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SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT TUE AUG 23 2011
BURN ADVISORY:
Agricultural burning burning is not recommended.
Prep burning is not allowed.
Propane flaming is allowed from 12:00pm to 5:00pm.
WEATHER DISCUSSION:
A large upper level high pressure system remains firmly entrenched over much of the
continental United States today while a broad upper level trough covers much of the Gulf
of Alaska. Minor impulses are periodically rippling out of the trough and into Canada
around the top of the US ridge.
One of those impulses brought heavy rain to western Washington yesterday and light rain
to parts of northwest Oregon last night. Over three inches of rain fell at Quillayute,
Washington during the past 24 hours. The Portland Airport picked up .17 inches and the
Salem Airport showed just a trace. CoCoRahS reports show measurable rain confined to
Clackamas County and northward, just trace amounts in Marion County, and no rain farther
south.
Satellite pictures show clouds confined to northwestern Oregon. Those clouds will be
slow to burn off to the north of about Aurora, areas south will see some sun by this
afternoon.
The morning surface analysis shows a rather flat pressure pattern over western Oregon.
Pressure gradients as of 8am included: Newport to Salem, 0.2 mb onshore; Salem to
Redmond, 0.5 mb offshore; Newport to Redmond, 0.3 mb offshore. The north/south Portland
to Medford gradient was flat.
The surface analysis also shows the California thermal trough re-intensifing. This will
return the Willamette Valley into a northerly transport pattern with the transition
occuring today. Transport winds will change from southwesterly to northerly or even
northeasterly by this evening. There may be an opportunity to catch a brief westerly
transport during the switch-over, but the window would be very small.
The morning Salem sounding showed a moist atmosphere with slight warming below 1500 feet
and cooling above. That means a less stable atmosphere than yesterday and consequently
higher afternoon mixing heights. Currently we expect the sun to break through and push
temperatures into the low 80's this afternoon which would put the maximum mixing height
at about 6000 feet.
TODAY'S FORECAST:
Morning clouds, then turning partly sunny.
Salem's high temperature today will be near 83.
Relative humidity drops to 50% by 2pm. Minimum relative humidity 44%.
Surface winds: Southerly 5-7 mph becoming northwesterly 5-7 by mid afternoon.
Transport winds: S 3 mph this morning, shifting to northerly during the afternoon.
As the winds shift there will be a fair amount of directional shear (ie. wind
direction changing with height) which may inhibit plume development.
Maximum mixing height: 6000 feet.
Sunset tonight: 8:05 pm
THREE-HOURLY DATA:
11am 2pm 5pm 8pm
Temperature: 74 80 83 77
Relative Humidity: 62% 51% 44% 56%
Surface Wind Direction: 180 230 300 310
Surface Wind Speed: 6 5 7 6
Transport Wind Direction: 190 270 360 020
Transport Wind Speed: 3 3 7 9
Estimated Mixing Height: 3100 4900 6000 1000
Ventilation Index: 9 15 42 9
EXTENDED DISCUSSION:
The ridge over the central US builds northwestward briefly tomorrow for warmer weather,
more sunshine, and an intensified, unfavorable, northerly transport wind pattern.
Additional impulses moving out of the Gulf of Alaska trough could provide burn
opportunities for Thursday or Friday, depending on timing.
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:230811:0829
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