[willamette-fcst] Silverton Hills Midday Forecast_Gary.doc

Thu Aug 4 11:43:07 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT THU AUG 4, 2016

BURN ADVISORY:

***STATE FIRE MARSHAL CONDITIONS POSSIBLE THIS AFTERNOON***

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A thermal trough over Willamette Valley will cause very poor burning conditions today, with northerly transport winds and low mixing heights. Conditions will approach fire marshal, mainly on the relative humidity and wind. Pressure gradients also suggest that air flow would converge over the valley.

Sunny and very warm.

Salem's high temperature today will be near 92°F (average is 84°F).

Relative humidity:  Falling below 30% about 4:00 p.m.

Surface winds: N 12-16 mph.

Transport winds: N 14-18 mph, increasing to 17-21 mph after 3:00 p.m.

Mixing height: Rising to 2000 feet about 1:00 p.m. and 3500 feet at 5:00 p.m.

Salem's sunset tonight: 8:34 p.m.

(Salem Airport data for Wednesday, August 3rd: High 83°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 73)

EXTENDED DISCUSSION:

An upper level low will be over Vancouver Island on Friday and it is not expected to move into Washington until Sunday night. This will provide a marine push on Friday and a possible good burning opportunity. Pressure gradient stacking appears to be the factor to watch. Current models indicate that transport winds will begin turning northwest just before noon and increasingly become northwest into early afternoon while mixing heights would be good. But they also show gradient stacking to be the one possible negative factor and will have to be monitored.

There is a chance of showers due to the upper low on Sunday through Tuesday which could hamper burning. Once the low moves out by mid-week we will have a warming trend. Each day next week shows some potential for burning which requires further monitoring.

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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