[willamette-fcst] Silverton Hills AM Forecast.doc

Tue Jul 26 08:39:48 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:40 AM PDT TUES JUL 26 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from 11:00am to 2:00pm with a 50 acre limit.

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

WEATHER DISCUSSION:

Upper level trough from yesterday has weakened and moved off to the east.  The current upper level pattern is a weak trough with generally zonal flow overhead.  Expect a tranquil day with skies partly to mostly cloudy under residual marine air.  Air mass is somewhat stable this morning but will destabilize by the late morning to early afternoon.  Expect generally light northerly flow.

This morning's sounding shows a fairly moist marine layer to 3500 ft.  However, it is not as moist as yesterday, so expect clearing to come earlier today.  Wind flow has already turned northerly and should remain so with only a slight turning to NNW during the afternoon.  Pressure gradients are light with equal gradient stacking from the Coast Range to the Cascades.

TODAY'S FORECAST:

Salem's high temperature today will be near 75.

Relative humidity drops to 50% by 2pm.  Minimum relative humidity 41%.

Surface winds:  NW to N at 5-10 mph.

Transport winds: Northerly at 8-12 mph.

Mixing height:  Rising above 3000 ft by 11 am and above 5000 ft by 2 pm.

Sunset tonight: 8:46 pm.

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm

     Temperature:                 68       72      75      70

     Relative Humidity:           61%      49%     41%     51%

     Surface Wind Direction:     360      340     340     320

     Surface Wind Speed:           6        7      10      11

     Transport Wind Direction:   360      350     350     360

     Transport Wind Speed:         8        9      10      12

     Estimated Mixing Height:   3300     5000   5000+    4500

     Ventilation Index:           26       45      50      54

EXTENDED DISCUSSION:

Little change in the weather pattern expected for the remainder of the week.  Look for weak westerly flow with a minor ridge nosing in late Wednesday and Thursday.  Wind flow will remain generally northerly with a little more offshore flow likely Thursday and Friday.  Temperatures will gradually warm through the week.

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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