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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE AUG 14 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from 2:00pm to 5:00pm with a 50 acre limit.
Propane flaming is allowed from 2:00pm to 5:00pm.
WEATHER DISCUSSION:
A weak disturbance moving through southern Canada and Washington will enhance onshore
flow a bit for slightly cooler temperatures than yesterday. The cool-down will be minor
and short lived, however, as temperatures will begin to climb again tomorrow.
The morning ODF surface pressure analysis shows a well-developed thermal trough extending
from the central valley of California to along the southern Oregon Coast. Surface high
pressure noses into western Washington and northwest Oregon. This is giving the
Willamette Valley a northerly transport flow and this is likely to continue throughout
the day.
Satellite pictures show nearly cloud free skies except for some low clouds and fog along
the central and northern Oregon coastline and along the Columbia River from Astoria to
about Kelso.
The morning Salem sounding shows a temperature trace very similar to yesterday morning’s
except a degree or two cooler. There is a weak inversion from about 1800 feet to about
2900 feet. This should mix out this afternoon and maximum mixing heights should reach
around 4000 feet. Low level winds on the sounding were mostly northerly or slightly
north-northeasterly and fairly brisk. Above about 2000 feet wind speeds were generally
between 15 and 20 mph.
Pressure gradients are fairly flat across western Oregon. Readings at 8:00am showed
0.9mb onshore from Newport to Salem and 1.2mb onshore from Salem to Redmond.
The limiting factor today will again be the north winds. These were quite brisk
yesterday and forced shutdown of some prep burning. Expect an unfavorable northerly
direction again today, with local areas of 12-16 mph wind speeds.
TODAY'S FORECAST:
Sunny.
Salem's high temperature today will be near 91.
Relative humidity drops to 50% by 10am. Minimum relative humidity 29%.
Surface winds: Northerly 5-9 mph.
Transport winds: Northerly 8-10 mph, increasing to 12-14 by late afternoon.
Maximum mixing height: 4000 feet.
Sunset tonight: 8:18 pm.
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 81 90 90 76
Relative Humidity: 37% 30% 31% 52%
Surface Wind Direction: 360 360 360 330
Surface Wind Speed: 7 7 9 7
Transport Wind Direction: 010 010 360 360
Transport Wind Speed: 8 10 12 14
Estimated Mixing Height: 2100 3000 4000 1500
Ventilation Index: 17 30 48 21
EXTENDED DISCUSSION:
As mentioned, temperatures will climb tomorrow and Thursday. Hot weather will continue
Friday and very likely Fire Marshal conditions will come into play. Strong high pressure
aloft and a thermal trough along the coastline will give easterly transport flow
Wednesday and Thursday. The thermal trough should be over the Valley Friday for
continued hot and humid weather. The hot spell is likely to come to an end with a round
of thunderstorms, possibly Friday night but more likely Saturday. Temperatures cool back
to near seasonal averages Saturday, Sunday and Monday.
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:140812:0845
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