[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Mon Aug 31 08:55:24 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT MON AUG 31, 2015

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from noon until 2 p.m. with a 50 acre limit.

WEATHER DISCUSSION:

The first significant rain of the summer fell over the weekend, with the Salem Airport receiving .27" on Saturday and .44" on Sunday.  The rain had mostly ended by noon Sunday with brisk west winds helping to speed up the drying process, of damp fields, Sunday afternoon.

A strong upper-level trough, in the Gulf of Alaska, is producing a westerly flow aloft over Oregon today.  Another frontal system will bring rain to much of western Washington and the NW corner of Oregon today, but it appears that the rain will stay just north of the Silverton Hills.

Surface gradients are southerly this morning and will turn southwesterly this afternoon.  SW transport winds and excellent mixing heights will make for favorable ventilation conditions today.  Wind speeds and the dampness of fields will be limiting factors for burning this afternoon, as well as an ongoing threat of light rain.

TODAY'S FORECAST:

Mostly cloudy.  Slight chance of light rain this afternoon and evening.

Salem's high temperature today will be near 76°F (average is 81°F).

Relative humidity:  Dropping below 60% by noon and below 50% by 2 p.m.

Surface winds: S 5-10 mph this morning; SW 5-10 mph this afternoon.

Transport winds: SW 10-15 mph this morning; SW 15 mph this afternoon.

Mixing height: Rising to 3000 feet by noon and to 5000 feet by 2 p.m.

Salem's sunset tonight: 7:51 p.m.

(Salem Airport data for Sunday, August 30th: High 73°F; Rainfall: .44")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

The upper-level trough in the Gulf of Alaska will spin another cold front onshore Tuesday.  Ahead of the front, SW transport winds and high mixing heights will make for favorable ventilation conditions, However, wind speeds may to too strong to allow for burning.  Light rain is expected to make it as far south as Salem later Tuesday afternoon and evening rainfall totals near one-tenth of an inch.

The upper-level trough is predicted to move over Washington and Oregon Wednesday through Friday, for continued cool and unstable conditions with scattered showers likely.  Rainfall totals will be mostly between one-tenth and one-quarter of an inch.  Drier and warmer weather is expected over the Labor Day weekend, with temperatures recovering to near average.

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

Pete Parsons

ODF Meteorologist
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