[willamette-fcst] Silverton Hills Midday Forecast.doc
Willamette Valley Field Burning Forecast
willamette-fcst at listsmart.osl.state.or.us
Fri Jul 29 11:49:15 PDT 2011
SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
11:48 AM PDT FRI JUL 29, 2011
BURN ADVISORY:
Agricultural burning is not recommended.
Prep burning is not allowed.
Propane flaming is not allowed.
WEATHER DISCUSSION:
A weak system is cutting across British Columbia with a dry westerly flow aloft over Oregon. The late-morning surface map showed weak onshore flow across the northwestern corner of Oregon, with northerly pressure-gradients in the Willamette Valley. Valley winds were mostly northerly near 10 mph.
Visible satellite imagery showed patchy marine clouds breaking up along the coast and along the Columbia River into the Portland area. Elsewhere, skies are clear across western Oregon. Temperatures are running about the same as 24 hours ago in the extreme northern Willamette Valley and 3-5 degrees warmer than late-morning yesterday across the central and southern valley.
The weather system moving across British Columbia is too far away to force onshore flow into the central and southern Willamette Valley today, so expect northerly winds to prevail. That should keep skies clear with temperatures warming into the mid 80s.
TODAY'S FORECAST:
Sunny and warm.
Salem's high temperature today will be near 86 degrees (normal is 83).
Relative humidity drops to near 30% by 5 p.m.
Surface winds: N 8-12 mph this afternoon.
Transport winds: N 15 mph this afternoon.
Mixing height: Rises to 4500 feet by 5 p.m.
Salem's sunset tonight: 8:42 p.m.
THREE-HOURLY DATA:
2 p.m. 5 p.m. 8 p.m.
Temperature: 80 85 78
Relative Humidity: 38% 31% 42%
Surface Wind Direction: 360 360 350
Surface Wind Speed: 8 10 10
Transport Wind Direction: 360 360 360
Transport Wind Speed: 13 15 18
Mixing Height: 4000 4500 2800
Ventilation Index: 52 68 50
EXTENDED DISCUSSION:
The flow aloft will turn southwesterly over the weekend, as an upper-level trough moves inland and across southern British Columbia. Low-level winds will likely stay northerly across the Willamette Valley with mostly sunny skies and little change in temperatures. Southwesterly flow aloft is forecast to increase just enough next week to induce weak onshore flow at times. That will keep temperatures near normal and could create some favorable 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
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