[willamette-fcst] Silverton Hills Forecast - Fri, August 13 2010

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST
OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE
9:00 AM PDT FRI AUG 13, 2010

...Fire Marshal Conditions are possible this afternoon and likely both Saturday and Sunday afternoons...

BURN ADVISORY:

     Recommended burn times for agricultural burning are from noon to 6:30pm
     Prep burning is not allowed.
     Propane flaming is not allowed.

WEATHER DISCUSSION:

An upper-level ridge is building just offshore with a dry north-northwesterly flow aloft over western Oregon.  The surface chart shows a thermal trough building northward, from southwestern Oregon, into the Willamette Valley this morning.  Satellite imagery shows low clouds along most of the immediate coastline, where the flow is still weakly onshore.  However, skies are clear just inland and over the entire Willamette Valley.

Gradients are already offshore this morning, from Redmond to Salem, and expected to increase this afternoon.  Down-sloping northeasterly winds and a full day of August sunshine will warm valley temperatures well into the 90s, with very low humidity levels.  Northeasterly transport winds are not conducive to good valley ventilation, so the prospect for any burning today is poor.  The focus of today's discussion is on next week's weather.  More on that in a moment...

TODAY'S FORECAST:

Sunny and warmer.  After reaching 88 degrees on Thursday, Salem's high temperature today will be near 93 degrees.  The mixing height will climb to 3000 feet about noon and likely top out near 5000 feet around 5 p.m.  No evening sea breeze is expected this evening, with persistent dry offshore flow, so surface temperatures will be slow to cool.  That will likely keep the mixing height above 1500 feet until about sunset.  Light northerly surface winds, this morning, will turn north-northeasterly and increase to about 10 mph this afternoon.  NE transport winds will increase to about 15 mph this afternoon. Relative humidity will drop to 50% by 11 a.m. and to near 21% around 5 p.m.  The ventilation index will climb to 80 this afternoon.

Silverton area sunset tonight: 8:16 pm

THREE-HOURLY DATA:

                                 11am     2pm     5pm     8pm
     Temperature:                 76       87      93      84
     Relative Humidity:           45%      29%     21%     29%
     Surface Wind Direction:     010      010     010     010
     Surface Wind Speed:           7        8      12       8
     Transport Wind Direction:   020      030     040     030
     Transport Wind Speed:         7       14      16      12
     Estimated Mixing Height:   2200     3800    5000    1500
     Ventilation Index:           15       53      80      18

EXTENDED DISCUSSION:

A strong upper-level ridge of high pressure will build over the Pacific Northwest Saturday through Monday.  Sunny skies and dry offshore flow will bring near-record warmth to the Willamette Valley. A weak upper-level trough is forecast to undercut the ridge and move across northern California on Sunday, possibly bringing some thunderstorms to the southern Oregon Cascades.

The ridge is forecast to peak over the region on Monday, with a stronger upper-level trough approaching the southern Oregon Coast by Tuesday.  That system may circulate thundershower activity further north, along the Cascades, and should initiate a cooling trend.  That may create an open burning opportunity, as early as Tuesday afternoon, with transport winds forecast to turn back onshore.

The long-range models are beginning to come into agreement that the upper-level trough will come onshore Wednesday and/or Thursday.  Increasing onshore flow and continued cooling may create an enhanced opportunity open burning opportunity one or both of those days. Temperatures will likely cool back to normal by Thursday.  Strong onshore flow is forecast on Friday with considerable marine clouds penetrating into the Willamette Valley.  Gradient-stacking may be too great for open burning by that time.

EXTENDED FORECAST:

Saturday: Sunny with near-record warmth. NE wind 5-10 mph. 58/97
Sunday: Sunny with near-record warmth. Light wind. 59/98
Monday: Sunny with near-record warmth. Light wind. 61/99
Tuesday: Sunny. A little cooler. Wind possibly becoming W 5-15 mph. 61/94
Wednesday: Partly cloudy and cooler. Wind W 5-15 mph. 57/87
Thursday: Partly cloudy. Slight chance of showers. 54/80
Friday: Morning clouds with afternoon clearing. 51/77

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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