[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:30 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:



Today's weather pattern is very similar to the past two days.  A weak upper-level trough is still just offshore with a dry southwesterly flow aloft over western Oregon.  Winds below 5000 feet are light and weakly onshore. There was not a significant influx of marine air into the Willamette Valley overnight.  In fact, Salem's morning sounding showed slight warming from the surface up through about 11,000 feet.



There are some significant changes beginning to show up in the overall weather pattern.  The surface map is indicating slightly stronger onshore flow, compared to yesterday morning, across western Oregon.  Mid-morning visible satellite imagery shows that marine clouds have penetrated inland into the Umpqua Basin and the southern Willamette Valley.  Patchy low clouds were also making their way up the Columbia River into the Portland area.  Skies were still mostly clear across Marion and northern Linn Counties.



Onshore gradients should further increase today, as the weak upper-level trough finally moves inland.  The strength and timing of the increasing onshore flow will need to be monitored closely today.  If marine air pours into the valley too quickly, then gradient-stacking may become unfavorable, and surface winds too strong, for burning. If the onshore flow is too weak, then low-level winds may stay northerly, as was the case yesterday. Slight cooling aloft will yield high afternoon mixing heights.



TODAY'S FORECAST:



Sunny and warm.  Patchy morning clouds possible.



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

Relative humidity drops below 50% by noon and to near 37% by 5 p.m.

Surface winds:  Light this morning; NNW 5-10 mph this afternoon.

Transport winds: Light north this morning; NNW 10 mph this afternoon.

Mixing height: Rises to 3000 feet about noon and to 5000 feet by 5 p.m.

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





THREE-HOURLY DATA:

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

     Temperature:                 73       80      84      74

     Relative Humidity:           53%      43%     37%     52%

     Surface Wind Direction:     Var      340     330     300

     Surface Wind Speed:           3        6       7      10

     Transport Wind Direction:   350      340     340     320

     Transport Wind Speed:         3       10      10      15

     Mixing Height:             2800     4200    5000    2000

     Ventilation Index:            8       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 temperature near normal and could create favorable burning conditions.  Expect dry weather, weak onshore flow, and moderate temperatures to continue through at least the first half of next week.  Long-range computer models are showing a stronger trough dropping into the region about next Friday.  That could create a burning opportunity and also bring some showers to the region.



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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