[willamette-fcst] Silverton Hills AM Forecast.doc

Thu Aug 23 08:48:37 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:45 AM PDT THU AUG 23, 2012

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from noon until 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from 11:00 a.m. until 5:00 p.m.

WEATHER DISCUSSION:

A strong upper-level trough will move inland today, across southern British Columbia and northern Washington, strengthening the dry and stable westerly flow aloft over Oregon.  The sounding over Salem this morning confirmed that the westerly winds aloft were already increasing.  Winds were still generally northerly below 5000 feet.

Marine low clouds are a little more expansive this morning across western Oregon with partly to mostly cloudy skies covering the Willamette Valley.  Some additional expansion of the marine deck is possible this morning.  Valley temperatures should only climb into the mid 70s today with skies becoming partly cloudy in the afternoon.

The air aloft will be cooling today, in response to the approaching dry upper-level trough.  That will make for relatively high mixing heights this afternoon.  Unlike yesterday, transport winds are forecast to turn northwesterly this afternoon, which is more favorable for open burning.  However, gradient-stacking and high low-level wind speeds may inhibit burning chances today and will need to be closely monitored.

(Salem Airport on Wednesday: High 78°F; Rainfall .00")

TODAY'S FORECAST:

Sunny skies with breezy north winds.

Salem's high temperature today will be near 74 degrees (normal is 82).

Relative humidity drops to 60% by noon and to near 40% by 5 p.m.

Surface winds:  N 5-10 mph this morning; becoming NW 7-13 mph this afternoon.

Transport winds: N 10-15 mph this morning; NW 12-18 mph this afternoon.

Mixing height: Rising to 3000 feet by 11 a.m. and to 5000 feet by 2 p.m.

Salem's sunset tonight: 8:04 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 64       70      73      65

     Relative Humidity:           63%      48%     40%     56%

     Surface Wind Dir/Speed:     N  6     N  8   NW  9   NW 12

     Transport Wind Dir/Speed:  NE 12     N 15   NW 15   NW 20

     Mixing Height:              3500     5000    5500    2500

     Ventilation Index:           42       75      83      50

EXTENDED DISCUSSION:

On Friday, the upper-level trough will push east of the region.  Drying northerly winds will make for unfavorable open burning conditions.  Clearing skies and warming aloft will help valley temperatures recover into the upper 70s.  A stronger trough is forecast to dig offshore Saturday with southwesterly flow aloft over Oregon.  Skies should be mostly sunny with valley temperatures climbing back into the 80s.  Long-range computer models continue to be inconsistent on their timing of bringing that trough onshore.  It may spread showers across the region as soon as Sunday afternoon, or it may weaken offshore and not come inland until early next week.

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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