[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT WED AUG 3, 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 what we saw on Tuesday.  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, with Salem's morning sounding nearly identical to yesterday.



Visible satellite imagery shows marine clouds along the northern and central coastline but clear skies over the Willamette Valley.  The surface map is indicating that onshore flow is very weak from the coast into a thermal trough, which extends from central Oregon into southwestern Oregon.



Onshore gradients should increase this afternoon, as the upper-level trough slowly approaches the coastline.  Cooling aloft will yield favorable mixing heights for burning this afternoon, but the strength and direction of transport winds will need to be closely monitored. Light northwesterly winds are forecast, below 5000 feet, with an abrupt shift to SSW winds at the mixing height.



TODAY'S FORECAST:



Sunny and warm.



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

Relative humidity drops below 50% by 11 a.m. and to near 30% by 5 p.m.

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

Transport winds: Light SW this morning; NW 4-8 mph this afternoon.

Mixing height: Rises to 3000 feet about noon and to 5500 feet by 5 p.m.

Salem's sunset tonight: 8:36 p.m.





THREE-HOURLY DATA:

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

     Temperature:                 70       78      83      74

     Relative Humidity:           46%      35%     30%     48%

     Surface Wind Direction:     Var      320     310     300

     Surface Wind Speed:           3        4       7      10

     Transport Wind Direction:   240      300     310     300

     Transport Wind Speed:         3        5       7      15

     Mixing Height:             2500     5000    5500    2300

     Ventilation Index:            8       25      39      35



EXTENDED DISCUSSION:



Some marine air is expected to penetrate into the Willamette Valley beginning this evening.  Gradient-stacking may inhibit burning on Thursday, if too much marine air pours into the valley tonight.  The weak upper-level trough is forecast to move across western Oregon Thursday, which would otherwise make for favorable burning conditions. A dry westerly flow aloft may also create favorable burning conditions on Friday.  Temperatures are only expected to cool a couple of degrees each day.



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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