[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT THU JUL 17, 2014



BURN ADVISORY:



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



WNW flow aloft is continuing to strengthen over Oregon.  At the surface, onshore flow is now spreading cooler across the entire state.  Midday temperatures in the northern Willamette valley are about 10 degrees cooler than 24 hours ago, but the air mass remains quite dry.  Relative humidity levels have already dropped to near 50% under sunny skies.  The weak overnight surge of marine air into the valley resulted in minor pressure-gradient-stacking this morning.  The strong July sun is already working to "balance" the onshore flow, which should happen early this afternoon.



The air aloft is forecast to continue to slowly cool this afternoon...resulting is significantly higher mixing heights compared to Wednesday.  The 11 a.m. PIBAL revealed light NW winds through the transport layer, which was about 2500 feet deep.  Mixing depths could increase to as much as 5000 feet by late this afternoon with some increase in the NW transport winds likely making for favorable burning conditions.



TODAY'S FORECAST:



Sunny but cooler.



Salem's high temperature today will be near 86°F (average is 83°F).

Relative humidity:  Dropping to near 30% by 5 p.m.

Surface winds: NW 5-10 mph this afternoon.

Transport winds: NW 8-15 mph this afternoon.

Mixing height: Rising to near 5000 feet by 5 p.m.

Salem's sunset tonight: 8:54 p.m.



(Salem Airport data for Wednesday, July 16th: High 96°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)



EXTENDED DISCUSSION:



The upper-level ridge will continue to flatten and give way to increasing westerly flow aloft on Friday.  Morning marine clouds will likely penetrate into the Willamette Valley, causing more significant pressure-gradient-stacking.  Afternoon sunshine should help to "balance" the onshore flow, with near average temperatures.  However, transport winds are forecast to turn mostly northerly, which is not conducive to burning.



The flow aloft is forecast to turn southwesterly over the weekend; yielding mostly cloudy skies and cooler temperatures.  A weak weather system may bring a few light showers to the region, but significant precipitation is not expected.  Southwesterly flow aloft is forecast to continue through at least early next week.  Moisture is limited, so the chance of showers will be minimal.  However, continued onshore flow will maintain moderate temperatures with possible burning opportunities.



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 15 mph.



     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 (ft) times

         the transport wind speed (mph) 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 (ODA) and the Oregon Department of Forestry (ODF).  For

information contact ODA at 503-986-4701.



To add/remove your email address from this list, please go to:



http://listsmart.osl.state.or.us/mailman/listinfo/willamette-fcst



Pete Parsons

ODF Meteorologist
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20140717/dba13245/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: Silverton Hills Midday Forecast_Pete.doc
Type: application/msword
Size: 34304 bytes
Desc: Silverton Hills Midday Forecast_Pete.doc
URL: <https://omls.oregon.gov/pipermail/willamette-fcst/attachments/20140717/dba13245/attachment.doc>