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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE JUL 31 2012
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is allowed from 12:00pm to 2:00pm with a 50 acre limit.
Propane flaming is allowed from 12:00pm to 5:00pm.
WEATHER DISCUSSION:
Weather patterns today are very similar to yesterday. The main difference is that the
low level air-mass is drier this morning. While there are low level clouds west of the
Cascades today they are not nearly as extensive as yesterday.
In the upper atmosphere a broad trough covers the northeastern Pacific Ocean and the
northwestern part of the United States. At low levels high pressure is centered offshore
while a semi-persistent trough of low pressure is centered in central Washington. In
addition, a weak thermal trough extends from the central Valley of California into
southwest Oregon. Winds in the Willamette Valley this morning are mostly
north-northwesterly.
The temperature profile on the morning Salem sounding was almost identical to yesterday.
The humidity profile, however, was drier – in line with the reduced cloudiness this
morning.
Gradient stacking continues today. Pressure gradient readings this morning are nearly
thes ame as yesterday morning. As of 8am Newport to Salem 0.5mb onshore compared to Salem
to Redmond at 2.8mb onshore. The Portland to Medford gradient was 3.1mb onshore.
Satellite pictures show clouds offshore of Washington associated with a very minor upper
level impulse riding up the west-southwest flow aloft. As this moves onshore to the
north of us it has the potential to draw some low level marine air into the valley and
shift low level transport winds to westerly. Computer models are not showing this
happening however and the probability of a burn opportunity this afternoon is low. Never
the less, it is something that bears watching and warrants launching pibals this afternoon
to monitor.
TODAY'S FORECAST:
Salem's high temperature today will be near 80.
Minimum relative humidity today 34%. (Relative humidity drops to 50% by 2pm.)
Surface winds: Northerly increasing to 7-10 mph this afternoon
Transport winds: Northerly around 10 mph.
Maximum mixing height: 4300 feet.
Sunset tonight: 8:39 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 69 78 80 69
Relative Humidity: 55% 38% 34% 51%
Surface Wind Direction: 350 350 360 330
Surface Wind Speed: 7 7 10 9
Transport Wind Direction: 020 010 360 350
Transport Wind Speed: 10 10 11 11
Estimated Mixing Height: 2200 3800 4300 2000
Ventilation Index: 22 38 47 22
EXTENDED DISCUSSION:
The basic pattern of morning clouds/afternoon sunshine should continue with a slow
warming trend through the week. A weak upper disturbance moving down from BC into the
northern Rockies will alter upper level winds tomorrow night and Thursday, but right now
there are no indications that this will produce a significant burn opportunity. Later in
the week there is potential for weak offshore flow which is not favorable for open field
burning in the Silverton Hills.
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:310712:0838
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