[willamette-fcst] Silverton Hills Midday Forecast.doc

Wed Aug 24 11:47:56 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT WED AUG 24, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from 2:00 p.m. until 5:00 p.m.

WEATHER DISCUSSION:

A strong upper-level ridge of high pressure remains anchored over the south-central United States.  The clockwise flow around it is directing a warm southwesterly flow aloft over Oregon.  A weak disturbance, embedded in that flow, will turn the upper-level winds more southerly today and introduce the best chance of thunderstorms so far this summer into western Oregon

Late-morning satellite imagery showed middle and high clouds advancing northward across the Willamette Valley.  A more significant band of clouds was pushing northward across Douglas Lane Counties.  Doppler radar showed some showers associated with these clouds but not any thunderstorms.  Daytime heating will further destabilize the air mass over Oregon, so afternoon thunderstorm development is likely, bands of clouds continues to migrate northward.  Most of the thunderstorms will be over the Cascades, but some storms are likely over the Willamette valley.

Increasing clouds may act to limit afternoon heating, so the high temperature forecast was lowered slightly from this morning's forecast.   Transport winds should remain northerly or slightly offshore this afternoon.  A pibal is scheduled for 2 p.m.  Winds will be highly variable, and possibly gusty, near showers and thunderstorms later this afternoon and evening.

TODAY'S FORECAST:

Warm and muggy. Increasing clouds with a chance of thunderstorms this afternoon through tonight.

Salem's high temperature today will be near 90 degrees (normal is 82).

Relative humidity drops to near 40% by 5 p.m.

Surface winds:  N 5-10 mph.

Transport winds: NNE 10-15 mph.

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

Salem's sunset tonight: 8:03 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 86      89      81

     Relative Humidity:           42%     40%     50%

     Surface Wind Direction:     360     360     330

     Surface Wind Speed:           6       8       7

     Transport Wind Direction:   010     010     340

     Transport Wind Speed:        14      15      12

     Mixing Height:             3000    4000    1500

     Ventilation Index:           42      60      18

EXTENDED DISCUSSION:

The upper-level disturbance that will bring a threat of showers and thunderstorms overnight is forecast to move northeast of the region by Thursday morning.  That should initiate a weak onshore surge and stabilize the air mass.  Southwesterly flow aloft will decrease the thunderstorm threat and cool temperatures a few degrees.  Afternoon transport winds are forecast to be northerly.

Friday looks to be a repeat of today, with the flow aloft backing to more southerly and reintroducing a chance of thunderstorms.  Burning opportunities are unlikely both days.

The flow aloft is forecast to turn southwesterly again this weekend, which should stabilize the air mass and bring a return of mostly sunny and dry weather.  Long-range models are still showing a possible burning opportunity for next Monday, as a system moving into southern British Columbia turns the flow aloft more westerly.

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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