[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Thu Sep 4 11:41:48 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:40 AM PDT THU SEP 4, 2014

BURN ADVISORY:

***  State Fire Marshal Conditions Likely by Mid-Afternoon Due to Low Humidity and Wind ***

Recommended times for agricultural burning are from now until 5:30 p.m.

Prep burning is not allowed.

WEATHER DISCUSSION:

A dry northerly flow over aloft over Oregon has cleared skies statewide.  At the surface, a thermal trough is building northward along the coast.  NNE winds will increase across the Willamette Valley this afternoon; drying out the low-levels of the air mass.  Temperatures will top out about 10 degrees warmer than on Wednesday.

TODAY'S FORECAST:

Sunny, breezy, and warmer.

Salem's high temperature today will be near 86°F (average is 80°F).

Relative humidity:  Dropping to 30% by 2 p.m. and to near 25% by 5 p.m.

Surface winds: NNE 10-20 mph this afternoon.

Transport winds: NE 15-25 mph this afternoon.

Mixing height: Near 3000 feet; rising to near 4500 feet by 5 p.m.

Salem's sunset tonight: 7:43 p.m.

(Salem Airport data for Wednesday, September 3rd: High 75°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 110)

EXTENDED DISCUSSION:

Northeasterly flow aloft will maintain sunny skies on Friday.  The surface thermal trough will remain along the coast with offshore flow and even warmer temperatures.  This warm spell should peak on Saturday, with Willamette Valley temperatures likely climbing into the mid-90s.

Early next week, the ridge is expected to weaken with increasing NW flow aloft forcing the surface thermal trough east of the Cascades.  That will initiate a cooling onshore flow into the interior of western Oregon.  No precipitation is expected, but temperatures will cool to near average with possible burning opportunities Monday and Tuesday.

An upper-level ridge is predicted to strengthen over Oregon during the second half of next week, for a return to offshore flow and warmer temperatures.  Longer-range models are maintaining a generally dry weather pattern, with above average temperatures, through the middle of September.

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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