[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT FRI JUL 27, 2012



BURN ADVISORY:



Recommended burn times for agricultural burning are from 3 p.m. to 6:30 p.m.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A strong "marine push" brought a surge of cool and moist air into the Willamette Valley overnight.  This morning's sounding over Salem showed significant cooling, compared to Thursday morning, below 6000 feet, with saturated air extending up to about 4000 feet.  As a result, extensive low clouds cover the valley this morning, with Pilot reports confirming that the low-cloud deck extends upward to around 4200 feet.  Continued deepening of the marine layer could produce local areas of light drizzle this morning.



Cloudy skies will likely persist into at least the early afternoon, with cooler surface temperatures suppressing mixing heights.  Even with some afternoon clearing, temperatures will be about 10 degrees below average today.  As is usually the case after a "marine push," there is significant pressure-gradient-stacking this morning, which is forecast to persist this afternoon.  That is unfavorable for open burning.



(Salem Airport data for Thursday, July 26th: High 85°F; Rainfall .00")



TODAY'S FORECAST:



Cloudy with a chance of light drizzle this morning.  Becoming partly sunny later this afternoon.  Much cooler.



Salem's high temperature today will be near 75 degrees (normal is 84).

Relative humidity drops to 60% by 2 p.m. and to near 50% by 5 p.m.

Surface winds:  S 3-6 mph; becoming NNW 4-8 mph this afternoon.

Transport winds: SW 5; becoming NNW 5-10 mph this afternoon.

Mixing height: Rising to 3000 feet by 3 p.m. and to 3500 feet by 5 p.m.

Salem's sunset tonight: 8:44 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 64       69      74      67

     Relative Humidity:           73%      63%     54%     68%

     Surface Wind Direction:     200      320     340     350

     Surface Wind Speed:           5        4       5       5

     Transport Wind Direction:   240      320     340     350

     Transport Wind Speed:         5        5       7       7

     Mixing Height:             2500     2800    3500    1800

     Ventilation Index:           13       14      25      13



EXTENDED DISCUSSION:



Little change in the overall weather pattern is forecast through next week, with a continued generally dry and stable southwesterly flow aloft.  Varying degrees of onshore flow, morning clouds, and afternoon sunshine could present additional burning opportunities.  The marine layer could deepen enough for areas of morning drizzle, especially late next week, but that should be the extent of any precipitation.  Onshore flow should hold temperatures to near or slightly below normal.



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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