[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 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:



Afternoon mixing heights will be slightly lower today, compared to yesterday, due to a building ridge of high pressure over Oregon.  Surface gradients are weakly onshore (2.8mb) from Newport to Salem but essentially flat (0.1mb onshore) from Salem and Redmond.  Daytime heating should turn transport winds from northeasterly to northerly this afternoon, which is more favorable for prep burning.  However, burning will be limited by the lack of onshore flow.



Smoke from multiple wildfires in SW Oregon is lifting to well over 8000 feet, where southwesterly winds aloft have been transporting it across much of central and eastern Oregon.  With the flow aloft turning more southerly, "elevated" wildfire smoke will begin migrating northward, across western Oregon, likely spreading over the Willamette Valley tonight.



TODAY'S FORECAST:



Sunny and very warm.  Increasing "elevated" wildfire smoke late.



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

Relative humidity:  Below 30%; dropping to near 20% by 5 p.m.

Surface winds:  N 5-10 mph this afternoon.

Transport winds: 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 onto the northern California coast on Wednesday, stall there through Friday, then lift northward across western Oregon this weekend.  South to southeasterly flow aloft will transport more wildfire smoke into the region along with increasing mid-level moisture and instability.  That will bring a good chance of thundershowers into the valley, especially south, beginning Wednesday evening.  These storms may initially be fairly dry, but the potential for wetting rains will increase through the week and into the weekend.  Temperatures should cool back to near average.



Long-range computer models continue to advertise a weather pattern change, around the middle of next week, to a dry and more stable westerly flow aloft.  That will end the thunderstorm threat and could create more substantial open 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
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