[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT THU AUG 18, 2011



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from noon until 2:00 p.m. with a 50 acre limit.

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



WEATHER DISCUSSION:



A weak upper-level disturbance is cut across southern British Columbia Wednesday afternoon and increased the onshore flow into western Oregon. Visible satellite imagery this morning showed that marine low clouds had moved inland, up the Columbia River, into the northern Willamette Valley.  Some low clouds had pushed about as far south as the Silverton Hills.



With the influx of cooler marine air overnight, there is some minor gradient-stacking this morning, with Newport-to-Salem at 0.6mb onshore and Salem-to-Redmond at 2.4mb onshore.  Westerly flow aloft is forecast for today, as a weak upper-level disturbance moves over western Washington.  Morning clouds will give way to a few high clouds in the afternoon.



Daytime heating should be enough to balance out the onshore gradients, but transport winds will need to be monitored closely and may stay too northerly to allow for open burning.  A weak sea breeze will lower mixing heights this evening with some marine air expected to penetrate up the Columbia River and into the northern Willamette Valley again overnight.



TODAY'S FORECAST:



Some morning low clouds, then mostly sunny.  A little cooler.



Salem's high temperature today will be near 81 degrees (normal is 82).

Relative humidity drops to 50% by 11 a.m. and to near 33% by 5 p.m.

Surface winds:  N 5-10 mph this morning; NNW 8-12 mph this afternoon.

Transport winds: N 10 mph this morning; NNW 10-15 mph this afternoon.

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

Salem's sunset tonight: 8:14 p.m.





THREE-HOURLY DATA:

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

     Temperature:                 68       76      79      71

     Relative Humidity:           50%      39%     33%     48%

     Surface Wind Direction:     360      350     340     320

     Surface Wind Speed:           5        8       9       9

     Transport Wind Direction:   360      350     340     340

     Transport Wind Speed:        10       12      14      16

     Mixing Height:             2700     3500    4700    2000

     Ventilation Index:           27       42      66      32



EXTENDED DISCUSSION:



A very weak upper-level trough will remain over Oregon on Friday with onshore flow likely strong enough to bring at least some morning marine clouds into the northern Willamette Valley.  Otherwise, skies will be sunny with transport winds likely remaining too northerly for open burning.



An upper-level flow will turn south-southwesterly this weekend; likely producing the warmest weather so far this summer.  Valley temperatures could be near 90 degrees, with north to northeasterly winds.  Some thunderstorm development is possible over the Cascades.



A weak cold front is forecast to move into southern British Columbia and northwestern Washington early next week, which may turn transport winds enough onshore to create a burning opportunity.



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



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20110818/6d5d9d46/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Silverton Hills AM Forecast.doc
Type: application/msword
Size: 32768 bytes
Desc: Silverton Hills AM Forecast.doc
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20110818/6d5d9d46/attachment.doc>