[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT THU AUG 30, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from noon until 5:00 p.m.



WEATHER DISCUSSION:



An upper-level trough is slowly dropping southward along the southern British Columbia coastline this morning, with a dry west-southwesterly flow aloft over Oregon. Low-level onshore flow is very weak, but areas of marine low clouds have managed to form across the Willamette Valley.  Skies are mostly sunny skies over the coastal range and along the coast.



The morning sounding over Salem showed northeasterly low-level winds and considerable warming aloft compared to 24 hours ago.  However, the air aloft is still cool enough to allow for good afternoon mixing.  The approaching upper-level trough will increase the southwesterly flow aloft over Oregon this afternoon.  Computer models differ on their forecasts of transport wind directions this afternoon, which will likely be the determining, factor regarding afternoon open field-burning today.  It is likely that some shift to a more northwesterly direction will occur later this afternoon.  That will be closely monitored with pibals. Gradient-stacking should become favorable for open burning this afternoon.



(Salem Airport on Wednesday: High 77°F; Rainfall .00")



TODAY'S FORECAST:



Areas of morning clouds, then mostly sunny.



Salem's high temperature today will be near 78 degrees (normal is 81).

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

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

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

Mixing height: Rising to 3000 feet by noon and to near 5000 feet by 5 p.m.

Salem's sunset tonight: 7:50 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 66       73      77      68

     Relative Humidity:           63%      45%     36%     49%

     Surface Wind Dir/Speed:     N  5     N  5   NW  7   NW 10

     Transport Wind Dir/Speed:   N 10     N 10   NW 12   NW 15

     Mixing Height:              2500     4000    5000    1800

     Ventilation Index:           25       40      60      27



EXTENDED DISCUSSION:



The upper-level trough to our north is forecast to swing inland over southern British Columbia on Friday, with a continued dry southwesterly flow aloft over Oregon.  Once again, areas of morning clouds will yield to mostly sunny skies with seasonal temperatures.  However, transport winds are forecast to stay generally northerly, which is not favorable for open field-burning.



A continued dry and weak westerly flow aloft is forecast for this weekend and early next week, with areas of morning clouds giving way to mostly sunny skies.  Temperatures should remain near seasonal normals.



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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