[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

Fri Aug 8 08:55:09 PDT 2014

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:50 AM PDT FRI AUG 8, 2014

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

WEATHER DISCUSSION:

A dry upper-level trough spread some middle and high clouds across NW Oregon this morning and will push east of the region this afternoon.  Weak onshore flow will keep temperatures near average with mostly sunny skies today.

The air aloft is cool, so daytime heating should yield high afternoon mixing heights.  Onshore pressure gradients are stronger from Salem to Redmond this morning than they are from Newport to Salem.  Daytime heating should be sufficient to balance these gradients this afternoon.  The limiting factor for burning, once again, today will be wind direction.  Low-level winds are northeasterly this morning and should turn northerly this afternoon.  It is possible that transport winds could turn north-northwesterly, but that is not predicted by this morning's computer models.

TODAY'S FORECAST:

Mostly sunny and seasonably warm.

Salem's high temperature today will be near 83°F (average is 83°F).

Relative humidity:  Dropping below 50% by 11 a.m. and to near 30% by 5 p.m.

Surface winds: N 5-10 mph this morning; N 7-13 mph this afternoon.

Transport winds: NNE 10-15 mph this morning; N 10-15 mph this afternoon.

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

Salem's sunset tonight: 8:29 p.m.

(Salem Airport data for Thursday, August 7th: High 86°F; Rainfall: .00")

(Maximum Ventilation Index expected today: 75)

EXTENDED DISCUSSION:

An upper-level trough is forecast to approach the northern California coast on Saturday, turning the flow aloft southeasterly over Oregon.  That will warm temperature back to above average, as onshore flow begins to relax.  On Sunday, increasing SE flow aloft will circulate more moisture and unstable air northward over Oregon.  Thundershowers are likely across southern Oregon and will migrate northward, possibly to over the Willamette Valley, Sunday night through Monday night.  Wetting rains are possible from these storms.

An approaching trough, from the Gulf of Alaska, will turn the flow aloft southwesterly on Tuesday and should push the thundershower threat east of the region.  Increasing onshore flow will cool temperatures back to near normal and may create a significant burning opportunity, if fields are not too damp.  Dry and warmer weather is predicted for the second half of next week, with mostly northerly transport winds.

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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