[willamette-fcst] Silverton Hills AM Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT THURS. AUG. 8, 2013



BURN ADVISORY:



Agricultural burning is not recommended today.

Prep burning is not allowed.



WEATHER DISCUSSION:



An upper-level closed low continues to spin just to our southwest off the coast today. Deep southeasterly flow aloft is in place over western OR as the closed low slowly moves northeast. Surface winds are light and mostly southerly, and the surface pressure gradient pattern in place is unfavorable. Low clouds currently cover most of the southern portion of the valley after the marine push from the west late yesterday.



Low clouds will mostly dissipate later this morning. However, partly to mostly cloudy skies will develop later and are generally expected through the afternoon, as moisture aloft is widespread out ahead of the upper closed low to the southwest. Some shower and thunderstorm activity is a possibility in the south valley and likely for SW/south central OR late in the day. Surface temperatures will be a bit cooler than yesterday, mixing heights will remain moderate, and transport winds will be mostly northerly later today at speeds of 5 to 15 mph. Surface winds will also likely become N late this afternoon.



(Salem Airport data for Wed., August 7: High 83°F; Rainfall 0)

(Maximum Ventilation Index expected today:  30)

TODAY'S FORECAST:



Expect partly to mostly cloudy skies through day. Cooler than average temperatures are expected, with low-level winds becoming mostly N but remaining weak.  Some light showers are possible late today on the south side. Winds could get erratic this afternoon in the vicinity of these showers.



Salem's high temperature today will be around 80.

Relative humidity will drop to the 40-50% range this afternoon.

Surface winds: N 5-10 mph.

Transport winds: N 5-15 mph.

Maximum mixing height:  2500 ft.

Sunset tonight: 8:28 p.m.



EXTENDED DISCUSSION:



The upper trough gradually approaches western OR on Friday, with partly to mostly cloudy skies expected to continue in the valley with a chance of light showers. Again, any significant rain will likely be confined to the southern portions through much of the day. Weak transport flows are also likely to continue, but there is a possibility of some onshore gradients late in the day. Evening showers may spread through the valley.



On Saturday, even more cloudiness and showery weather is expected as the upper trough likely moves into western OR. Shower activity will likely increase over the valley late in the day and mixing heights will be low. A drying trend is currently advertised on model guidance beginning Sunday, but cloudy conditions are expected in the morning.



The National Weather Service's digital forecast is available at:



http://forecast.weather.gov/MapClick.php?lat=44.90549606158295&lon=-122.8106689453125&site=pqr&unit=0&lg=en&FcstType=text





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.



Doug Wesley

ODF Meteorologist


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20130808/0361017c/attachment.html>