[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT THU JUL 19, 2012



BURN ADVISORY:



Recommended burn times for agricultural burning are from noon to 7:00 p.m.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



An upper-level trough remains parked off the southern Oregon coast with a moist southeasterly flow aloft over Oregon.  The Salem sounding this morning showed a warm layer of air aloft, between 4000 and 8000 feet, which is keeping the lower levels of the air mass stable over the Willamette Valley.  However, as the upper-level trough slowly begins to lift northeastward this afternoon, cooling aloft will begin to destabilize the air mass.  That may lead to some sunbreaks but will also increase the chance of showers.  There is a slight chance that the air mass will destabilize enough for thunderstorm development late this afternoon and evening, but the odds of that are much greater over the Cascades.



(Salem Airport on Wednesday:  High temperature was 73 degrees; no rainfall)



TODAY'S FORECAST:



Cloudy and muggy this morning. Some sunbreaks possible this afternoon with a chance of showers.  Slight chance of afternoon and evening thunderstorms. Rainfall totals likely less than one-tenth of an inch.



Salem's high temperature today will be near 78 degrees (normal is 83).

Relative humidity drops to 50% around 5 p.m.

Surface winds:  Light; becoming WNW 5-10 mph this afternoon and evening.

Transport winds: Light; becoming W 6-12 mph this afternoon but shearing to southeasterly at the top of the mixing layer.

Mixing height: Rising to near 3500 feet around 2 p.m. and 4500 feet at 5 p.m.

Salem's sunset tonight: 8:52 p.m.



THREE-HOURLY DATA:

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

     Temperature:                 66       73      77      72

     Relative Humidity:           73%      58%     50%     66%

     Surface Wind Direction:     Var      300     290     270

     Surface Wind Speed:           3        5       6       7

     Transport Wind Direction:   Var      300     270     260

     Transport Wind Speed:         3        6       8       8

     Mixing Height:             1800     3500    4500    1800

     Ventilation Index:            5       21      36      14



EXTENDED DISCUSSION:



The pesky upper-level trough will finally lift northeastward across Oregon on Friday, with some showers likely in the morning and a risk of a thunderstorm.  A drier southwesterly flow aloft should begin to clear skies Friday afternoon.  Temperatures will remain a few degrees below normal.

Another upper-level trough is forecast to drop southeastward, from the Gulf of Alaska, over the weekend, but it should remain northwest of the state.  That will return the region to a more typical summer-time pattern of morning clouds with afternoon clearing.  However, temperatures will likely remain slightly below normal. There is a slight chance of showers by Monday, as the trough swings onshore across Washington and extreme northern Oregon.



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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