[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT THU AUG 9, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from 1 p.m. until 3 p.m. with a 50 acre limit.

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



WEATHER DISCUSSION:



A weak upper-level disturbance, centered over the southern Oregon coast at midday, is circulating considerable clouds northward over the Willamette Valley.  The system is forecast to migrate northeastward, across western Oregon, this afternoon.  That will maintain at least partly cloudy skies and should maintain general lifting of the upper-level air mass.



Gradient stacking has eased today and may balance out, with daytime heating, later this afternoon.  Transport winds are forecast to be northerly this afternoon, but there is a chance that they will turn enough onshore to create a limited burning opportunity.  Pibals are scheduled to begin at 2:00 p.m.



TODAY'S FORECAST:



Partly sunny this afternoon.  A little warmer.



Salem's high temperature today will be near 82 degrees (normal is 83).

Relative humidity drops to 50% by 2 p.m. and to near 40% by 5 p.m.

Surface winds:  N 8-13 mph.

Transport winds: N 10-16 mph.

Mixing height: Rising to 3000 feet by 2 p.m. and to near 3800 feet by 5 p.m.

Salem's sunset tonight: 8:26 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 76      81      74

     Relative Humidity:           48%     40%     50%

     Surface Wind Dir/Speed:     N 10    N 12   NW 12

     Transport Wind Dir/Speed:   N 12    N 15   NW 17

     Mixing Height:              3000    3800    2000

     Ventilation Index:           36      57      34



EXTENDED DISCUSSION:



A weak upper-level trough will move across western Oregon tonight and slightly reinforce the marine layer.  Expect morning clouds, on Friday, with afternoon sunshine.  Once again, there is a chance of a late-day limited burning window, but morning gradient stacking will need to be overcome and northerly transport winds will need to turn onshore.  Little change to the weather pattern is forecast through early next week, with continued possible limited late-day burning opportunities.



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/20120809/03737c8d/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Silverton Hills Midday Forecast.doc
Type: application/msword
Size: 32256 bytes
Desc: Silverton Hills Midday Forecast.doc
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20120809/03737c8d/attachment.doc>