[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:40 AM PDT TUE JUL 19, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are from 9:00am to 7:00pm.

Prep burning is allowed from 12:00pm to 2:00pm with a 50 acre limit.

Propane flaming is allowed from 11:00am to 5:00pm.



WEATHER DISCUSSION:



Upper level trough will finally move through the region this morning leaving cool, NW'rly flow over the region through this afternoon and evening.  A few showers are still possible this morning before dissipating this afternoon.  Onshore flow will remain fairly moderate today keeping clouds from breaking up too much and keeping temperatures a little cooler than yesterday.



Morning upper air sounding over Salem generally showed a deep marine layer to 14,000 ft with minor stability near the surface.  This stable layer should dissipate by mid morning, allowing mixing heights to rise to 5000 ft by noon.  Surface and transport winds show a W to NW flow.  Pressure gradients are over 4 mb from Newport to Redmond with even gradient stacking in the Coast Range and Cascades.



TODAY'S FORECAST:



Mostly cloudy with a chance of showers.  Chance of showers decreases this afternoon.



Salem's high temperature today will be near 71.

Relative humidity drops to 50% by 4pm.  Minimum relative humidity 49%.

Surface winds:  W to NW at 6-12 mph.

Transport winds: W to NW at 10-15 mph.

Maximum mixing height:  5000+ ft.

Sunset tonight: 8:53 pm





THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm

     Temperature:                 66       70      71      66

     Relative Humidity:           67%      54%     49%     54%

     Surface Wind Direction:     280      280     280     290

    Surface Wind Speed:           6        8      11      10

     Transport Wind Direction:   290      280     280     280

     Transport Wind Speed:        10       12      14      13

     Estimated Mixing Height:   4000    5000+   5000+    3500

     Ventilation Index:           40       60      70      46



EXTENDED DISCUSSION:



Upper level pattern remains somewhat trough-like on Wednesday with onshore flow weakening and turning a little more northerly.  Next upper level low drops down from north of Vancouver Island on Thursday and strengthens the onshore flow again.  Expect a chance of light showers or sprinkles but not like the past weekend.  Trough moves off to the east Friday giving way to a weak ridge for Friday afternoon and Saturday.  Winds turn northerly again.



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.



Nick Yonker

ODF Meteorologist



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