[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 31, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from noon until 2:00 p.m. with a 50 acre limit.

Propane flaming is not allowed.



WEATHER DISCUSSION:



An upper-level trough is centered over Vancouver Island, British Columbia this morning and will very slowly drift eastward today.  Water vapor imagery shows an associated moisture band extending southwestward across most of Washington and western Oregon.  However, visible satellite imagery only shows a corresponding very narrow band of mainly high clouds.  Only patchy marine clouds formed this morning and should quickly evaporate with daytime heating.



The morning sounding over Salem was almost identical to yesterday morning, so 3000-foot mixing heights should be reached about noon again today.  Northeasterly transport winds this morning will turn northerly this afternoon.  Like yesterday, computer models are struggling with whether or not low-level winds will turn northwesterly later this afternoon.  Minor gradient-stacking this morning should balance out this afternoon.



(Salem Airport data for Thursday, Aug. 30th: High 80°F; Rainfall .00")



TODAY'S FORECAST:



Mostly sunny.



Salem's high temperature today will be near 79 degrees (normal is 81).

Relative humidity drops to 50% by noon and to near 30% by 5 p.m.

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

Transport winds: N 5-10 mph; becoming N-NW 10-15 mph late this afternoon.

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

Salem's sunset tonight: 7:48 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 65       74      79      68

     Relative Humidity:           54%      40%     30%     53%

     Surface Wind Dir/Speed:     N  5     N  7   NW  7   NW  8

     Transport Wind Dir/Speed:   N  5     N 10   NW 10   NW 14

     Mixing Height:              2500     4000    4500    1800

     Ventilation Index:           13       40      45      25



EXTENDED DISCUSSION:



The upper-level trough to our north will push eastward across southern British Columbia on Saturday with the dry and stable flow aloft turning westerly.  Another weak trough will maintain the dry westerly flow aloft Sunday and Monday.  Patchy morning clouds should give way to mostly sunny skies with near-normal temperatures.



Long-range computer models are showing the development of an upper-level ridge over the region Tuesday-Friday of next week.  That would produce above normal temperatures and offshore flow, which is not conducive to open field-burning opportunities.



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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