[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:30 AM PDT THU SEP 1, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A dry north-northwesterly flow aloft will direct an upper-level disturbance across British Columbia later today and tonight.  At the surface, high pressure is building into western Washington and northwestern Oregon, with northerly pressure-gradients across the Willamette Valley.



Late-morning visible satellite imagery showed clear skies across western Oregon. ODF Sodar, located in the central coastal range, is showing northeasterly winds from just above the surface up to 3000 feet.  That is in-line with model sounding forecasts, which show NNE transport winds increasing across the Willamette Valley this afternoon.  A pibal is scheduled for 2 p.m. to verify the predicted transport winds.



Sunny skies will help temperatures recover close to normal this afternoon.  Warming aloft will lead to much lower mixing heights compared to Wednesday.



TODAY'S FORECAST:



Sunny and warmer.



Salem's high temperature today will be near 78 degrees (normal is 80).

Relative humidity drops to near 34% by 5 p.m.

Surface winds:  N 10-15 mph this afternoon.

Transport winds: NNE 15 mph this afternoon.

Mixing height: Rises to 3000 feet by 2 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 7:49 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 71      77      70

     Relative Humidity:           41%     34%     44%

     Surface Wind Direction:     360     360     350

     Surface Wind Speed:          10      12      10

     Transport Wind Direction:   010     360     360

     Transport Wind Speed:        12      17      17

     Mixing Height:             3000    4000    1500

     Ventilation Index:           36      68      26



EXTENDED DISCUSSION:



An upper-level ridge is forecast to build onshore Friday and over Oregon Saturday.  Temperatures will warm into the low 80s Friday and to near 90 Saturday.  As the ridge slides east of the state, increasing southerly flow aloft may bring isolated thunderstorms to the Cascades on Sunday, with temperatures likely reaching the low 90s.



The weather pattern for next week is not favorable for open burning, with a broad ridge of high pressure forecast to remain over the region.   Expect dry and quite warm conditions.  If the ridge shifts far enough to the east, then afternoon sea breezes could provide limited burning opportunities, but the threat of thunderstorms would also increase.



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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