[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT WED OCT 3, 2012



BURN ADVISORY:



...State Fire Marshal Burn-Ban Conditions are expected this afternoon due to low relative humidity (≤ 30%) and increasing 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.  The dry cold front extended from SE Idaho across northern Nevada this morning.  In its wake, an unseasonably cold and dry Canadian air mass is settling into eastern Oregon, where many locations were well below freezing early this morning.



Cool and dry air is now pouring through the Columbia Gorge, into the northern Willamette Valley, via brisk easterly winds gusting to near 30 mph in Portland.  Strong NE pressure gradients will force the dry air southward across all of western Oregon today.  The combination of low relative humidity and increasing NE winds should put the Willamette Valley into State Fire Marshal Burn-Ban Conditions this afternoon.



Salem's sounding this morning showed NE winds from just above the surface up through 5000 feet.  Warm air aloft should keep afternoon mixing heights from climbing much above 3000 feet, even with plenty of sunshine.



(Salem Airport data for Tuesday, Oct. 2nd: High 71°F; Rainfall .00")



TODAY'S FORECAST:



Sunny.  Turning Breezy.



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

Relative humidity:  Dropping below 30% by 1 p.m. and to near 20% by 5 p.m.

Surface winds:  N 5-15 mph; becoming NE 12-20 mph this afternoon.

Transport winds: NE 15 mph; becoming NE 15-25 mph this afternoon.

Mixing height: Rising to near 3000 this afternoon.

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



THREE-HOURLY DATA:

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

     Temperature:                 56       67      70      55

     Relative Humidity:           43%      24%     20%     41%

     Surface Wind Dir/Speed:      N 8    NE 15   NE 15   NE 15

     Transport Wind Dir/Speed:  NE 15    NE 20   NE 25   NE 20

     Mixing Height:              1000     2300    3000    1200

     Ventilation Index:           15       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 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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