[willamette-fcst] Silverton Hills Midday Forecast.doc

Tue Jul 19 11:41:08 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT TUE JUL 19, 2011

BURN ADVISORY:

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

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

Propane flaming is allowed from now to 5:00pm.

WEATHER DISCUSSION:

Upper level trough is moving east of the region and will leave cool, NW'erly flow over the area through this afternoon and evening.  Air mass will be mostly unstable.  Onshore flow will remain fairly moderate today keeping clouds from breaking up too much and keeping temperatures a little cooler than yesterday.

Radar is showing this morning's showers dissipating and moving east of the region.  Would not likely see anymore shower activity this afternoon.  Satellite picture shows extensive cloud cover over western Oregon with little noticeable breakup.  Pressure gradients are onshore at 1.4 mb Newport to Salem and onshore at 2.2 mb from Salem to Redmond.

TODAY'S FORECAST:

Mostly cloudy.

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:

                                 2pm     5pm     8pm

     Temperature:                 70      71      66

     Relative Humidity:           54%     49%     57%

     Surface Wind Direction:     280     280     290

     Surface Wind Speed:           8      11      10

     Transport Wind Direction:   280     280     280

     Transport Wind Speed:        12      14      13

     Estimated Mixing Height:   5000+   5000+    3500

     Ventilation Index:           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.  Temperatures warm and 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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