[willamette-fcst] Silverton Hills Midday Forecast_doug.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

12:00 PM PDT FRI AUG 16, 2013



BURN ADVISORY:



Recommended burn times for agricultural burning are 2:00 p.m. to 6:30 p.m.

Prep burning is not allowed.



WEATHER DISCUSSION:



Mostly cloudy skies and very humid conditions continue to dominate over the Silverton Hills and Willamette Valley today, ahead of the stationary upper trough off the Oregon coast. Light showers have mostly moved off to the north and east late this morning.



Surface heating will again be limited today, due primarily to the cloud cover. The 3000+ feet mixing level likely will be reached by 1-2 pm.



Like yesterday, a favorable wind profile is in place with mostly southwesterly transport flow through the afternoon, likely shifting to westerly late in the day. Surface winds, now weak, will also be westerly later today. Currently surface pressure gradients are weakly positive with 2.7 mb Newport-Salem and 0.5 mb Salem-Redmond. Gradient stacking and potential down-mixing could again be an issue later as the latter gradient continues to increase this afternoon (areas east of the Cascades are heating up faster than the local valley/hills). The gradient pattern will continue to be monitored closely.



High surface humidities are persistent and this is another limiting factor again today. The readings appear to be likely to stay well above 50% at least through mid-afternoon, as we won't have a significant drying mechanism until at least tomorrow afternoon.





TODAY'S FORECAST:



Mostly cloudy today.  A few light showers are possible through the afternoon, but no significant rain expected.



Salem's high temperature today will be around 80.

Relative humidity:  Drops to near 55% by 4 p.m.

Surface winds:  Light and variable, becoming W 5-10 mph this afternoon.

Transport winds: SW to W 10 mph this afternoon.

Mixing height: Rising to 3000-3500 feet later this afternoon.

Salem's sunset tonight: 8:16 p.m.



(Salem Airport data for Thursday, Aug. 15th: High 80°F; Rainfall T)

(Maximum Ventilation Index expected today: 30)



EXTENDED DISCUSSION:



The upper trough to the west will weaken and elongate this weekend, and generally westerly upper flow will be in place by Sunday with a drying trend at low levels. Significant cloud cover over the hills is doubtful for late Saturday, and not expected Sunday afternoon, although some onshore flow is expected both evenings. Morning cloud cover is also expected both days.



Broad upper ridging will likely set in early next week with mostly dry and warm conditions at low levels. Northerly low-level flow appears likely to take over with potentially a westerly transport component late in the days on Monday/Tuesday.



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 (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 and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.



Doug Wesley

ODF Meteorologist

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