[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Tue Aug 4 08:53:35 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT TUE AUG 4, 2015

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from 11 a.m. until 1 p.m. with a 50 acre limit.

WEATHER DISCUSSION:

A drier and more stable SW flow aloft has cleared skies across western Oregon this morning, except for patchy marine low clouds along the coast and in the northern Willamette Valley.  Satellite imagery shows a layer of elevated smoke, from wildfires across northern California and southern Oregon, stretching from the SW to NE across virtually all of Oregon. That smoke layer is expected to slowly migrate eastward over the next 48 hours; gradually clearing from the skies over NW Oregon.

Onshore flow has created only minor pressure gradient-stacking this morning, which should be overcome by daytime heating.  Cooling aloft will also make for higher mixing heights today.  The limiting factor for open burning today will be transport wind direction, which is expected to start out northerly but may turn northwesterly this afternoon.

TODAY'S FORECAST:

Mostly sunny but hazy due to a layer of wildfire smoke aloft.

Salem's high temperature today will be near 84°F (average is 84°F).

Relative humidity:  Dropping below 50% by 11 a.m. and to near 30% by 5 p.m.

Surface winds: N 5-10 mph this morning; NNW 7-12 mph this afternoon.

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

Mixing height: Rising to 3000 feet by 11 a.m. and to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:35 p.m.

(Salem Airport data for Monday, August 3rd: High 84°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

An upper-level trough, dropping southward to over southern British Columbia, will turn the flow aloft westerly on Wednesday.  Increasing low-level onshore flow will bring some marine clouds into the Willamette Valley Wednesday morning, but expect mostly sunny skies in the afternoon.  Temperatures will likely be a few degrees cooler with continued good afternoon mixing.  Once again, there will be potential for open burning, with the limiting factors being pressure gradient-stacking and transport wind direction.

A weak upper-level ridge is expected to turn the flow aloft northwesterly on Thursday with north to NE transport winds making burning unlikely.  On Friday, another upper-level trough will turn the flow aloft to the SW and may increase the onshore flow enough for an afternoon burning opportunity.

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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