[willamette-fcst] Silverton Hills Midday Forecast_Pete.doc

Sun Sep 20 11:53:58 PDT 2015

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:50 AM PDT SUN SEP 20, 2015

*** Supplemental Weekend Forecast. ***

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

Increasing westerly flow aloft has forced extensive low clouds across western Washington and extreme NW Oregon.  However, skies are still mostly sunny from Marion County southward across the interior of western Oregon.  That has allowed temperatures to warm into the lower 70s at midday.

A cold front is expected to push inland across western Washington this afternoon with increasing clouds across NW Oregon.  Cooling aloft should combine with daytime heating to push mixing heights above 3000 feet by early this afternoon.  As of late this morning, surface gradients have become favorable to minimize down-mixing of smoke (11 a.m. readings:  1.5mb onshore from Newport to Salem with 0.0mb gradient from Salem to Redmond).

South to SW Surface and transport winds are predicted to become mostly westerly this afternoon and northwesterly this evening.  Winds will be closely monitored with PIBALS, beginning around noon.

TODAY'S FORECAST:

Mostly sunny with near-average temperatures.

Salem's high temperature today will be near 80°F (average is 76°F).

Relative humidity:  Near 55% at midday; dropping to near 40% by 5 p.m.

Surface winds: S 5-10 mph at midday; veering to W 5-10 mph later today.

Transport winds: SW 5-10 mph at midday; veering to W 7-12 mph later today.

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

Salem's sunset tonight: 7:14 p.m.

(Salem Airport data for Saturday, September 19th: High 81°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 55)

EXTENDED DISCUSSION:

A weak cold front should bring a significant marine push overnight along the areas of very light rain.  Rainfall totals are expected to range from a trace to just a few hundredths of an inch.  Look for just some afternoon clearing on Monday with temperatures struggling to reach the low-70s.  Expect unfavorable gradient-stacking and mostly northerly winds.

Very little change is in store for Tuesday with morning clouds and afternoon clearing.  Temperatures will remain slightly below average, with continued unfavorable gradients and mostly northerly winds.

The flow aloft should turn more southwesterly by Wednesday, ahead of a developing weather system.  That may create more favorable burning conditions Wednesday afternoon, with transport winds turning to the WSW.  There may be another burning opportunity Thursday afternoon, depending on the timing of an approaching cold front.  Wetting rains are expected Thursday night with showers lasting into Friday.

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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