[willamette-fcst] Silverton Hills AM Forecast_Gary.doc

Thu Sep 22 09:17:38 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:40 AM PDT THU SEPT 22, 2016

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper level low centered over northern California will move into Nevada today as another system approaching in the Pacific nudges it eastward. The low caused the thunder yesterday and last night but as it is forced to move on so does the instability.

The thunderstorm last night in Salem was quite localized and east of I-5 rainfall reports received are mainly a trace. Relative humidity will be slow to fall today and mixing height again slow to rise, but are expected to reach burnable levels. South winds will gradually turn west. The primary limiting factor for burning may be the pressure gradients though they are not far from balancing now and if we receive enough sunshine they should balance too.

TODAY's FORECAST

Mostly cloudy with partial clearing in afternoon.

Salem's high temperature today will be near 69°F (average is 75°F).

Relative humidity:  Dropping to 60% about 2:00 p.m. and near 50% at 5:00 p.m.

Surface winds: S 5-8 mph becoming SW about 2:00 p.m. then W 9-12 mph by 5:00 p.m.

Transport winds: S 5-8 mph becoming SSW about 2:00 p.m. then W 9-12 mph by 5:00 mph.

Mixing height: Near 1000 feet at 11:00 a.m. rising to 2500 feet about 2:00 p.m. and 3000 feet by 5:00 p.m.

Salem's sunset tonight: 7:08 p.m.

(Salem Airport data for Wednesday, September 23rd: High 73°F; Rainfall: .44")

(Maximum Ventilation Index expected today: 36)

EXTENDED DISCUSSION:

The approaching system contains significant moisture but is weakening. It will likely cause a period of rain in the middle of the day on Friday to limit burning potential. North or northeast winds will dominate the transport layer over the weekend but no rain is expected. Monday and Tuesday appear to be possible burn days, especially Tuesday with northwest winds and maximal drying before that.

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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