[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Tue Sep 1 11:38:47 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT MON SEP 1, 2015

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A strong upper-level trough, in the Gulf of Alaska, is increasing the WSW flow aloft over the region.  A slow-moving cold front is spreading light rain across much of western Washington and the NW tip of Oregon.

Ahead of the cold front, mostly sunny skies and southerly winds helped temperatures warm to near 70°F across the Willamette Valley by late this morning.  Warm surface temperatures have already pushed mixing heights above 3000 feet, and mixing heights above 5000 feet can be expected this afternoon.

An 11 a.m. PIBAL showed SSW transport winds, which are expected to veer to the WSW by late this afternoon.  Another PIBAL is scheduled for noon.  Good mixing and SW winds should provide favorable burning conditions today, although the dampness of fields will continue to be a limiting factor.

Light rain will likely push across the Willamette Valley by tonight, with overnight rainfall totals expected to be less than one-tenth of an inch.

TODAY'S FORECAST:

Increasing clouds.  Chance of light rain this evening and overnight.

Salem's high temperature today will be near 75°F (average is 80°F).

Relative humidity:  Dropping to 45% around 2 p.m.

Surface winds: SW 5-10 mph.

Transport winds: SW 5-10 mph; becoming WSW 10-15 mph later this afternoon.

Mixing height: Rising above 5000 feet this afternoon.

Salem's sunset tonight: 7:49 p.m.

(Salem Airport data for Monday, August 31st: High 76°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 90)

EXTENDED DISCUSSION:

Wednesday through Friday, an upper-level trough is predicted to move over Oregon.  That will keep temperatures below average with scattered showers and a chance of afternoon thundershowers.  Spotty and light rainfall totals are generally expected, but brief intense showers are possible.  Predicted WNW transport winds may provide some burning opportunities Wednesday and Thursday, if fields remain dry and there is not too much shower activity.  Transport winds are expected to turn mostly northerly by Friday.

Drier and warmer weather will return over the Labor Day weekend, with temperatures recovering to near average.  There are indications that we could see several days of sunny and very warm weather, beginning next week, with the potential for a period of offshore flow.

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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