[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Tue Aug 25 11:55:54 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT TUE AUG 25, 2015

BURN ADVISORY:

*** State Fire Marshal Burn-Ban Conditions Possible This Afternoon. ***

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

SW flow aloft and very weak onshore flow at the surface slowly cleared the bulk of the wildfire smoke from NW Oregon.  Lyons and Sweet Home still have slightly elevated smoke levels at midday, but all NW Oregon air monitoring stations are indicating "good" air quality. Isolated thunderstorm development is possible over the Cascades this afternoon.  Otherwise, expect fair skies and slightly warmer temperatures today, due to less wildfire smoke.

Onshore gradients are very weak and slightly negatively stacked.  Newport to Salem was +0.8mb and Salem to Redmond was +1.9mb at 11 a.m.  Gradients should become more favorably stacked later this afternoon.  NNW transport are expected this afternoon but may stay too northerly to allow for burning.  In addition, the air mass is very dry.  Relative humidity values should drop below 30% this afternoon, so wind speeds of 15 mph or greater will put the valley into State Fire Marshal Burn-Ban conditions.  A PIBAL is scheduled for 2 p.m.

TODAY'S FORECAST:

Partly cloudy and warm.  Still a little hazy near the Cascades.

Salem's high temperature today will be near 85°F (average is 81°F).

Relative humidity:  Dropping below 30% by 2 p.m.

Surface winds: N 5-15 mph this afternoon.

Transport winds: NNW 7-15 mph this afternoon.

Mixing height: Rising to 3000 feet about 1 p.m. and to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:02 p.m.

(Salem Airport data for Monday, August 24th: High 84°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

An upper-level trough, centered about 800 miles off the northern Oregon coast, will move very little through Thursday.  Continued dry SW flow aloft will maintain partly cloudy and warm weather with just a chance for isolated afternoon thundershowers over the Cascades.  Wildfire smoke should remain east of the region.  Increasing onshore flow may create favorable burning conditions in the afternoons, especially by Thursday.

The upper-level trough is expected to advance towards the coast on Friday.  Increasing onshore flow will bring clouds, cooler temperatures, and the threat of showers.  Computer models have slowed down their timing for the onset of showers, so another burning opportunity may be possible Friday afternoon, ahead of the rain.  Over one-half inch of rain is possible, in the Silverton Hills, Friday night and Saturday.  Another system may bring additional showers on Sunday.

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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