[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:30 AM PDT WED SEP 12, 2012



BURN ADVISORY:



...Fire Marshal conditions may be reached this afternoon...

(relative humidity is expected to drop well below 30% and wind speeds may increase to at or above 15 mph)



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



An upper-level ridge is strengthening over the Pacific Northwest, with a surface thermal trough building northward along the southern Oregon Coast.  Offshore flow is bringing sunny skies to the entire state today, along with warmer temperatures. Some haze, from area wildfires, is possible across northwestern Oregon this afternoon.



Warmer air aloft will suppress mixing heights today.  In addition, brisk northeasterly winds and very low relative humidity will make for unfavorable burning conditions this afternoon.



TODAY'S FORECAST:



Sunny, warmer, and breezy.  Possibly turning hazy.



Salem's high temperature today will be near 84 degrees (normal is 78).

Relative humidity will drop well below 30% this afternoon.

Surface winds: NE 10-15 mph.

Transport winds: NE 15-25 mph.

Mixing height: Slowly climing to near 3000 feet by 5 p.m.

Salem's sunset tonight: 7:27 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 76      82      72

     Relative Humidity:           25%     19%     29%

     Surface Wind Dir/Speed:    NE 12   NE 15   NE 10

     Transport Wind Dir/Speed:  NE 15   NE 20   NE 15

     Mixing Height:              2500    3200    1500

     Ventilation Index:           38      64      23



EXTENDED DISCUSSION:



The center of a broad upper-level ridge of high pressure is forecast to shift over Idaho on Thursday, with a strong surface thermal trough likely remaining over western Oregon.  Look for valley temperatures to climb into the low 90s with slackening northeasterly transport winds.  The thermal trough is forecast to shift into eastern Oregon on Friday, as a weak upper-level trough turns the flow aloft southwesterly.  That may create more favorable burning conditions, as the transport winds turn onshore and cooling aloft improves mixing.  Valley temperatures should cool back into the mid 80s.



The dry upper-level trough is forecast to move across Oregon on Saturday, with increasing onshore flow likely bringing some morning marine clouds into the valley and helping to cool high temperatures back to near normal.  The flow aloft is forecast to turn more northerly on Sunday, which appears to be the beginning of another period of quite warm and continued dry weather.  A return to offshore flow is forecast for much of next week, which would be unfavorable for 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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20120912/1f706643/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Silverton Hills Midday Forecast.doc
Type: application/msword
Size: 31744 bytes
Desc: Silverton Hills Midday Forecast.doc
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20120912/1f706643/attachment.doc>