[willamette-fcst] Silverton Hills Midday Forecast_Gary.doc

Wed Sep 21 09:47:57 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT WED SEPT 21, 2016

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is allowed.

WEATHER DISCUSSION:

Showers late yesterday afternoon appear to have gone no farther east than the Salem area, so fields are likely drier today except that fog this morning is starting them moist. The upper trough over western Oregon will gradually sink southward into northern California and Nevada tonight and Thursday. Enough cold air remains from the trough to make showers possible again today but the scenario would likely be similar to yesterday, starting out on higher terrain and drifting into part of the valley.

Otherwise, winds will be N most of today and most of the models tend to keep it N through the afternoon, but there is a chance that a NNW transport wind can be realized by mid-afternoon for a possible burn opportunity. Pressure gradients are slightly negative but balancing should occur once warming occurs by early afternoon. Due to the fog mixing heights will also start rising slowly but excellent heights are likely by late afternoon. The primary limiting factor today will be the wind but the direction in afternoon is uncertain.

TODAY's FORECAST

Fog and low clouds dissipating during the morning then sunny.

Salem's high temperature today will be near 70°F (average is 76°F).

Relative humidity:  Dropping to 60% about 1:00 p.m. and near 40% at 4:00 p.m.

Surface winds: N 5-10 mph.

Transport winds: N 8-10 mph increasing to 10-14 mph after 2:00 p.m. and possibly turning NNW.

Mixing height: Near 1000 feet at 11:00 a.m. rising to 2500 feet about 2:00 p.m. and 5000 feet by 5:00 p.m.

Salem's sunset tonight: 7:10 p.m.

(Salem Airport data for Tuesday, September 21st: High 72°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 32)

EXTENDED DISCUSSION:

Thursday still looks like a better burning opportunity since transport winds will begin the day northwesterly and then transition to west. Gradients will again questionable and there will be a few showers in the area, but overall the afternoon conditions appear to encourage burning. Friday looks more likely to be wet as a decaying cold front arrives from the Pacific with showers around noon. Winds do not look especially favorable for burning this weekend, certainly not on Sunday, but so far, early next week still looks like a better week to burn with very small chances of showers from day to day.

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

Gary Votaw

ODF Meteorologist

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