[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Fri Sep 15 08:59:12 PDT 2017

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT FRI SEP 15, 2017

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

Weak offshore flow, in the wake of yesterday's dry upper-level trough, brought wildfire smoke back into sections of the valley.  Pressure gradients remain slightly offshore this morning, and valley nephelometer readings are somewhat elevated, especially along the Cascade foothills and near Portland.

A weak upper-level ridge will provide sunshine and seasonably warm temperatures today.  The air aloft has warmed since Thursday but is still cool enough for good afternoon mixing.  Surface pressure gradients should turn weakly onshore this afternoon, which will help to clear the low-level wildfire smoke from the valley and may create a limited burning opportunity.

TODAY'S FORECAST:

Sunny and warm.  Areas of wildfire smoke, mainly this morning.

Salem's high temperature today will be near 80°F (average is 77°F).

Relative humidity:  Dropping to 50% by noon and to near 30% by 5 p.m.

Surface winds: Light this morning; N 5-10 mph this afternoon.

Transport winds: NE 5-10 mph this morning; NNW 8-13 mph this afternoon.

Mixing height: Rising to 3000 feet by 1 p.m. and to 5000 feet by 5 p.m.

Salem's sunset tonight: 7:22 p.m.

(Salem Airport data for Thursday, September 14th: High 78°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 65)

EXTENDED DISCUSSION:

Friday night through Saturday, offshore flow will likely transport wildfire smoke, from the Cascades, back into the valley.  Otherwise, skies will be partly cloudy with continued seasonably warm temperatures.  A strengthening westerly flow aloft should force a weak surface thermal trough east of the valley Saturday night, with a switch to onshore flow improving air quality.

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.  The timing of this system has been slowed down, so there may be a midday burning opportunity, before rain spreads across the valley in the afternoon.

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 wet weather is expected to extend through most of next week.  Long-range computer models are indicating a return to a drier weather pattern for 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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