[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 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. Marine low clouds continued to blanket the coast many of the coastal range gaps at midday.  However, onshore flow is weak across the interior of western Oregon (2.8 mb from Newport to Redmond), with mostly clear across the Willamette Valley.  As a result, valley temperatures are running about 7-12 degrees warmer than 24 hours ago.  Afternoon highs are expected to reach the mid to upper 80s.



Even with much warmer surface temperatures today, warm air aloft should cap maximum mixing heights at or below 4500 feet this afternoon.  At the surface, a broad thermal trough extends from central Washington, through central Oregon, into southwestern Oregon.  Weak onshore pressure gradients are favorably "stacked" to keep smoke elevated.  Light northerly surface and transport winds will need to be monitored, because the afternoon sea breeze may turn them enough onshore to allow for open burning. Pibals are scheduled to begin at 1 p.m.



TODAY'S FORECAST:



Sunny and warmer.



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

Relative humidity drops 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 4500 feet by 5 p.m.

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



THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 80      87      78

     Relative Humidity:           46%     35%     47%

     Surface Wind Direction:     350     340     300

     Surface Wind Speed:           3       7       6

     Transport Wind Direction:   340     330     310

     Transport Wind Speed:         4       8       8

     Mixing Height:             3500    4500    1800

     Ventilation Index:           14      36      14



EXTENDED DISCUSSION:



Sunny and warm weather is expected to continue on Thursday, with a very weak upper-level trough forecast to promote a stronger afternoon sea breeze.  That increases the chance that transport winds will turn enough onshore for afternoon field-burning.



On Friday, some morning marine clouds will likely penetrate into the Willamette Valley.  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 dry southwesterly flow aloft is expected through much of next week.  Look for varying degrees of onshore flow and morning clouds, with plenty of 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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