[willamette-fcst] Silverton Hills AM Forecast.doc

Wed Sep 26 08:53:19 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT WED SEP 26, 2012

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

An upper-level ridge of high pressure is building back over the Pacific Northwest this morning.   At the surface, a thermal trough is beginning to push northward along the southern Oregon coast, with weak pressure gradients across the Willamette Valley.  Visible satellite imagery shows low clouds extending inland from the north coast, along the Columbia River, into the northern and central Willamette Valley.  Skies are mostly clear over the remainder of the state, with the exception of areas of wildfire smoke, mainly from the Cascades eastward.

The Salem sounding this morning and local pilot reports indicate that the marine layer is only about 2000 feet thick, so skies should clear by early afternoon.  The air aloft has warmed several degrees since yesterday, so maximum mixing heights will be lower today.  North to northeast transport winds will be unfavorable for burning.

(Salem Airport data for Tuesday, Sept. 25th: High 75°F; Rainfall .00")

TODAY'S FORECAST:

Morning clouds with afternoon sunshine.

Salem's high temperature today will be near 75 degrees (normal is 74).

Relative humidity drops to near 60% around noon and to near 30% by 5 p.m.

Surface winds:  N to NE 5-10 mph.

Transport winds: N to NE around 10 mph.

Mixing height: Rising to near 2000 feet by noon and 3000 feet by 5 p.m.

Salem's sunset tonight: 7:01 p.m.

THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 58       69      75      62

     Relative Humidity:           64%      41%     30%     50%

     Surface Wind Dir/Speed:     NE 5      N 6     N 8     N 6

     Transport Wind Dir/Speed:  NE 10    NE 10    N 12    N 10

     Mixing Height:              1500     2500    3000    1500

     Ventilation Index:           15       25      36      15

EXTENDED DISCUSSION:

Little change in the weather pattern is forecast for Thursday, with a broad upper-level ridge of high pressure remaining parked over the Pacific Northwest.  Expect sunny skies after areas of morning fog.  Maximum temperatures should be about 5 degrees warmer than today.  Continued northerly transport winds will be unfavorable for burning.

On Friday, the upper-level ridge is forecast to shift eastward, to over the Rockies.  Skies should remain mostly sunny, but increasing westerly flow aloft will force the thermal trough into eastern Oregon.  That may turn transport winds enough onshore in the afternoon to create a burning opportunity.  A weak cold front will pass mainly north of the state Friday night, with no precipitation expected in the Willamette Valley.  High pressure is forecast to quickly rebuild across the region over the weekend.

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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