[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT THU AUG 4, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from noon until 5:00 p.m.



WEATHER DISCUSSION:



A weak upper-level trough is finally moving onshore today.  That has forces the surface thermal trough into Central Oregon and increased the low-level onshore flow into western Oregon.  Satellite imagery showed low clouds still blanketing the southern Willamette Valley and Umpqua Basin late this morning.  Some low clouds were visible as far north as southern Marion County, with sunshine over the north valley.



The marine push into the south valley has resulted in a rather unusual pressure pattern across the Willamette Valley.  Gradients are northerly from Portland to Salem and southerly from Eugene to Salem.  That is helping to give low-level winds more of a westerly component over southern Marion and extreme northern Linn Counties.  That is a pattern more favorable for burning, if it persists into the afternoon.



Onshore gradients should further increase today, as the weak upper-level trough moves across western Oregon.  Slight cooling aloft will yield high afternoon mixing heights but could also trigger thunderstorm development over the Cascades.  Transport winds will need to be monitored closely to see if they have and maintain enough of a westerly component for open burning.  In any case, look for an influx of cool marine air to rapidly lower mixing heights this evening.



TODAY'S FORECAST:



Sunny and warm.



Salem's high temperature today will be near 85 degrees (normal is 84).

Relative humidity drops to near 37% by 5 p.m.

Surface winds:  NW 5-10 mph this afternoon.

Transport winds: NW 10 mph this afternoon.

Mixing height: Rises to 5000 feet by 5 p.m.

Salem's sunset tonight: 8:35 p.m.





THREE-HOURLY DATA:

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

     Temperature:                 80      84      74

     Relative Humidity:           43%     37%     52%

     Surface Wind Direction:     340     320     300

     Surface Wind Speed:           7       7      10

     Transport Wind Direction:   340     330     320

     Transport Wind Speed:        10      10      15

     Mixing Height:             4200    5000    2000

     Ventilation Index:           42      50      30



EXTENDED DISCUSSION:



A dry westerly flow aloft is forecast for Friday, with another weak upper-level trough moving into British Columbia.  Moderate onshore flow will keep temperatures near normal.  There is a slight chance that northerly low-level winds will turn enough onshore to allow for limited afternoon burning.  Little change in the weather pattern is forecast through early next week.



Long-range computer models are now waffling on their forecasts for late next week, with some showing a fairly strong trough moving onshore and others building a ridge over the west coast.  Therefore, confidence in the forecast beyond early next week is low.



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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