[willamette-fcst] fbs0727a.docx

Wed Jul 27 08:56:27 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT WED JUL 27, 2016

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A very weak upper level trough is crossing the northern Oregon coast this morning but it is causing very few clouds and having essentially no effect on the lower levels. Otherwise, we can expect very weak westerly flow aloft today and low level conditions similar to those on Tuesday. Mixing heights will again only reach 3000 feet by about 2:00 p.m. but pressure gradients will very likely balance this afternoon. The primary limiting factor for burning opportunity this afternoon will be consistent north transport winds, or even slightly east of north. State Fire Marshal conditions are possible this afternoon due to expected winds and low relative humidity.

Sunny and very warm.

Salem's high temperature today will be near 90°F (average is 84°F).

Relative humidity:  Dropping below 60% by 10:00 a.m. and to 30% by 5:00 p.m.

Surface winds: N 9-14 mph.

Transport winds: N 12-17 mph.

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

Salem's sunset tonight: 8:44 p.m.

(Salem Airport data for Tuesday, July 26th: High 88°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 68)

EXTENDED DISCUSSION:

A ridge will build slightly northward from California on Thursday before an upper trough approaches from the northwest by Friday night. Temperatures will reach the lower 90s on Thursday and Friday and transport winds will continue northerly. The trough may approach early enough to provide enough westerly component and a burn opportunity on Friday afternoon. Another trough on Monday will push more marine air back into the valley which will deepen the mixing layer and provide better burning potential into early next week.

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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