[willamette-fcst] Silverton Hills Forecast - Fri, Jul 09 2010

[Willamette Valley Ag/Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT FRI JUL 09 2010

BURN ADVISORY:

…State Fire Marshal Conditions may be reached this afternoon due to high temperatures and
low relative humidities…

Recommended burn times for agricultural burning are from 11:00am to 6:00pm.
Prep burning is not allowed. 
Propane flaming burning is not allowed.									
										
WEATHER DISCUSSION:

A strong ridge of high pressure remains over the Pacific Northwest this morning.  A broad
surface thermal trough extends from the Willamette Valley to the Cascades with light winds
across the interior of western Oregon. It was a very warm night with valley temperatures
staying in the 60s.  As a result, mid-morning temperatures are running about 3-8 degrees
warmer than 24 hours ago.  Once again this morning, visible satellite imagery shows low
clouds and fog banked up against the entire length of the Oregon Coast with temperatures
there in the low to mid 50s.

A very weak upper-level disturbance, just off the northern California coast, is continuing
to circulate considerable mid and high-level moisture northward over Oregon. That has
combined with daytime heating the past couple of days to produce some fairly potent
thunderstorm development over southwestern and south-central Oregon.  That will be the
case again later today, but the convective activity should stay east and south of the
Willamette Valley.

The surface thermal trough is forecast to shift into central Oregon this evening, which
will initiate a weak marine push into the Willamette Valley.  However, sunny skies and
very warm air aloft will lift temperatures well into the 90s, before the cooling evening
sea breeze kicks in.

TODAY:

Sunny and continued hot.  Light north winds turning northwesterly and increasing this
evening.  High near 95 degrees.  Relative humidities expected to fall below 30% in the mid
to late afternoon.

Salem Sunset: 8:59 pm	
							
										
THREE-HOURLY DATA:		

                                 11am     2pm     5pm     8pm			
     Temperature:                 82       91      95      85  			
     Relative Humidity:           44%      32%     25%     35%     			
     Surface Wind Direction:     360      350     340     300 		
     Surface Wind Speed:           5        7       8      11    			
     Transport Wind Direction:   360      360     350     330 
     Transport Wind Speed:         4        9      10      12  
     Estimated Mixing Height:   1500     3200    4000    1500
     Ventilation Index:            6       29      40      18		
										
EXTENDED DISCUSSION:

A dry northwesterly flow aloft will prevail during the extended forecast period.  Weak
onshore flow will cool temperatures back into the 80s over the weekend.  A stronger
upper-level trough is forecast to swing across southern British Columbia Monday, which
will increase the onshore flow across western Oregon and cool temperatures back to normal
or slightly below.  A similar system is forecast to bring another surge of marine air into
the valley late Wednesday and Thursday, which will keep temperatures near normal.

EXTENDED FORECAST:	
     
Saturday:  Mostly Sunny but cooler.  Wind: NW 5-15 mph.  57/85
Sunday:    Mostly Sunny.  Wind: NW 5-15 mph.  55/87
Monday:    Morning clouds with afternoon clearing. Much cooler.  55/77
Tuesday:   Morning clouds.  Mostly Sunny in the afternoon.  53/78
Wednesday: Partly cloudy.  52/81
Thursday:  Morning clouds.  Afternoon clearing.  52/78
Friday:    Brief morning clouds.  Mostly Sunny in the afternoon.  53/84
										
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.