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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT MON AUG 15 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from now until 1:00pm with a 50 acre limit.
Propane flaming is allowed from now until 5:00pm.
WEATHER DISCUSSION:
A weak upper trough over the Pacific Northwest is mostly pulling out to the east but a
portion is hanging back well off the coastline. This is leaving the region under a fair
weather pattern. At the surface a well developed thermal trough has developed over
California's central valley. This is a typical summer north transport wind situation.
Satellite pictures are showing just some fair weather cumulus clouds over the area.
As of late morning temperatures in the mid Willamette Valley were in the upper 60's and
low 70's. Most weather stations were showing a very light north or northeast wind.
At 11am the Newport to Salem pressure gradient was 0.4 mb onshore; Salem to Redmond was
1.2 mb onshore for a total Newport to Redmond gradient of 1.6 mb. This compares to the
Portland to Medford pressure difference which was 2.8 mb northerly.
Computer models indicate a generally light north or even north-northeast transport wind
through the afternoon. This matches will with the light coast to interior pressure
gradient and the stronger north to south, or Portland to Medford pressure gradients being
observed.
THIS AFTERNOON'S FORECAST:
Mostly sunny the rest of the afternoon.
Salem's high temperature today will be near 77.
Minimum relative humidity today will be about 37%.
Surface winds: NNW to NNE 4-7 mph.
Transport winds: N 3-5 mph increasing to 5-8 mph this evening.
Maximum mixing height:
Sunset tonight: 8:19 pm
THREE-HOURLY DATA:
2pm 5pm 8pm
Temperature: 73 78 73
Relative Humidity: 43% 35% 44%
Surface Wind Direction: 340 360 330
Surface Wind Speed: 5 7 7
Transport Wind Direction: 360 360 360
Transport Wind Speed: 3 5 8
Estimated Mixing Height: 4800 6100 1000
Ventilation Index: 14 30 8
EXTENDED DISCUSSION:
A ridge of high pressure now over the southwestern US will expand northwestward for
warmer temperatures and plenty of sunshine into the weekend.
Temperatures will also warm in California enhancing the thermal trough and keeping the
Willamette Valley in a generally northerly transport wind pattern. Any westerly flow
would have to be associated with an afternoon sea breeze and would need to be monitored
on a day-to-day basis.
The National Weather Service’s digital forecast is available at:
<a
href="http://forecast.weather.gov/MapClick.php?lat=44.905&lon=-122.810">http://forecast.weather.gov/MapClick.php?lat=44.905&lon=-122.810</a>.
Notes:
1. Mixing height, as used here, is the lowest height at which the
potential temperature exceeds the equivalent 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.
Jim Little
ODF Meteorologist
jlwx:150811:1147
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