[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT MON AUG 17, 2015



BURN ADVISORY:



***State Fire Marshal Burn-Ban Conditions should be reached this afternoon   (due to low humidity and wind).***



Agricultural burning is not recommended.



Prep burning is not allowed.



WEATHER DISCUSSION:



A strengthening upper-level ridge in the Gulf of Alaska is producing a dry NW flow aloft over Oregon.  Warm air aloft will suppress afternoon mixing and allow temperatures to climb to around 90°F.  Air quality is generally good today across NW Oregon, but N-NE winds in the mixing layer may bring additional wildfire smoke into the region this afternoon and tonight.



At the surface, a thermal trough will build over SW Oregon today...leading to increasing north winds in the Willamette Valley this afternoon.  The combination of brisk north winds and dropping humidity levels will likely put the valley into State Fire Marshal Burn-Ban Conditions around mid-afternoon.



TODAY'S FORECAST:



Sunny and very warm.  Hazy at times due to wildfire smoke.



Salem's high temperature today will be near 91°F (average is 82°F).

Relative humidity:  Dropping below 30% around 3 p.m.

Surface winds: N 10-20 mph this afternoon.

Transport winds: N 15-20 mph this afternoon.

Mixing height: Rising to 3000 feet about 2 p.m. and to 3500 feet by 5 p.m.

Salem's sunset tonight: 8:15 p.m.



(Salem Airport data for Sunday, August 16th: High 86°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 70)



EXTENDED DISCUSSION:



The upper-level ridge is predicted to strengthen over the region on Tuesday. Further warming aloft will keep mixing heights suppressed with afternoon temperatures climbing into the upper 90s (possibly breaking daily records).  Dry north winds will slacken slightly.  State Fire Marshal Conditions are likely Tuesday afternoon due to hot temperatures and low humidity.



On Wednesday, the upper-level ridge will weaken with minor cooling aloft over Oregon.  The surface thermal trough is expected to shift east of the Cascades in the afternoon...initiating onshore flow into NW Oregon and capping temperatures in the mid-90s.  It is unlikely that this transition will occur soon enough in the day to create a burning opportunity, with the valley likely staying near State Fire Marshal Burn-Ban conditions in the afternoon.



On Thursday, a strengthening NW flow aloft will bring more significant onshore flow and cooling to western Oregon.  Temperatures are expected to drop back close to average.  A burning opportunity is possible but will depend on the timing of the marine push.



Marine clouds should penetrate into the Willamette Valley Friday morning but clear in the afternoon.  No precipitation is expected, but temperatures will struggle to hit 80°F.  Afternoon NW transport winds may provide another burning opportunity, if there is not too much gradient-stacking from the previous day's marine push.



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
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