[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Wed Jul 6 11:49:28 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT WED JUL 6, 2016

BURN ADVISORY:

Recommended times for agricultural burning are from now until 7 p.m.

Prep burning is not allowed.

WEATHER DISCUSSION:

A dry and stable NW flow aloft is bringing only high clouds to NW Oregon today.  Onshore flow is weak but has favorable gradient-stacking to keep smoke elevated.  Cool air aloft will provide excellent mixing this afternoon, so the limiting factor for burning today be the strength of the onshore flow.  Winds are predicted to strengthen and turn northwesterly later this afternoon, which could provide a burning opportunity.  The first PIBAL of the season is scheduled for 2 p.m.

TODAY'S FORECAST:

Mostly sunny.

Salem's high temperature today will be near 79°F (average is 80°F).

Relative humidity:  Dropping to 40% by 5 p.m.

Surface winds: Light and variable; becoming NW 5-10 mph this afternoon.

Transport winds: N 3-7 mph; becoming NW 5-15 mph this afternoon.

Mixing height: Above 3000 feet; rising to 6000 feet by 5 p.m.

Salem's sunset tonight: 9:00 p.m.

(Salem Airport data for Tuesday, July 5th: High 77°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 90)

EXTENDED DISCUSSION:

On Thursday, an approaching cold front will turn transport winds mostly westerly.  Daytime mixing should be excellent with favorable onshore pressure-gradients to keep smoke elevated.  Clouds will be on the increase, but rain is expected to hold off until Thursday evening.  A significant burning window is likely, ahead of the cold front, Thursday afternoon.

Rain Thursday night will be followed by showers on Friday, as an upper-level trough moves onshore.  Rainfall totals may exceed one-tenth of an inch. Showery and unseasonably cool weather is expected on Saturday with a chance of a thundershower.  Additional rainfall may exceed one-quarter inch.  Showers should end on Sunday with dry weather on Monday.  Another cold front may present a burning opportunity Monday afternoon, if fields are dry enough. Temperatures will remain well below average through the extended period.

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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