[willamette-fcst] Silverton Hills Midday Forecast.doc

Wed Sep 21 11:56:22 PDT 2011

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT WED SEP 21, 2011

BURN ADVISORY:

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

A strong and broad upper-level ridge of high pressure remains anchored over the Rockies with light southwesterly winds aloft over Oregon.  At the surface, pressure gradients were still offshore as of late this morning across western Oregon with light easterly winds continuing to funnel through the Columbia Gorge into the northern Willamette Valley.  Winds are light southeasterly in the central and southern Willamette Valley.

Satellite imagery shows a cold front extending from the northern tip of Vancouver Island, British Columbia to about 350 miles west of the northern Oregon coast.  Some mid-level clouds from this system have spread as far south and east as the northwestern corner of Oregon.  Skies are sunny across the remainder of the state, including most of the coastline, due to the offshore low-level winds.

Very slow eastward progress of the offshore weather system is forecast for this afternoon.  Warm air aloft will keep mixing heights suppressed.  Valley temperatures are running about 5-9 degrees warmer than 24 hours ago, so afternoon highs should climb into the upper 80s.

Offshore transport winds are forecast to slacken by mid-afternoon and could possibly turn weakly onshore by late-afternoon.  Pibals are scheduled to begin at 1 p.m.  Due to the very warm air aloft, mixing heights will struggle to reach 3000 feet today.  That will limit burning opportunities, even if transport winds eventually turn weakly onshore later this afternoon.

TODAY'S FORECAST:

Partly cloudy and warmer.

Salem's high temperature today will be near 87 degrees (normal is 76).

Relative humidity drops to near 30% by 5 p.m.

Surface winds:  Light SE; becoming light S later this afternoon.

Transport winds: Light SE; possibly turning light SW later this afternoon.

Mixing height: Slowly rising to 3000 feet by 5 p.m.

Salem's sunset tonight: 7:12 p.m.

THREE-HOURLY DATA:

                                2 p.m.  5 p.m.  8 p.m.

     Temperature:                 81      85      71

     Relative Humidity:           35%     31%     49%

     Surface Wind Direction:     160     180     210

     Surface Wind Speed:           4       3       4

     Transport Wind Direction:   Var     230     230

     Transport Wind Speed:         4       5       5

     Mixing Height:             2500    3200    1200

     Ventilation Index:           10      16       6

EXTENDED DISCUSSION:

The strong Rocky Mountain ridge is forecast to weaken slightly Thursday and Friday with increasing southwesterly flow aloft over Oregon.  The surface thermal trough should push well east of the Cascades with slight cooling aloft and southwesterly transport winds likely creating favorable afternoon burning conditions both days.

The flow aloft, and at transport level, is forecast to become more southerly, on Saturday, as an upper-level trough digs offshore.  Computer models are showing rain-making weather systems moving onshore Sunday through Tuesday of next week with a break from the rain next Wednesday and Thursday.

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 15mph.

     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 times

         the transport wind speed 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 and the Oregon Department of Forestry.  For information contact ODA at 503-986-4701.

Pete Parsons

ODF Meteorologist

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