[willamette-fcst] Silverton Hills Midday Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT FRI JUL 15, 2011



BURN ADVISORY:



Recommended burn times for agricultural burning are now until 7:00 p.m.

Prep burning is allowed from now until 2:00 p.m. with a 50 acre limit.

Propane flaming is allowed from now until 5:00 p.m.



WEATHER DISCUSSION:



A dry and warmer west-southwesterly flow aloft has set up ahead of another cold front that is rapidly approaching the coastline.  Late-morning satellite imagery showed cloudy skies covering the coast but mostly sunny skies east of the coast range.  Some light rain fell at times this morning at Astoria, on the extreme northern coast, but dry weather prevailed across the rest of the state.



Late this morning, Willamette Valley temperatures were approaching 70 degrees (about 3-5 degrees warmer than 24 hours ago).  High temperatures should approach the 80 degree mark this afternoon, which is still a couple of degrees below normal.  Look for increasing clouds this afternoon with transport winds veering to more westerly.  Mixing heights should climb above 5000 feet for good smoke dispersal conditions.



TODAY'S FORECAST:



Increasing clouds but warmer.



Salem's forecast high temperature:  80 degrees

Surface winds:  S 3-7 mph; becoming WSW 5-10 mph by late this afternoon

Transport winds: SSW 5-9 mph; becoming SW 7-12 mph by late this afternoon

Maximum mixing height:  5500 feet

Relative humidity:  Drops to near 38% by 5 p.m.

Salem sunset tonight: 8:56pm



THREE-HOURLY DATA:



                                 2pm     5pm     8pm

     Temperature:                 76      79      72

     Relative Humidity:           44%     38%     53%

     Surface Wind Direction:     210     250     270

     Surface Wind Speed:           6       7       8

     Transport Wind Direction:   220     250     270

     Transport Wind Speed:         8      10      12

     Estimated Mixing Height:   4500    5500    2500

     Ventilation Index:           36      55      30



EXTENDED DISCUSSION:



A cold front will likely spread rain across the northern Willamette Valley in the pre-dawn hours Saturday.  Steady rain should turn to showers by Saturday afternoon.  This system has a fair amount of moisture with it, for mid July, so rainfall totals could locally exceed one-quarter of an inch.



Cool weather, with at least a chance for showers, will continue into the middle of next week.  An upper-level trough is forecast set up camp, just offshore, Sunday and Monday, before swinging inland on Tuesday.  That will likely be followed by another upper-level trough on Wednesday.  Some drying and warming of the air mass is forecast for Thursday, as the upper-level trough exits to the east.



The next upper-level trough is forecast to swing inland a little further to the north, into southern British Columbia, on Friday.  That could set up a favorable burning pattern, if any fields are ready.  There are indications that a drier and warmer weather pattern may be setting up beginning about Monday, July 25th, so summer may be on the way.  Have a great weekend.



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



-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20110715/d8e15850/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Silverton Hills Midday Forecast.doc
Type: application/msword
Size: 30720 bytes
Desc: Silverton Hills Midday Forecast.doc
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20110715/d8e15850/attachment.doc>