[willamette-fcst] Silverton Hills AM Forecast.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

8:55 AM PDT THU OCT 4, 2012



BURN ADVISORY:



...State Fire Marshal Burn-Ban Conditions may be reached this afternoon due to the combination of low relative humidity (≤ 30%) and wind (≥ 15 mph).



Agricultural burning is not recommended.

Prep burning is not allowed.

Propane flaming is not allowed.



WEATHER DISCUSSION:



Northwesterly flow aloft is continuing, with an unseasonably cold and dry Canadian air mass over Oregon.  Most locations east of the Cascades fell below freezing this morning.  Meacham, in NE Oregon, dropped into the mid teens, with Redmond, in central Oregon, dipping into the low 20s.  Meanwhile, temperatures were not as cold across western Oregon, but some spots in the Willamette Valley saw the upper 30s.



A surface thermal trough is trying to build northward along the southern Oregon coast, which is forcing cool and very dry air west of the Cascades, via east to northeast winds.  Pressure gradients have relaxed some today, but NE winds may still be strong enough to put the Willamette Valley into State Fire Marshal Burn-Ban Conditions this afternoon.  Some wildfire smoke is also getting transported across NW Oregon, making for hazy conditions.



Satellite imagery shows clear skies across all of Oregon this morning, with the exception of low clouds and fog along the southern coast.  The morning sounding over Salem showed a few degrees of warming aloft, compared to yesterday (from just above the surface through 6000 feet).  Warm air aloft should keep mixing heights to below 3000 feet this afternoon.



(Salem Airport data for Wednesday, Oct. 3rd: High 74°F; Rainfall .00")



TODAY'S FORECAST:



Sunny, hazy, and breezy.



Salem's high temperature today will be near 75 degrees (normal is 70).

Relative humidity:  Dropping below 30% by 11 a.m. and below 20% by 2 p.m.

Surface winds:  NE 5-10 mph; increasing to near 15 mph this afternoon.

Transport winds: NE 15-20 mph; increasing to 20-25 mph this afternoon.

Mixing height: Rising to near 2800 feet this afternoon.

Salem's sunset tonight: 6:46 p.m.



THREE-HOURLY DATA:

                               11 a.m.   2 p.m.  5 p.m.  8 p.m.

     Temperature:                 65       73      73      61

     Relative Humidity:           26%      17%     19%     30%

     Surface Wind Dir/Speed:    NE 10    NE 15   NE 15   NE 12

     Transport Wind Dir/Speed:  NE 17    NE 23   NE 25   NE 20

     Mixing Height:              1800     2800    2800    1200

     Ventilation Index:           31       64      70      24



EXTENDED DISCUSSION:



Little change in the weather pattern is expected through Saturday, with continued sunny skies and dry offshore flow through the transport layer.  Offshore flow should weaken early next week, but there are no indications that this will create any burning opportunities.  The upper-level ridge is forecast to maintain dry and stagnant weather through at least next Thursday.



A very weak weather system may put a dent in the strong ridge over the region next Friday; possibly turning transport winds onshore enough to create a burning opportunity.  That may also be the beginning of a pattern change to a more fall-like regime.



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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