[willamette-fcst] Silverton Hills Midday Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT TUES. JUL 30, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



Upper troughing continues just off the coast, with southwesterly winds aloft over the area. Mostly sunny skies are in place with some low cloudiness hanging on over the coastal region and portions of the north valley. Pressure gradients are continuing to be very weak and surface winds are very light.



Mostly sunny skies and seasonal temperatures will dominate the area today. Transport winds will be northerly but generally weak through early afternoon, with some onshore flow possibly developing late this afternoon as gradients become somewhat more favorable. A mixed layer of greater than 3000 feet deep is also expected this afternoon. We will closely monitor the situation via surface observations, PIBALS and model guidance updates.



(Salem Airport data for Monday, July 29th: High 81°F; Rainfall .00")

(Maximum Ventilation Index expected today:  35)

TODAY'S FORECAST:



Expect mostly sunny skies and seasonal temperatures.



Salem's high temperature today will be in the mid 80s.

Relative humidity will be 30-35% late this afternoon.

Surface winds: Light.

Transport winds: N to NNW at 5-15 mph.

Maximum mixing height:  3500 ft.

Sunset tonight: 8:41 p.m.



EXTENDED DISCUSSION:



Upper troughing is expected to strengthen over the area on Wednesday and slowly extend eastward, with cooler conditions and morning low clouds more prevalent. Partly cloudy skies with some light shower development over the valley late in the day is expected, with some onshore flow expected late. Partly to mostly cloudy conditions are also expected Thursday as the upper trough continues to move inland, with a continued chance of showers and potential for an onshore flow pattern.



On Friday the onshore, unsettled pattern will likely continue with relatively cool low-level conditions and a likelihood of morning low clouds. As the late-week period approaches, we will provide more details on the flow and moisture pattern.



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.



Doug Wesley

ODF Meteorologist


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