[willamette-fcst] Silverton Hills Forecast - Wed, Jul 21 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
12:00 PM PDT WED JUL 21 2010

BURN ADVISORY:

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

A weak upper-level trough is shifting east of the Cascades at midday with a dry
northwesterly flow aloft over western Oregon.  That is maintaining the onshore flow into
the Willamette Valley.  Late-morning visible satellite imagery showed some morning marine
low clouds persisting in mainly the extreme northern Willamette Valley and along much of
the northern and central coast.  The remainder of the state had mostly sunny skies. 
Midday valley temperatures were mostly in the low to mid 60s.

In the wake of the weak trough, a dry northwesterly flow aloft will help to clear the
remaining marine clouds from the north valley, with sunny skies helping afternoon
temperatures recover into the low 80s.

The late-morning surface map showed the morning gradient-stacking already beginning to
balance out, with 1.8 mb onshore from Newport to Salem and 3.0 mb onshore from Salem to
Redmond.  Continued heating this afternoon will likely be sufficient to reverse the
stacking of the onshore flow, creating favorable lift for smoke dispersion, but transport
winds will likely remain too northerly for open burning.

THIS AFTERNOON’S FORECAST:

Becoming sunny.  A little warmer. Salem's high temperature today will be near 83 degrees. 
The mixing height will climb to near 3000 feet in the early afternoon with a maximum near
3800 feet around 5 p.m.  Surface winds becoming NW 5-12 mph this afternoon and evening. 
Transport winds NNW at 10 mph this afternoon, backing to NW at 10 mph this evening.
Relative humidity will drop to 50% by 1 p.m. and to near 35% by late this afternoon.

Salem's sunset tonight: 8:51 pm	
										
EXTENDED DISCUSSION:

Another weak upper-level trough will maintain the moderately strong onshore flow pattern
Thursday.  Once again, transport winds are forecast to remain too northerly for open
burning opportunities.  A stronger ridge of high pressure is forecast to build over the
region Friday and Saturday.  Transport winds will turn northerly Friday, and perhaps
slightly offshore Saturday, with temperatures climbing to well above normal.

The upper-level 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 much of the state…mainly 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 an open burning opportunity Monday afternoon, if the flow aloft
is not too southerly.  An increasing southerly component to the upper-level flow is
forecast by Tuesday, which will introduce at least a chance of a shower or thundershower
into the valley.  The flow is forecast to turn more westerly by the middle of next week,
with temperatures cooling back to near normal.  That transition could also create open
burning opportunities.

EXTENDED FORECAST:	
     
Thursday: Morning clouds with afternoon clearing.  Wind: NNW 5-15 mph.  53/80
Friday: Sunny and turning much warmer.  Wind: N 5-15 mph.  50/88
Saturday: Sunny and very warm.  56/95  
Sunday: Mostly sunny. Very warm. Chance of t-storms near the Cascades. 58/92  
Monday: Partly cloudy.  Slight chance of showers or t-storms.  58/86  
Tuesday: Partly cloudy.  Chance of showers or t-storms.  55/85  
Wednesday: Partly cloudy.  53/82  
										
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