[willamette-fcst] Silverton Hills Midday Forecast.doc

Wed Aug 8 08:55:17 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT WED AUG 8, 2012

BURN ADVISORY:

Recommended burn times for agricultural burning are from noon until 6:30 p.m.

Prep burning is allowed from noon until 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from noon until 5:00 p.m.

WEATHER DISCUSSION:

A broad upper-level ridge of high pressure remains anchored over the Southwestern United States with a south-southwesterly flow aloft over Oregon and Washington.  A weak disturbance in the force shifted the surface thermal trough east of the Cascades Tuesday evening, initiating a "marine push" into the Willamette Valley.  That system is moving over northwestern Washington this morning and will reinforce the marine layer over NW Oregon today.

Satellite imagery this morning shows extensive coverage of marine low clouds over northwestern Oregon.  The Salem sounding showed several degrees of cooling, from just above the surface up through 10,000 feet, compared to Tuesday morning.  Pilot reports this morning indicated that the marine clouds are only about 1000 feet thick with the top of the cloud deck near 3000 feet.  That is usually indicative of an early break-up of the cloud deck, but continued strong onshore flow should delay that process into the afternoon.

Look for the look clouds to give way to sunshine around the mid-afternoon.  Temperatures should be several degrees cooler than on Tuesday, but cooler air aloft will result in higher afternoon mixing heights.  As is usually the case after a "marine push," there is significant pressure gradient stacking east of the Willamette Valley this morning.  That is not conducive to good burning conditions, and it is unlikely that the valley will get enough heating to effectively reverse the gradient stacking this afternoon.  Light south to southwesterly winds will switch to the north-northwest this afternoon.

(Salem Airport on Tuesday, August 7th: High 84°F; Rainfall .00")

TODAY'S FORECAST:

Morning clouds giving way to sunshine by mid-afternoon. Cooler.

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

Relative humidity drops to 60% around noon and to near 40% by 5 p.m.

Surface winds:  Light south; becoming NNW 5-12 mph this afternoon.

Transport winds: SW 3-5 mph; becoming NNW 7-14 mph this afternoon.

Mixing height: Rising to 3000 feet by noon and to near 5000 feet by 5 p.m.

Salem's sunset tonight: 8:28 p.m.

THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 66       75      78      71

     Relative Humidity:           63%      47%     42%     57%

     Surface Wind Direction:     180      340     350     340

     Surface Wind Speed:           3        5       7       9

     Transport Wind Direction:   240      340     340     340

     Transport Wind Speed:         3        6       9      12

     Mixing Height:             2800     4500    5000    3000

     Ventilation Index:            8       27      45      36

EXTENDED DISCUSSION:

Another weak upper-level trough will maintain the onshore flow pattern across western Oregon on Thursday, with morning clouds giving way to afternoon sunshine.  Gradient stacking may, once again, be unfavorable for open burning, and transport winds will have a significant northerly component.  The flow aloft should weaken and turn more westerly on Friday, with more sunshine lifting temperatures to slightly above normal.  However, transport winds are forecast to remain mostly northerly.  Little change to the overall weather pattern is forecast through early next week.

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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