[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Wed Sep 25 08:57:43 PDT 2013

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT WED SEP 25, 2013

BURN ADVISORY:

Recommended times for agricultural burning are from now until 5:00 p.m.

WEATHER DISCUSSION:

A strong upper-level trough will maintain cool and showery conditions across western Oregon today.  The center of circulation is dropping southward just off the central Oregon coast this morning and will take the strongest shower activity into SW Oregon this afternoon.  Additional rainfall totals in the Silverton Hills should be only near one-tenth of an inch, with northerly flow aloft likely bringing partial clearing to the Willamette Valley later today.

Satellite imagery shows clouds blanketing most of the state this morning with radar and surface reports indicating scattered showers across the Willamette Valley.  Surface winds are light southwesterly this morning but should increase and turn northwesterly this afternoon.

TODAY'S FORECAST:

Showers and continued unseasonably cool.  Partial afternoon clearing.

Salem's high temperature today will be near 61 degrees (average is 74).

Relative humidity:  Remaining above 60% throughout the day.

Surface winds: W 5-10 mph this morning; NW 7-15 mph this afternoon.

Transport winds: W 10 mph this morning; NW 10-15 mph this afternoon.

Mixing height: Above 5000 feet throughout the day.

Salem's sunset tonight: 7:03 p.m.

(Salem Airport data for Tuesday, Sept. 24th: High 61°F; Rainfall: .40")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

A dry and cool northerly flow aloft will clear skies on Thursday.  After a chilly and perhaps foggy morning, daytime temperatures should only recover into the mid 60s, even with some afternoon sunshine.  A weak upper-level ridge is forecast to move over the region on Friday.  The southern end of a cold front is forecast to drag across Washington and northern Oregon Friday afternoon.  It will likely keep skies mostly cloudy and may be strong enough to produce light rain as far south as the Silverton Hills.  Temperatures will remain below average.

A strengthening trough, in the Gulf of Alaska, is forecast to turn the flow aloft southwesterly over the weekend.  Expect mostly cloudy skies with increasing southerly winds.  Rain may extend south into the region on Saturday and is likely on Sunday.  Temperatures will remain slightly below normal.

A flat upper-level ridge is forecast to keep Oregon on the southern edge of the storm track through the middle of next week, with possibly warmer and drier weather late next week.

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.

Pete Parsons

ODF Meteorologist
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