[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 FRI OCT 5, 2012



BURN ADVISORY:



...State Fire Marshal Burn-Ban Conditions have been met in the northern Willamette Valley, due to the combination of low relative humidity (≤ 30%) and wind (≥ 15 mph), and may spread south this afternoon.



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A strong upper-level ridge of high pressure, centered in the Gulf of Alaska, is producing a dry and stable northerly flow aloft over Oregon.  A surface thermal trough is building northward along the Oregon coast.  That is continuing to draw very dry air into the Willamette Valley through the Columbia Gorge and the Cascades passes.



East winds were gusting to over 20 mph in the northern Willamette Valley this morning, as far south as Aurora and McMinnville.  Brisk east winds and low humidity levels have already put north valley locations, from about Woodburn north, into State Fire Marshal Burn-Ban Conditions.  Increasing north to northeast winds may extend the burn-ban conditions into the central and southern valley again this afternoon.



Satellite imagery shows clear skies across most of Oregon.  However, low clouds and fog have moved from the southern Oregon coast into portions of the southwest interior.  Patchy low clouds also extend northward along the coast to about Tillamook. Wildfire smoke is continuing to make for hazy conditions across most of the state, including the Willamette Valley.



TODAY'S FORECAST:



Sunny but hazy.  Increasingly breezy, but not as windy as yesterday.



Salem's high temperature today will be near 75 degrees (normal is 69).

Relative humidity:  Dropping below 20% by 2 p.m.

Surface winds:  NE 5-10 mph; increasing to 10-15 mph this afternoon.

Transport winds: NE 10 mph; increasing to 12-20 mph this afternoon.

Mixing height: Rising to near 4000 feet this afternoon.

Salem's sunset tonight: 6:44 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 73      73      60

     Relative Humidity:           19%     17%     29%

     Surface Wind Dir/Speed:    NE 12   NE 12   NE  7

     Transport Wind Dir/Speed:  NE 16   NE 18   NE 10

     Mixing Height:              4000    4000    1200

    Ventilation Index:           64      72      12



EXTENDED DISCUSSION:



Sunny and stable conditions are expected to persist through Wednesday of next week, with lighter, mostly northeasterly, winds.  Some air quality models are showing a significant increase in smoke and haze for the Willamette Valley, especially during the overnight periods, but that will depend largely on the amount of new smoke produced by area wildfires.



The upper-level ridge is forecast to finally push east of the state late next week, which would open the door for fall storms to begin coming onshore.  Typically, the initial weather system is fairly weak, and the latest computer guidance is showing it splitting as it moves onshore late Friday.  It is unlikely to produce much rainfall but could create an opportunity to burn any remaining fields on Friday.  Much stronger systems are forecast to quickly follow, bringing more significant rain to the valley.



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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