[willamette-fcst] Silverton Hills AM Forecast.doc

Wed Aug 24 08:55:44 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:54 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.  That will increase mid-level moisture and instability, with some thunderstorm development likely.

The Salem sounding this morning showed several degrees of warming, compared to 24 hours ago, from the surface through 10,000 feet.  Low-level winds were offshore, and satellite imagery this morning showed mostly clear skies over the northern two-thirds of Oregon.  Transport winds are forecast to remain northerly or slightly offshore today, with increasingly southerly flow aloft.  Mixing heights will be slow to rise due to warmer air aloft.  Look for temperatures to be several degrees warmer than yesterday, with highs likely climbing into the low 90s.  In addition, high dew-point temperatures will make it feel muggy.

Daytime heating will combine with the increasing southerly flow aloft to spawn afternoon thunderstorm development.  Storms will form first over southern Oregon and migrate northward, later this afternoon and evening, into northern Oregon.  The best chance for thunderstorms will be over the Cascades, but some storms could make their way over the Willamette Valley.

TODAY'S FORECAST:

Sunny early; increasing clouds this afternoon with a chance of thunderstorms this evening and overnight. Warmer and muggy.

Salem's high temperature today will be near 91 degrees (normal is 82).

Relative humidity drops to near 60% around 11 a.m. and to near 37% 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:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 76       86      90      81

     Relative Humidity:           60%      42%     37%     50%

     Surface Wind Direction:     360      360     360     330

     Surface Wind Speed:           4        6       8       7

     Transport Wind Direction:   020      010     010     340

     Transport Wind Speed:        10       14      15      12

     Mixing Height:             2000     3000    4000    1500

     Ventilation Index:           20       42      60      18

EXTENDED DISCUSSION:

The flow aloft is forecast to turn more southwesterly on Thursday, which will decrease the thunderstorm threat and cool temperatures a few degrees.  Transport winds are forecast to remain north to northeasterly.  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 bring a return of mostly sunny and warm weather.  Long-range models are 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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