[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Tue Aug 13 11:33:09 PDT 2013

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT TUE AUG 13, 2013

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from 1 p.m. to 3 p.m. with a 50 acre limit.

WEATHER DISCUSSION:

A dry and stable southwesterly flow aloft has weakened the onshore flow today.  Patchy marine clouds quickly gave way to sunshine this morning across the Willamette Valley, with late-morning temperatures running about 5 degrees warmer than 24 hours ago.  The air aloft is also warmer, so afternoon mixing heights should not exceed 4000 feet, even with high temperatures expected to climb into the mid 80s.

Onshore pressure gradients were evenly stacked late this morning (Newport to Salem was 1.5 mb and Salem to Redmond was 1.3 mb, as of 11 a.m.) and should turn slightly favorable for burning this afternoon.  However, prevailing northerly transport winds will not likely allow for significant burning.  PIBALS will begin at 2 p.m. to monitor for any shift or increase in transport winds that would allow for open burning.

TODAY'S FORECAST:

Sunny and warmer this afternoon.

Salem's high temperature today will be near 85 degrees (normal is 83).

Relative humidity:  Drops to near 30% by 5 p.m.

Surface winds:  N 5-10 mph this afternoon.

Transport winds: N 6-12 mph this afternoon.

Mixing height: Rising to 3000 feet by 1 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 8:21 p.m.

(Salem Airport data for Monday, Aug. 12th: High 78°F; Rainfall .00")

(Maximum Ventilation Index expected today: 40)

EXTENDED DISCUSSION:

Increasing high clouds are expected on Wednesday, as an approaching upper-level trough increases the southwesterly flow aloft.  That will likely lead to very favorable open burning conditions fairly early Wednesday afternoon.  This could be a significant burning opportunity, with southwesterly transport winds and mixing heights rising to near or above 5000 feet.

A weak cold front is forecast to stall as it moves onshore Wednesday night. Some drizzle or light rain is possible along the coast, but little or no precipitation is expected in the Willamette Valley. More clouds and onshore flow will likely bring slight cooling on Thursday, but conditions may remain favorable for additional burning Thursday afternoon.

A stronger front is forecast to move onshore Friday.  If the timing is right, it could create another significant burning opportunity, Friday afternoon, with southwesterly transport winds and mixing heights rising to near or above 5000 feet.

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 15mph.

     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 and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.

Pete Parsons

ODF Meteorologist
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