[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Thu Sep 14 11:57:51 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT THU SEP 14, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed from now until 2 p.m. with a 50-acre limit.

WEATHER DISCUSSION:

A dry upper-level trough will slowly exit the region this afternoon with a dry NW flow aloft clearing skies. Cool air aloft will provide excellent mixing.  Weak onshore gradients are nearly balanced and should become positively-stacked this afternoon.  However, transport winds may stay mostly northerly, potentially limiting burning opportunities.  The first PIBAL is scheduled for noon.

TODAY'S FORECAST:

Mostly sunny with mild temperatures.

Salem's high temperature today will be near 76°F (average is 78°F).

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

Surface winds: NNW-N 5-10 mph.

Transport winds: NNW 8-15 mph.

Mixing height: Rising to 6000 feet by 5 p.m.

Salem's sunset tonight: 7:24 p.m.

(Salem Airport data for Wednesday, September 13th: High 77°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 80)

EXTENDED DISCUSSION:

A weak upper-level ridge will bring sunny and warmer weather on Friday with high temperatures near 80°F.  The air aloft will remain cool enough for good daytime mixing with mostly northerly transport winds.

On Saturday, transport winds should start out weakly offshore.  However, the latest computer guidance shows a strengthening westerly flow aloft turning transport winds northwesterly Saturday afternoon.  That could create a burn opportunity and will need to be closely watched.

A major change in the weather pattern is expected on Sunday, as the first fall-like storm of the season gets pushed onshore.  Low-level winds will increase and veer to the S-SW Sunday morning.  A cold front is predicted to spread light rain across the Willamette Valley Sunday afternoon.  Right now, it appears that this system will arrive too soon in the day to allow for a burning opportunity, but its timing will need to be closely watched.

A stronger weather system is expected to bring more significant rain on Monday with temperatures cooling into the low-60s.  Rainfall totals may exceed one-half inch.  Cool and showery weather is expected to extend at least through the middle of next week.  Long-range computer models are indicating a return to a drier weather pattern the final week of the month.

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.

     5. In accordance with OAR 837-110-0090, all field burning shall be
         banned when any two of the following criteria are present:
         A. Temperature of 95 degrees Fahrenheit or above
         B. Relative humidity of 30 percent or below
         C. Wind speed of 15 miles per hour or higher

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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