[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 WED JUL 25, 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 2 p.m.



WEATHER DISCUSSION:



A dry, warm and stable southwesterly flow aloft is continuing over Oregon today.  The Salem sounding this morning showed very little change in the air mass, above 3000 feet, compared to 24 hours ago.  However, the air mass below 3000 feet has undergone considerable warming and drying.  Marine low clouds blanket the coast and many coastal range gaps again this morning.  Unlike yesterday, skies are mostly sunny in the Willamette Valley.  That will allow temperatures to warm much more quickly today, with highs expected to reach the mid to upper 80s this afternoon.



Even with much warmer surface temperatures today, warm air aloft should cap maximum mixing heights near 4000 feet this afternoon.  Pressure gradients are very weakly onshore this morning, but a typical summer-time sea breeze should increase the onshore flow slightly this afternoon.  Pressure-gradient stacking will likely be favorable for sustaining lifting of air mass today.  Transport wind directions will likely be the main limiting factor for open field-burning this afternoon, possibly maintaining a significant northerly component.



(Salem Airport data for Tuesday: High temperature 81 degrees; Rainfall .00")



TODAY'S FORECAST:



Sunny and warmer.



Salem's high temperature today will be near 87 degrees (normal is 84).

Relative humidity drops to 60% by noon and to near 35% by 5 p.m.

Surface winds:  Light north; becoming NNW 3-7 mph this afternoon.

Transport winds: Light north; becoming NNW 4-8 mph this afternoon.

Mixing height: Rising to 3000 feet around 1 p.m. and to 4000 feet by 5 p.m.

Salem's sunset tonight: 8:46 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 71       80      87      78

     Relative Humidity:           61%      46%     35%     47%

     Surface Wind Direction:     360      350     340     300

     Surface Wind Speed:           3        3       7       6

     Transport Wind Direction:   360      340     330     310

     Transport Wind Speed:         4        4       8       8

     Mixing Height:             1800     3200    4000    1800

     Ventilation Index:            7       13      32      14



EXTENDED DISCUSSION:



A very weak upper-level trough will slightly increase the southwesterly flow aloft on Thursday.  Sunny and warm weather is expected to continue, with increasing onshore flow, in the afternoon.  Transport wind directions are forecast to become more westerly, which would be favorable for open field-burning.  Marine clouds will likely penetrate into the Willamette Valley Friday morning with areas of light drizzle possible.  Even with afternoon sunshine, temperatures should cool back to slightly below normal.  Pressure gradient-stacking could be unfavorable for open field-burning.



Little change in the weather pattern is expected through this weekend, with morning clouds and afternoon sunshine.  Temperatures should be near 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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