[willamette-fcst] Silverton Hills Midday Forecast.doc

Fri Aug 3 11:57:49 PDT 2012

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

11:55 AM PDT FRI AUG 3, 2012

BURN ADVISORY:

...State Fire Marshal Conditions may be met late today (humidity and wind)...

Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.

WEATHER DISCUSSION:

The upper-level trough that brought onshore flow the past couple of days dropped into eastern Montana and western North Dakota this morning. A building offshore ridge of high pressure is producing a warmer, drier and stable northerly flow aloft over Oregon.

Late-morning visible satellite imagery showed marine low clouds persisting along the immediate coastline but rapidly evaporating inland.  Expect sunny skies across the Willamette Valley this afternoon with temperatures likely topping out at least 5 degrees warmer than on Thursday.

The ODF SODAR, located near Elkton in the coast range, was showing brisk NE winds from just above the surface up to 4000 feet late this morning. All forecast models are calling for further strengthening of the offshore flow across northwestern Oregon today, which will combine with warming aloft to make for highly unfavorable burning conditions.

TODAY'S FORECAST:

Sunny, warmer, and breezy this afternoon.

Salem's high temperature today will be near 87 degrees (normal is 84).

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

Surface winds:  N 7-12 mph; becoming NNE 10-16 mph this afternoon.

Transport winds: NE 10-15 mph; increasing to 16-22 mph this afternoon.

Mixing height: Rising to 3000 feet by 1 p.m. and to near 4000 feet by 5 p.m.

Salem's sunset tonight: 8:35 p.m.

THREE-HOURLY DATA:

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

     Temperature:                 80      86      80

     Relative Humidity:           41%     29%     38%

     Surface Wind Direction:     010     020     010

     Surface Wind Speed:          10      14      13

     Transport Wind Direction:   030     040     030

     Transport Wind Speed:        17      20      20

     Mixing Height:             3500    4000    2000

     Ventilation Index:           60      80      40

EXTENDED DISCUSSION:

The upper-level ridge is forecast to move directly over Oregon on Saturday with a surface thermal trough pushing northward over the region.  A full day of sunshine will combine with very warm air aloft and offshore flow to produce the highest temperatures so far this year.  Highs in the Willamette Valley should be in the middle and upper 90s.  State Fire Marshal Conditions will likely be met Saturday afternoon due to hot temperatures and low relative humidity.

The upper-level ridge is forecast to shift eastward by Sunday, with increasing southerly flow aloft destabilizing the air mass and introducing monsoonal moisture into the region.  Increasing middle and high clouds will block some sunshine, resulting in a few degrees of cooling, but highs should still climb well into the 90s.

An upper-level disturbance is forecast to rotate northward, across Oregon, late Sunday.  That will bring a chance of thunderstorms to most of the state, including the Willamette Valley, especially Sunday night.  The flow aloft is forecast to turn southwesterly Monday and Tuesday.  That should stabilize the air mass and push the thunderstorms east of the region.  Increasing onshore flow will cool temperatures back to normal by Tuesday; possibly creating burning opportunities.

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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