[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT TUE AUG 6, 2013



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is allowed from 2 p.m. to 4 p.m. with a 50 acre limit.



WEATHER DISCUSSION:



An upper-level ridge is rebuilding over Oregon today.  That will slightly suppress mixing heights, compared to Monday.  Surface gradients are still weakly onshore from Newport to Salem but have turned offshore between Salem and Redmond. In response to the decrease in onshore flow, marine low clouds are confined mostly to the immediate coastline this morning.



The morning sounding over Salem showed NE winds from just above the surface up through 2000 feet.  With daytime heating, transport winds will likely back from northeasterly to more northerly this afternoon, but burning opportunities will be limited by the lack of onshore flow.



Smoke from the multiple wildfires in SW Oregon is being transported well up into the atmosphere, where southwesterly winds aloft (above 8000 feet) have been transporting it northeastward across much of central Oregon and eastern Oregon.  As the flow aloft turns more southerly today, that "elevated" smoke will begin migrating northward across western Oregon; likely spreading over the Willamette Valley by tonight.



TODAY'S FORECAST:



Sunny and very warm.  Increasing wildfire smoke aloft late.



Salem's high temperature today will be near 93 degrees (normal is 84).

Relative humidity:  Drops below 50% by 11 a.m. and below 30% by 2 p.m.

Surface winds:  N 2-6 mph this morning; N 5-10 mph this afternoon.

Transport winds: NNE 4-8 mph this morning; N 8-12 mph this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 8:31 p.m.



(Salem Airport data for Monday, Aug. 5th: High 93°F; Rainfall .00")

(Maximum Ventilation index expected today: 54)



EXTENDED DISCUSSION:



An upper-level trough is forecast to move over northwestern California on Wednesday and stall there at least through Friday.  South to southeasterly flow aloft will likely continue to transport "elevated" wildfire smoke across most of western Oregon.  Even though transport winds should remain generally northerly, settling and down-mixing of wildfire smoke may deteriorate air quality in the Willamette Valley.  There will also be an increasing chance of thundershowers, with temperatures cooling to near average.  Burning opportunities will likely by very limited.



Long-range computer models are consistent is showing a weather pattern change, around the middle of next week, to a more westerly flow aloft.  That will end the thunderstorm threat and could create more substantial 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 (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/20130806/faba8aca/attachment.html>