[willamette-fcst] Silverton Hills AM Forecast.doc

Wed Sep 21 08:56:44 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT WED SEP 21, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

A strong and broad upper-level ridge of high pressure remains anchored over the Rockies with light southwesterly winds aloft over Oregon.  At the surface, pressure gradients are offshore across western Oregon with light easterly winds coming down the Columbia Gorge.  Winds are light in the Willamette Valley, but the Salem sounding, from early this morning, showed light easterly winds from just above the surface up to about 2000 feet.

Satellite imagery shows an impressive-looking frontal system extending from the southern British Columbia coast to about 400 miles west of the northern Oregon coast.  Some mid-level clouds from this system were streaming across the northwestern corner of Oregon, but skies remained clear over the remainder of the state.

The latest computer guidance is forecasting very little eastward progress of the offshore weather system today.  There may be some increase in mid-level clouds, but warm air aloft will suppress mixing heights.  Valley temperatures are running 5-9 degrees warmer than 24 hours ago, so afternoon highs should climb into the upper 80s.

Due to the advancing cold front, transport winds are forecast to slacken by mid-afternoon; possibly turning weakly onshore in the late-afternoon.  Mixing heights will struggle to reach 3000 feet today.  Transport winds will need to be monitored closely for the possibility of a limited burning opportunity.

TODAY'S FORECAST:

Partly cloudy and warmer.

Salem's high temperature today will be near 87 degrees (normal is 76).

Relative humidity drops below 50% by noon and to near 30% by 5 p.m.

Surface winds:  Light SE this morning; becoming light SW late this afternoon.

Transport winds: Light SE this morning; turning light SW late this afternoon.

Mixing height: Below 2000 feet this morning; rising to 3000 feet by 5 p.m.

Salem's sunset tonight: 7:12 p.m.

THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 69       81      85      71

     Relative Humidity:           53%      35%     31%     49%

     Surface Wind Direction:     150      Var     250     250

     Surface Wind Speed:           3        3       4       4

     Transport Wind Direction:   150      Var     250     250

     Transport Wind Speed:         3        3       4       7

     Mixing Height:             1300     2500    3200    1200

     Ventilation Index:            4        8      13       8

EXTENDED DISCUSSION:

The strong Rocky Mountain ridge is forecast to weaken slightly Thursday and Friday with increasing southwesterly flow aloft over Oregon.  The surface thermal trough should push well east of the Cascades with slight cooling aloft and southwesterly transport winds likely creating favorable afternoon burning conditions both days.

The flow aloft, and at transport level, is forecast to become more southerly, on Saturday, as an upper-level trough digs offshore.  Some computer models are showing rain-making weather systems moving onshore Sunday through Tuesday of next week.  Longer-range charts are indicating a return of dry weather beginning about next Wednesday.

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 times

         the transport wind speed 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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