[willamette-fcst] Silverton Hills Forecast - Fri, Jul 16 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT FRI JUL 16 2010

BURN ADVISORY:

Recommended burn times for agricultural burning are from 12:00pm to 6:00pm.
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:

A weak upper-level trough is moving across southern British Columbia and initiated a weak
marine push into western Washington and northwestern Oregon Thursday evening. 
Temperatures across the Willamette Valley are running 5-10 degrees cooler than 24 hours
ago, with the morning sounding showing several degrees of cooling from the surface up to
about 8000 feet.  Visible satellite imagery confirmed that marine low clouds penetrated
across most of western Washington overnight and also made it up the Columbia River into
the northern Willamette Valley.  A few low clouds were making as far south as the
Silverton Hills at mid-morning.

There may yet be some increase in the marine clouds, before daytime heating acts to break
up the marine deck later this morning.  The air aloft is still warm, so mixing heights
will be slow to climb today.  The surface analysis shows the thermal trough extending from
eastern Washington across central Oregon and into southwestern Oregon.  Cool marine air
continues to poor into the Willamette Valley, with considerable gradient-stacking
developing across western Oregon.

As the upper-level trough pushes eastward, westerly winds aloft will maintain the
low-level onshore flow across northwestern Oregon.  Daytime heating will promote an
afternoon sea breeze, which should turn transport winds from northerly to northwesterly. 
It is uncertain if the significant morning gradient-stacking can be overcome enough for
open burning later this afternoon, but conditions look favorable for early afternoon prep
burning.

TODAY’S FORECAST:

Sunny but cooler.  After reaching 89 degrees on Thursday, Salem's high temperature today
will be near 83 degrees.  The mixing height will climb to 3000 feet in the early afternoon
with a maximum near 4000 feet around 5 p.m.  Surface winds will be north at 5-10 mph this
morning, becoming NW 5-15 mph this afternoon and evening.  Transport winds will be north
at 10 mph this morning, becoming NW 10-15 mph in the late afternoon and evening. Relative
humidity will drop to 50% by 11am and to near 30% this afternoon.

Salem's sunset tonight: 8:55 pm	
										
THREE-HOURLY DATA:		

                                 11am     2pm     5pm     8pm			
     Temperature:                 69       78      82      73  			
     Relative Humidity:           47%      36%     30%     43%     			
     Surface Wind Direction:     360      350     340     320 		
     Surface Wind Speed:           9        9      10      12    			
     Transport Wind Direction:   360      360     340     310 
     Transport Wind Speed:        10       10      12      15  
     Estimated Mixing Height:   2000     3200    4000    1500
     Ventilation Index:           20       32      48      22                          
				
										
EXTENDED DISCUSSION:

A weak upper-level trough will maintain a typical summer-time pattern over the weekend
with morning clouds giving way to afternoon sunshine and near-normal temperatures.  The
trough is forecast to strengthen early next week with the onshore flow possibly becoming
strong enough to produce areas of morning drizzle or light showers.  Otherwise dry
conditions should prevail, but temperatures will cool to below normal with considerable
marine low clouds.  The trough is forecast to back away from the coastline by late next
week with southwesterly flow aloft and temperatures returning to normal.  The marine layer
may be too deep for open burning, during much of the extended period, but there should be
opportunities for prep burning.

EXTENDED FORECAST:	
     
Saturday: Morning clouds, then mostly sunny.  50/80  
Sunday: Morning clouds, then mostly sunny.  50/77  
Monday: Morning clouds, then partly sunny.  49/79  
Tuesday: Morning clouds, then partly sunny.  51/77  
Wednesday: Morning clouds, then partly sunny.  51/76
Thursday: Brief morning clouds, then mostly sunny.  52/82
Friday: Mostly sunny.  54/85 
										
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.