[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT WED JUL 11, 2012



BURN ADVISORY:



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



A typical summer-time weather pattern has set up over the western United States.  A large upper-level ridge is centered over southern Nevada with a dry and warm southwesterly flow aloft over Oregon.  At the surface, a weak thermal trough extends from eastern Washington southward across central Oregon, with weak northerly pressure gradients across western Oregon.



Midday satellite imagery shows low clouds and fog banked up along most of the immediate Oregon coastline.  Some low clouds also extend a few miles up the Columbia River.  Otherwise, skies are generally clear over the remainder of the state, with the exception of haze from area wildfires.



Temperatures in the Willamette Valley are running about 10 degrees warmer than 24 hours ago, with late-morning readings already in the mid 70s.  Warmer air aloft will allow valley temperatures to climb into the upper 80s by late this afternoon (Salem's high on Tuesday was 83 degrees).  Some spots could reach 90.  Warm air aloft should keep mixing heights from climbing much above about 3500 feet this afternoon.  Northerly surface and transport winds will be unfavorable for field-burning.



TODAY'S FORECAST:



Sunny, hazy and warm.



Salem's high temperature today will be near 88 degrees (normal is 81).

Relative humidity drops to 50% by 1 p.m. and to near 35% by 5 p.m.

Surface winds:  N 7-12 mph; becoming NNW 7-12 mph this evening.

Transport winds: N 8-13 mph; becoming NNW 8-13 mph this evening.

Mixing height: Rising to a maximum near 3500 feet late this afternoon.

Salem's sunset tonight: 8:58 p.m.



THREE-HOURLY DATA:

                               2 p.m.  5 p.m.  8 p.m.

     Temperature:                82      87      80

     Relative Humidity:          43%     36%     46%

     Surface Wind Direction:    360     360     320

     Surface Wind Speed:          8       8       7

     Transport Wind Direction:  010     360     340

     Transport Wind Speed:       10      12      12

     Mixing Height:            3000    3500    1500

     Ventilation Index:          30      42      18



EXTENDED DISCUSSION:



A weak upper-level trough is forecast to increase the southwesterly flow aloft on Thursday.  That should cool the air aloft; leading to better daytime mixing.  Onshore flow will also increase; with some morning marine clouds likely making it into the Willamette Valley.  Look for several degrees of cooling, with highs dropping back to the low to mid 80s. A weak upper-level trough is forecast to remain over Oregon Friday through Sunday.  Expect areas of morning clouds with afternoon clearing and near-normal temperatures.  There is slight chance of sprinkles or morning drizzle, over the weekend, but no significant precipitation is expected.



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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