[willamette-fcst] Silverton Hills Midday Forecast_Gary.doc

Fri Aug 12 11:31:47 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:30 AM PDT FRI AUG 12, 2016

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper level ridge is just offshore this morning while a thermal trough sits over the valley. This will cause hot and dry conditions today and with only light breezes. Mixing heights will also be slow to rise today due to such warm air aloft. Fire marshal conditions are likely to begin about 3:00 p.m. as the temperature and relative humidity reach critical thresholds. Pressure gradients also indicate that winds will be converging over the valley. The main limiting factor for burning is fire marshal conditions by mid-afternoon.

Sunny and hot.

Salem's high temperature today will be near 97°F (average is 83°F).

Relative humidity:  Falling below 30% about 3:00 p.m.

Surface winds: N 4-8 mph.

Transport winds: N 6-9 mph turning NNW around 5:00 p.m.

Mixing height: Rising to 3000 feet about 3:00 p.m. and 3500 feet at 5:00 p.m.

Salem's sunset tonight: 8:22 p.m.

(Salem Airport data for Thursday, August 11th: High 89°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 31)

EXTENDED DISCUSSION:

A weak marine push will arrive by Saturday evening and cool the valley down into early next week. Models suggest that the best burning opportunity of next week may be Monday as a northwest transport wind may develop in the afternoon. The upper ridge looks to build again through the week with possible northeast winds developing again.

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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