with a 50 acre limit.

Propane flaming is allowed from now until 5 p.m.

WEATHER DISCUSSION:

A cool but generally dry upper-level trough is over the Pacific Northwest this morning.  The Salem sounding this morning showed a deep marine layer extending from the surface up to almost 7000 feet.   The lowest levels of the air mass have dried some since Tuesday, so cloud bases are up at nearly 6000 feet and are breaking up late this morning.

Late-morning satellite imagery showed a band of low clouds, associated with another upper-level disturbance, rotating into northwestern Oregon.  That system is helping to break up the marine clouds over the Willamette Valley but also adding to their vertical development.  It is possible that some light showers could develop this afternoon.

The outlook for open burning this afternoon remains favorable.  There was still some minor gradient-stacking late this morning, which will need to get balanced out to ensure good smoke evacuation over the Cascades.  That could happen by early this afternoon.  Due to the very cool air aloft, mixing heights are likely already above 3000 feet.  An 11 a.m. pibal showed light westerly winds up to 3800 feet, before the balloon was lost in the clouds.  As mixing heights rapidly climb this afternoon, transport winds should increase in speed and slowly veer to northwesterly.

TODAY'S FORECAST:

Mostly cloudy.  Slight chance of a shower.

Salem's high temperature today will be near 70 degrees (normal is 81).

Relative humidity drops to near 50% from 2 p.m. through 5 p.m.

Surface winds:  W 5 mph; becoming NW 6-12 mph this afternoon.

Transport winds: W 5 mph; becoming NW 10-15 mph this afternoon.

Mixing height: Rises to near 7000 feet by 2 p.m.

Salem's sunset tonight: 7:51 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 67      68      63

     Relative Humidity:           50%     50%     60%

     Surface Wind Direction:     310     320     330

     Surface Wind Speed:           6      10      10

     Transport Wind Direction:   315     330     340

     Transport Wind Speed:        12      15      20

     Mixing Height:             7000    6500    3000

     Ventilation Index:           84      98      60

EXTENDED DISCUSSION:

The upper-level trough is forecast to push into Idaho Thursday. The flow aloft will warm and turning more northerly, as an upper-level ridge begins to build over the region.  Transport winds will likely turn northerly on Thursday and north-northeasterly on Friday, with high temperatures warming back into the low 80s by Friday.  Open burning is highly unlikely both days.

A weak upper-level ridge is forecast to move directly over Oregon on Saturday and begin pushing east of the state on Sunday.  Temperatures will continue to warm with highs likely climbing into the low 90s by Sunday.  Increasing southerly flow aloft may introduce a chance of thunderstorms into the region by Sunday afternoon.

Increasing southwesterly flow aloft will bring some minor cooling on Monday and could initiate enough of an afternoon sea breeze to allow for limited open burning.  Little chance to the overall weather pattern is forecast for the remainder of next week, with a continuation of warm and dry conditions.  There could be enough onshore flow for limited burning opportunities, which will need to be monitored on a day-to-day basis.

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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