[willamette-fcst] Silverton Hills Forecast - Thu, Jul 15 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT THU JUL 15 2010

BURN ADVISORY:

Recommended burn times for agricultural burning are from 12:00pm to 7:00pm.
Prep burning is allowed from 12:00pm to 2:00pm with a 50 acre limit. 
Propane flaming is allowed from 12:00pm to 5:00pm.	
										
WEATHER DISCUSSION:

Late-morning visible satellite imagery showed low clouds continuing to blanket the
immediate coastline, from Washington southward to about Newport.  The low clouds had
backed off from the southern Oregon Coast but continued to cover the offshore coastal
waters.  That is an indication of increasing onshore flow, in response to a weak
upper-level disturbance cutting across southern British Columbia.  The surface thermal
trough is continuing to drift eastward with onshore pressure gradients increasing from
Newport to Salem (now 2.5 mb) and turning onshore from Salem to Redmond (0.5 mb).  Those
onshore pressure gradients should further increase this afternoon with a favorable
gradient-stacking pattern for eastward smoke evacuation.

Temperatures are running about 3 degrees warmer, than 24 hours ago, in the north valley
and up to 7 degrees warmer across the central and south valley. However, the air aloft is
also warmer than yesterday, so mixing heights will likely not exceed about 4000 feet
today.  The thermal trough is forecast to shift east of the Cascades later this afternoon,
which will initiate a northwesterly marine push this evening.  That will likely hold
maximum temperatures in the 80s, which should cap high temperatures in the northern
Willamette Valley in the mid to upper 80s across the north valley, but the south valley
may hit the 90 degree mark.

FORECAST:

Sunny and warm this afternoon, with a high temperature in the upper 80s.  Surface winds N
5-10 mph, becoming NW 5-15 mph late this afternoon and evening. Transport winds N 8 mph,
becoming NW 10 mph late this afternoon and evening.  The maximum mixing height should
climb to near 4000 feet later this afternoon, with the ventilation index rising to near
40.  The minimum relative humidity will be near 29%.

Salem’s sunset tonight: 8:56 pm	
								
EXTENDED DISCUSSION:

An upper-level trough is forecast strengthen over the Pacific Northwest during the
extended forecast period.  Increasing onshore flow will cool temperatures to near normal
Friday and to below normal over the weekend and much of next week.  There is even a threat
of showers by the middle of next week, mainly north and along the coast.

EXTENDED FORECAST:	
     
Friday: Sunny but cooler.  High near 83. Wind: NW 5-15 mph. 
Friday Night: Increasing clouds.  Low near 51. Evening NW wind 5-15 mph
              decreasing overnight.
Saturday: Mostly sunny.  High near 80. Wind: NW 5-15 mph in the afternoon. 
Saturday Night: Mostly cloudy.  Low near 51.  
Sunday: Morning clouds, then mostly sunny.  High near 79.  
Monday: Morning clouds, then partly sunny.  High near 77.  
Tuesday: Mostly cloudy.  Slight chance of showers.  High near 75.  
Wednesday: Mostly cloudy.  Chance of showers.  High near 72.  
										
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.