[willamette-fcst] Silverton Hills Midday Forecast.doc

Wed Aug 17 11:58:20 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:57 AM PDT WED AUG 17, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is allowed from 1:00 p.m. until 5:00 p.m.

WEATHER DISCUSSION:

Oregon remains under the influence of a dry southwesterly flow aloft.  Late-morning satellite imagery showed clear skies over the state.  Willamette Valley temperatures were in the low 70s (about 1-3 degrees warmer than 24 hours ago).  The surface map showed a broad thermal trough extending from central Washington, through central Oregon, into southwestern Oregon.  That was combining with high pressure, centered offshore, to produce northwesterly pressure-gradients across the Willamette Valley.

A weak upper-level disturbance is forecast to cut across southern British Columbia late this afternoon and evening; increasing the onshore flow into western Oregon.  That should cap valley temperatures in the mid 80s (close to where they peaked on Tuesday).  With warm air aloft, mixing heights will be slow to rise this afternoon but should eventually climb above 4000 feet.

There was only minor gradient-stacking late this morning, with Newport-to-Salem at 1.5mb onshore and Salem-to-Redmond at 2.3mb onshore.  Afternoon heating should be more than enough to balance that out.  However, transport winds may not turn enough onshore to allow for open burning.  The latest computer guidance shows northerly winds, through early this afternoon, with only a slight shift to NNW by late this afternoon.  A sea breeze will rapidly lower mixing heights this evening with some marine air expected to penetrate up the Columbia River and into the northern Willamette Valley overnight.

TODAY'S FORECAST:

Sunny and warm.

Salem's high temperature today will be near 86 degrees (normal is 82).

Relative humidity drops to near 25% by 5 p.m.

Surface winds:  Becoming NNW 5-10 mph this afternoon.

Transport winds: Becoming NNW 6-10 mph this afternoon.

Mixing height: Rises to 3000 feet by 1 p.m. and to 4700 feet by 5 p.m.

Salem's sunset tonight: 8:15 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 82      85      74

     Relative Humidity:           28%     24%     38%

     Surface Wind Direction:     350     340     320

     Surface Wind Speed:           6       7       9

     Transport Wind Direction:   350     340     330

     Transport Wind Speed:         7       9      14

     Mixing Height:             3700    4700    2000

     Ventilation Index:           26      42      28

EXTENDED DISCUSSION:

A very weak upper-level trough will be over Oregon on Thursday with onshore flow likely strong enough to bring at least some morning marine clouds into the Willamette Valley.  Skies should clear in the afternoon with daytime highs around 80 degrees.  Gradient-stacking will be less favorable for burning and transport winds are forecast to return to northerly in the afternoon.  Little change is forecast for Friday.

An upper-level ridge is forecast to build over Oregon this weekend; likely producing the warmest weather so far this summer.  Valley temperatures could climb into the low 90s, by Sunday, with north to northeasterly winds.  The upper-level ridge will shift east of the state early next week.  Increasing onshore flow could create burning opportunities.  It is also possible that the flow aloft may turn enough southerly to introduce a chance of thunderstorms over the Cascades, which is not helpful for open burning.

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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