[willamette-fcst] Silverton Hills Forecast - Thu, Aug 12 2010

[Willamette Valley Field Burning Forecast]

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

BURN ADVISORY:

     Recommended burn times for agricultural burning are from 1:00pm
     to 6:30pm.
     Prep burning is allowed from 1:00pm to 5:00pm with a 50 acre limit. 
     Propane flaming is allowed from 1:00pm to 5:00pm.		
										
WEATHER DISCUSSION:

An upper-level ridge is building just offshore with a dry north-northwesterly flow aloft
over western Oregon.  The surface chart shows a trough extending from eastern Washington
through north-central Oregon with weak northwesterly gradients across the Willamette
Valley.  Gradient-stacking is not an issue today with 2.0 mb onshore from Newport to Salem
and just 0.7 mb onshore from Salem to Redmond.  It is the overall lack of onshore flow
that will likely inhibit open burning today.

Midday satellite imagery shows low clouds along most of the Washington and Oregon
coastlines with only minor penetration of marine clouds up the Columbia River to about St.
Helens.  Skies were sunny over the entire Willamette Valley with temperatures running
about 2-5 degrees warmer than 24 hours ago.  Transport winds are forecast to be fairly
light and likely too northerly to provide adequate ventilation for open burning today. 
Transport winds will be monitored closely, with a pibal scheduled for 2:30 p.m.

TODAY’S FORECAST:

Sunny and warmer.  Salem's high temperature today will be near 87 degrees.  The mixing
height will climb to 3000 feet about 2 p.m. and likely top out near 3700 feet around 5
p.m.  An evening sea breeze should drop the mixing height to near 1500 feet by shortly
before sunset.  Surface and transport winds be NNW at 5-10 mph this afternoon, then turn
NW 5-10 mph this evening. Relative humidity will drop to 30% around 5 p.m.  The
ventilation index will climb to only 22 this afternoon.

Silverton area sunset tonight: 8:17 pm	
										
EXTENDED DISCUSSION:

A strong upper-level ridge of high pressure will build over the Pacific Northwest Friday
through Monday with mostly clear skies and dry offshore flow boosting high temperatures to
well above normal.  Weak disturbances are forecast to undercut the ridge and move into
northern California.  They might circulate enough moisture northward for a chance of
afternoon thunderstorms over the southern Cascades.  Otherwise, dry conditions are
expected.

The ridge is forecast to shift east of the region by next Wednesday with the surface flow
turning back onshore.  That will cool temperatures back closer to normal and could create
open burning opportunities.  An upper-level trough may be strong enough to produce showers
or thundershowers in the Willamette Valley next Wednesday or Thursday with strong onshore
flow cooling temperatures to below normal late next week.  

EXTENDED FORECAST:	
     
Friday: Sunny and very warm. NE wind 5-15 mph. 55/93
Saturday: Sunny with near-record warmth. NE wind 5-10 mph. 58/97
Sunday: Sunny with near-record warmth. Light wind. 59/98
Monday: Sunny with near-record warmth. Light wind. 61/99
Tuesday: Sunny. A little cooler. Wind becoming W 5-15 mph. 61/94
Wednesday: Partly cloudy and cooler. Wind W 5-15 mph. 56/84
Thursday: Mostly cloudy and cooler. Chance of showers. 54/75  
  
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