[willamette-fcst] Silverton Hills Midday Forecast_Gary.doc

Tue Sep 13 11:49:33 PDT 2016

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:45 AM PDT TUE SEPT 13, 2016

BURN ADVISORY:

***STATE FIRE MARSHAL CONDITIONS ARE LIKELY THIS AFTERNOON***

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

An upper level low centered over California will move into Nevada today which will result in weakening NE winds aloft over Oregon. However, they will still be strong enough down the slopes of the Cascades to cause very dry and quite breezy conditions. The resulting thermal trough will remain over the valley today with little chance for burning. Fire marshal conditions are possible this afternoon, certainly in the relative humidity but possibly also in the wind.

Sunny and warm.

Salem's high temperature today will be near 84°F (average is 78°F).

Relative humidity:  Dropping to 20% about 1:00 p.m. to a low of 15% by 5:00 p.m.

Surface winds: NNE 13-16 mph with gusts to 22 mph.

Transport winds: NE 17-22 mph.

Mixing height: Rising above 5000 feet by 2:00 p.m.

Salem's sunset tonight: 7:25 p.m.

(Salem Airport data for Monday, September 12th: High 81°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 140)

EXTENDED DISCUSSION:

The upper low will continue moving NE into the northern Rockies, allowing a marine push for the valley late on Wednesday. Mixing heights will still be excellent with favorable pressure gradients. Transport winds will turn from SW to W or NW during the afternoon and current models show wind speeds of only 4-8 mph. No rain is expected through Friday but winds will likely turn N on Thursday and Friday, limiting burning those days.

Weather models indicate a cold front approaching from the northwest on Saturday. Transport winds will increasingly be westerly during the afternoon, but the two models showing this feature differ on the speed on which it arrives. Clouds will be increasing with a slight chance of rain late. The most limiting factor for burning might be the mixing height if clouds arrive soon enough prevent sufficient warming or that rain arrives too soon, but current timing is encouraging for a burn opportunity.

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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