[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Thu Sep 29 11:55:48 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT THU SEP 29, 2016

BURN ADVISORY:

Recommended times for agricultural burning are from 2 p.m. until 5 p.m.

Prep burning is not allowed.

WEATHER DISCUSSION:

Strengthening SW flow aloft is spreading high clouds across the region today, while lower levels of the air mass remain quite dry.  Cooling aloft will improve mixing this afternoon and cap surface temperatures near average.

Negatively-stacked onshore pressure gradients (1.4mb from Newport to Salem and 3.4mb from Salem to Redmond late this morning) should balance out later today.  Northerly transport winds are predicted to turn enough northwesterly, later this afternoon, to allow for the burning of all remaining fields.  A PIBAL is scheduled for 3 p.m.

TODAY'S FORECAST:

Mostly sunny.

Salem's high temperature today will be near 72°F (average is 72°F).

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

Surface winds: N 5 mph; becoming NNW 5-10 mph later this afternoon.

Transport winds: N 5-10 mph; becoming NW 8-12 later this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and 4000 feet by 5 p.m.

Salem's sunset tonight: 6:55 p.m.

(Salem Airport data for Wednesday, September 28th: High 75°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 48)

EXTENDED DISCUSSION:

A transition to a fall-like weather pattern should accompany the end of the 2016 field-burning.  Increasing SW transport winds could provide one last burning opportunity, ahead of a cold front, on Friday.  Showers are possible as soon as Friday afternoon and are likely this weekend, as temperatures progressively drop to well-below average.  Showers should end on Monday, but another Pacific storm system is slated to spread rain back across the region Monday night and Tuesday.

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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