[willamette-fcst] fbs0901a.docx

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT THU SEPT 1, 2016


BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



Clouds will allow cool and moist air to persist in the valley today while a trough approaches the Pacific coast. The trough will likely bring rain to the area late today. There are several potential issues that may prevent burning today and will require monitoring. One is whether we can warm up enough to raise the mixing height and lower the relative humidity enough to reach burnable levels? Are the fields dry enough to consider burning, though reports indicate that the east side of the valley received less than 0.05 inch of rain on Wednesday. Pressure gradients are not currently balanced though models suggest that there is a chance they will. All considered, the chance to burn looks fair by around 2:00 p.m. and then if the rain holds off then the chances improve.



Mostly cloudy with areas of rain late in the afternoon.



Salem's high temperature today will be near 68°F (average is 80°F).

Relative humidity:  About 70% by noon, falling to near 60% about 2:00 p.m.

Surface winds: S to SSW 3-6 mph.

Transport winds: SW 4-7 mph becoming W late in afternoon.

Mixing height: Rising to 3000 feet by 2:00 p.m. then 4000 feet at 5:00 p.m.

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



(Salem Airport data for Wednesday, August 31st: High 70°F; Rainfall: .17")

(Maximum Ventilation Index expected today: 28)



EXTENDED DISCUSSION:



The trough will finally move inland tonight and rain appears likely at times tonight and Friday. Conditions might be conducive for burning on Friday afternoon except that a drying day will likely be necessary. The extended outlook is for onshore flow to persist through the next work week and the primary questions each day would be rain chances and pressure gradients.



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 15 mph.



     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 (ft) times

         the transport wind speed (mph) 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 (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.



To add/remove your email address from this list, please go to:



http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst



Gary Votaw

ODF Meteorologist

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