[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 THU JUL 26, 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.

The Salem sounding this morning showed very little change from Wednesday morning.  However, the marine layer along the coast has deepened, and marine low clouds were able to penetrate into the northern and southern Willamette Valley this morning.  In contrast, most of Marion County and northern Linn County had clear skies at mid-morning.  Some low clouds could briefly form in the Silverton Hills region, but otherwise expect another sunny and warm day.



Temperatures are likely to be a couple of degrees cooler today, with highs expected to reach the mid 80s this afternoon. A very weak upper-level trough is forecast to initiate a significant afternoon sea breeze today, which should turn transport winds onshore and create favorable smoke dispersal conditions for field-burning.



The air aloft is still quite warm, so mixing heights will not reach 3000 feet until about 2 p.m. and will likely top out between 4000 and 4500 feet this afternoon.  Daytime heating should be sufficient to overcome minor pressure-gradient stacking this morning.



(Salem Airport data for Wednesday, July 25th: High 88° F; Rainfall .00")



TODAY'S FORECAST:



Patchy brief morning clouds.  Becoming sunny and warm.



Salem's high temperature today will be near 86 degrees (normal is 84).

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

Surface winds:  Light NW; becoming NW 5-10 mph this afternoon.

Transport winds: Light NW; becoming WNW 5-10 mph this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to 4000 feet by 5 p.m.

Salem's sunset tonight: 8:45 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 71       80      85      74

     Relative Humidity:           59%      46%     37%     54%

     Surface Wind Direction:     340      290     300     280

     Surface Wind Speed:           3        4       8       8

     Transport Wind Direction:   330      270     290     280

     Transport Wind Speed:         3        6       8      10

     Mixing Height:             1800     3200    4300    1800

     Ventilation Index:            5       19      34      18



EXTENDED DISCUSSION:



On Friday, morning marine clouds will likely penetrate across the entire Willamette Valley and last through much of the morning.  The deeper marine layer will cool temperatures back to below normal, even with afternoon sunshine.  Pressure-gradient-stacking and transport wind directions may become unfavorable for open field-burning, but that will depend on the timing and extent of the influx of marine air.



Little change in the generally dry and stable southwesterly flow aloft is expected through much of next week.  Look for varying degrees of onshore flow, morning clouds, and afternoon sunshine.  There is also a chance that the marine layer will be thick enough for areas of drizzle some mornings.  Temperatures should be near to 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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