[willamette-fcst] Silverton Hills AM Forecast_Pete.doc

[Willamette Valley Field Burning Forecast]

SILVERTON HILLS FIELD BURNING FORECAST

OREGON DEPARTMENT OF FORESTRY WEATHER OFFICE

9:00 AM PDT MON SEP 9, 2013



BURN ADVISORY:



Agricultural Burning is not recommended.

Prep burning is not allowed.



WEATHER DISCUSSION:



A strengthening offshore upper-level ridge of high pressure will produce dry and stable northerly flow aloft over Oregon today.  The morning sounding over Salem showed several degrees of warming, compared to 24 hours ago, from just above the surface up to 10,000 feet.  That will make for lower daytime mixing heights today.



Satellite imagery shows marine low clouds penetrating up the Columbia River into the northern Willamette Valley; extending about as far south as the Silverton Hills.  At the surface, a thermal trough will build northward along the Oregon coast today, with increasing north to NE winds in the Willamette Valley this afternoon.  Any morning clouds should quickly give way to sunny skies, with afternoon temperatures climbing to well above average.  The combination of lowering humidity and increasing north winds may put the valley into State Fire Marshal Burn-Ban conditions late in the day.



TODAY'S FORECAST:



Patchy morning clouds.  Sunny and warm again this afternoon.



Salem's high temperature today will be near 87 degrees (normal is 79).

Relative humidity:  Drops below 60% by 11 a.m. and to near 30% by 5 p.m.

Surface winds: N 6-12 by late this morning; increasing to N 10-15, with gusts to around 20 mph, this afternoon.

Transport winds: NNE 10 mph this morning; becoming NNE 20 mph this afternoon.

Mixing height: Rising to 3000 feet by 2 p.m. and to 4000 feet by 5 p.m.

Salem's sunset tonight: 7:33 p.m.



(Salem Airport data for Sunday, Sept. 8th: High 87°F; No Rain)

(Maximum Ventilation Index expected today: 80)



EXTENDED DISCUSSION:



A strong ridge of high pressure will build over Oregon Tuesday and Wednesday, with the coastal thermal trough moving over the Willamette Valley.  That will bring progressively warmer weather, with high temperatures likely climbing into the low-to-mid 90s.  North winds will slacken, but the combination of warm temperatures and low humidity could put the valley into State Fire Marshal Burn-Ban conditions in the afternoons.



Late this week, some of the long-range computer models weaken the upper-level ridge enough to switch the low-level winds onshore.  That could create a burning opportunity, but confidence is low at this time.  A more significant change to a cooler onshore flow pattern is advertised for early next week.



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



Pete Parsons

ODF Meteorologist
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