[willamette-fcst] Silverton Hills Forecast - Fri, Jul 23 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT FRI JUL 23 2010

BURN ADVISORY:

     Recommended burn times for agricultural burning are from now to 7:00pm.
     Prep burning is not allowed. 
     Propane flaming is not allowed.		
										
WEATHER DISCUSSION:

Late-morning visible satellite imagery showed a couple of pockets of remaining marine low
clouds in the northern and central Willamette Valley, with the largest area of clouds
centered just north of the Silverton Hills.  The remainder of the state had sunny skies,
including along the entire coast.  The surface map shows only a trickle of onshore flow
remaining across northwestern Oregon and offshore flow developing, into a building thermal
trough, across southwestern Oregon.  There is just 0.1 mb of onshore gradient remaining
from Newport to Salem and 1.3 mb onshore gradient from Salem to Redmond.  Gradients have
turned offshore from Eugene to North Bend and are becoming strong northerly in the
Willamette Valley.  The south valley was getting northerly winds gusting to nearly 25 mph
late this morning, with the south coast seeing gusts to over 30 mph.

An upper-level ridge of high pressure is building over the region today with the air aloft
rapidly warming and drying.  Once the remaining low clouds evaporate, temperatures should
warm rapidly in the Willamette Valley.  Even though late-morning readings are running as
much as 10 degrees colder than 24 hours ago, afternoon highs should end up several degrees
warmer than yesterday.  The late start to the sunshine, however, may keep northern valley
highs in the mid 80s, instead of the previously forecast upper 80s.

FORECAST:

Becoming sunny and warmer with brisk north winds, especially south.  After reaching 81
degrees on Thursday, Salem's high temperature today will be near 86 degrees.  The mixing
height will climb to near 3000 feet in the early afternoon top out near 3800 feet late
this afternoon.  Northerly surface and transport winds increase to 10-20 mph this
afternoon, with higher gusts possible. Relative humidity will drop to near 27% late this
afternoon.  The combination of low relative humidity and increasing northerly winds may
put the Silverton Hills region into State Fire Marshal conditions by late this afternoon. 
The ventilation index will climb to around 60 this afternoon.

Salem's sunset tonight: 8:49 pm	
										
EXTENDED DISCUSSION:

The upper-level ridge of high pressure is forecast to continue building over the region
Saturday.  Transport winds will turn slightly offshore, with valley temperatures climbing
into the 90s.  The ridge is forecast to slide east of the region by Sunday with increasing
south-southwesterly flow aloft introducing a chance of showers or afternoon thunderstorms
to southern Oregon.  These storms should stay south and east of the Willamette Valley.

Increasing southwesterly flow aloft is forecast to eventually turn transport winds onshore
early next week.  That could present open burning opportunities but may also circulate
afternoon showers and thunderstorms northward along the Cascades.  Increasing onshore flow
will likely cool temperatures back to near normal during the second half of next week. 
That could also create open burning opportunities.

EXTENDED FORECAST:	
     
Saturday: Sunny and very warm.  Wind: NNE 5-15 mph.  56/94  
Sunday: Mostly sunny. Very warm. Chance of t-storms near the Cascades. 58/94  
Monday: Partly cloudy.  Slight chance of showers or t-storms.  58/86  
Tuesday: Partly cloudy.  Slight chance of showers or t-storms.  55/82  
Wednesday: Morning clouds, then mostly sunny.  53/80
Thursday: Morning clouds, then mostly sunny. 52/78
Friday: Morning clouds, then sunny.  50/83

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