[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT MON AUG 19, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

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



WEATHER DISCUSSION:



A flat upper-level ridge of high pressure is producing a dry and stable westerly flow aloft over Oregon.  Expect sunshine and increasing north winds this afternoon.



A weak weather system, cutting across southern British Columbia, will slightly cool the air aloft; making for good afternoon mixing.  Onshore pressure gradients are already favorable to suppress down-mixing of smoke, so the limiting factors for burning today will be transport wind direct and low-level wind speeds.



Surface wind speeds will likely exceed 10 mph later this afternoon and may become strong enough to inhibit vertical plume development.  Northerly transport winds are forecast to possibly turn northwesterly this afternoon, which would be more favorable for burning.  PIBALS are scheduled to begin at 1 p.m. to monitor both low-level wind speeds and transport wind direction.



TODAY'S FORECAST:



Sunny and warm.  Increasing north winds; possibly turning northwesterly.



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

Relative humidity:  Drops to near 35% by 5 p.m.

Surface winds:  N 10-15 mph this afternoon; possibly veering to NW.

Transport winds: N 10-15 mph this afternoon; possibly veering to NW.

Mixing height: Rising to near 5000 feet by 5 p.m.

Salem's sunset tonight: 8:11 p.m.



(Salem Airport data for Sunday, Aug. 18th: High 87°F; Rainfall .00")

(Maximum Ventilation Index expected today: 75)



EXTENDED DISCUSSION:



The upper-level ridge is forecast to strengthen over Oregon on Tuesday, with slackening winds aloft.  Warmer air aloft will make for lower afternoon mixing heights, even with surface temperatures climbing into the mid to upper 80s.  Northerly transport winds will limit burning opportunities.



The flow aloft will turn weakly southwesterly on Wednesday, with an upper-level disturbance, over northern California, likely triggering afternoon thundershowers across southern Oregon.  Sunny skies and northerly transport winds should continue to prevail over NW Oregon with temperatures warming into the low 90s.  A significant burning opportunity is not likely.



The upper-level disturbance over northern California is forecast to lift northeastward, into SE Oregon, by Thursday afternoon, which will trigger thunderstorms across much of southern and eastern Oregon.  Dry conditions will continue across NW Oregon, but increasing westerly flow aloft will initiate a cooling trend.  Transport winds may turn enough onshore to provide a burning opportunity.



Increasing southwesterly flow aloft is forecast for Friday.  Dry conditions should continue, with an approaching weak weather system possibly generating enough onshore flow to create another 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 (ft) times

         the transport wind speed (mph) 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/20130819/eb1d5f46/attachment.html>