[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT FRI AUG 10, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

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

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



WEATHER DISCUSSION:



A weak upper-level trough moved onshore this morning and deepened the marine layer.  Low clouds cover much of western Oregon this morning, with the exception of extreme southern Oregon and sections of the Willamette Valley.  Marine clouds may still fill in the areas of the valley that were getting some early morning sunshine.



Salem's morning sounding showed some minor cooling from 2500-4500 feet, but some additional cooling is likely with the current influx of marine air.  That will make for slightly higher mixing heights later today.  However, gradient stacking has increased this morning, which is not conducive to good smoke ventilation from the valley.  Morning clouds should give way to sunshine this afternoon, with near average temperatures.  Open burning is unlikely today, due to the combination of unfavorable gradient stacking and forecast brisk northern transport winds.



(Salem Airport on Thursday: High 83°F; Rainfall .00")



TODAY'S FORECAST:



Mostly clouds; turning sunny and breezy in the afternoon.



Salem's high temperature today will be near 82 degrees (normal is 83).

Relative humidity drops to 50% by 2 p.m. and to near 40% by 5 p.m.

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

Transport winds: N 10-14 mph this morning; N 12-18 mph this afternoon.

Mixing height: Rising to 3000 feet by noon and to near 4500 feet by 5 p.m.

Salem's sunset tonight: 8:25 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 67       76      82      74

     Relative Humidity:           61%      47%     38%     50%

     Surface Wind Dir/Speed:     N  5     N 10    N 12   NW 12

     Transport Wind Dir/Speed:   N 12     N 14    N 16   N  18

     Mixing Height:              2700     3800    4500    2500

     Ventilation Index:           32       53      72      45



EXTENDED DISCUSSION:



A warm-up is expected this weekend, as an upper-level ridge builds over the region.  Sunshine and offshore flow will likely push valley highs into the 90s.  A very weak upper-level trough is forecast to put a dint in the ridge on Monday; turning the low-level flow onshore and possibly creating a burning opportunity.  Temperatures will likely cool back into the 80s, but no precipitation is expected.



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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