[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT WED OCT 3, 2012



BURN ADVISORY:



...State Fire Marshal Burn-Ban Conditions are in effect due to low relative humidity (≤ 30%) and brisk N to NE wind (≥ 15 mph).



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



Northwesterly flow aloft, around a building upper-level ridge in the Gulf of Alaska, drove a dry cold front southward across eastern Washington and eastern Oregon on Tuesday.  In its wake, an unseasonably cold and dry Canadian air mass is settling into eastern Oregon.



Cool and dry air is pouring through the Columbia Gorge, into the northern Willamette Valley, via strong easterly winds gusting to near 40 mph in Portland.  Brisk NE wind, gusting to near 30 mph, have also picked up in the central Willamette Valley.  Smoke from wildfires is being transported across NW Oregon, with air quality gauges showing an increase in particulate levels late this morning.  Warm air aloft should keep afternoon mixing heights from climbing much above 3000 feet, even with hazy sunshine.



TODAY'S FORECAST:



Sunny, hazy, and breezy.



Salem's high temperature today will be near 70 degrees (normal is 70).

Relative humidity:  Dropping below 20% by 5 p.m.

Surface winds:  NE 12-20 mph this afternoon.

Transport winds: NE 20-30 mph this afternoon.

Mixing height: Rising to near 3000 this afternoon.

Salem's sunset tonight: 6:48 p.m.



THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 67      70      55

     Relative Humidity:           20%     18%     41%

     Surface Wind Dir/Speed:    NE 15   NE 15   NE 15

     Transport Wind Dir/Speed:  NE 20   NE 25   NE 20

     Mixing Height:              2300    3000    1200

     Ventilation Index:           46      75      24



EXTENDED DISCUSSION:



Little change in the weather pattern is expected through Saturday, with continued sunny skies and dry offshore flow through the transport layer.  Offshore flow should slacken by Sunday; possibly turning briefly onshore Monday, before turning offshore again during the middle of next week.  It is uncertain whether this brief switch in wind direction will present a burning opportunity.



The upper-level ridge is forecast to maintain dry and fairly stagnant conditions through Thursday of next week.  However, there are indications that the first in a series of fall storms may move onshore next Friday.



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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