[willamette-fcst] Forecast - Monday, August 4th, 2008

[Willamette Valley Ag/Burning Forecast]

Daily Smoke Management Forecast




Oregon Department of Agriculture
Smoke Management Program
Weather Outlook and Field Burning Advisory for Willamette Valley Growers and Fire Districts.

...Corrected wording in the extended forecast...

...Fire Marshal conditions may be met this afternoon due to low humidity and wind...

Issued: 
     Monday, August 4th, 2008 at 9:00am.

Burn Advisory:
     Agricultural burning is not recommended.
     Preparatory burning is not allowed.
     Propane flaming is not allowed.
     Stack burning is not allowed.

Weather Discussion:
     A 52-day dry spell in Salem was broken Friday, as a weak
     cold front moved south across the Willamette Valley and
     dropped .03 inches of rain at the Salem Airport.  The storm
     produced much greater precipitation totals in the
     Northeastern Willamette Valley, where upslope winds enhanced
     that rainfall.  Up to .70 inches of rain fell, in the
     Corbett area, at the western end of the Columbia Gorge, and
     troutdale picked up about one-tenth of an inch.

     A weak upper-level trough, over Southern British Columbia,
     provided onshore flow and moderate temperatures across
     Western Oregon this past weekend.  Valley highs were in the
     70s both days with morning clouds and afternoon sunshine. 
     The upper-level low pressure area had slid eastward this
     morning, and high pressure was rapidly building over the
     west coast.

     Weak onshore flow continued to push a shallow layer of low
     clouds onto the coastline, and locally into the coastal
     mountain gaps, this morning.  Some low clouds made it up the
     Columiba River into the Portland area, while the remainder
     of the valley had clear skies this morning.  Valley minimums
     were mostly near 50 degrees this morning.  Hillsboro was one
     of the colder spots with a low of 47...followed closely by
     Eugene at 48.  The Portland Airport was the warm spot with a
     minimum of 56.

     Visible satellite imagery this morning showed a blanket of
     low clouds west of the coast range with patchy low clouds
     along the Columbia River into the Portland area.  The
     remainder of the state had cloud-free skies.  California
     wildfire smoke was streaming over SW Oregon and
     northeastward across the southern half of Central and
     Eastern Oregon.

     The ODA surface analysis showed a thermal trough forming in
     the Willamette Valley with weak northerly gradients across
     Western Oregon.  Northerly surface winds will prevail and
     increase this afternoon across the western valleys. 
     Increasing offshore flow will drop relative humidities below
     30 percent this afternoon, so Fire Marshal conditions may
     locally be met, if north winds increase to 15 mph or greater.
     The north winds should be strongest in the south valley.

     With the air aloft rapidly warming today, mixing heights
     will struggle to reach 3000 feet.  A full day of sunshine
     and the lack of onshore flow will combine to lift valley
     temperatures into the upper 80s.

Surface Winds:
     N 5-10 this morning, N 7-15 this afternoon.
Transport Winds:
     NNE 10 this morning, NNE 10 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 3000 feet.  Ventilation index 30.
High Temperature:
     Salem\'s high temperature today will be near 88.
Humidities:
     Relative humidity drops to 50% by 11am.
     Minimum relative humidity will be near 26%.
Sunrise/Sunset:
     Salem sunset tonight: 8:34pm; sunrise tomorrow: 6:03am.

Extended Outlook:
     The upper-level ridge axis will be from SW Idaho through
     Central Washington Tuesday with weak southerly flow aloft
     over Oregon.  That will increase the warming over the state
     and introduce a thundershower threat to Southern Oregon as
     early as Tuesday evening.  Southerly flow aloft will
     increase Wednesday, as the upper-level ridge shifts eastward
     to over Western Montana.  That will likely result in scattered
     thunderstorm development along most of the Cascade Range.

     If the flow aloft becomes southeasterly, thunderstorms could
     also make their way over the Willamette Valley Wednesday
     afternoon and evening.  The surface flow will turn weakly
     onshore Wednesday, as the thermal trough shifts east of the
     Cascades.  That will bring some cooling to Western Oregon
     and could create a burning opportunity, if the flow aloft is
     not too southerly.

     The upper-level winds are forecast to become more
     southwesterly Thursday and Friday, which will increase the
     onshore flow across Western Oregon with the bulk of the
     thundershower activity from the Cascades eastward Thursday
     and well east of the region Friday.  That would also create
     another potential burning opportunity.  An upper-level
     trough is forecast to move onshore over the weekend for even
     cooler conditions and increased onshore flow.

Tomorrow (05 Aug):  Sunny and Hot.  55/95

Wed (06 Aug):  Mostly Sunny.  Slight Chance of PM T-storm.  56/91

Thu (07 Aug):  Partly Cloudy and Cooler.  Slight Chance of T-storms near the Cascades.  57/85

Fri (08 Aug):  Mostly Sunny and Cooler.  57/83

Sat (09 Aug):  Partly Sunny with Continued Cooling.  54/79

Sun (10 Aug):  Morning Clouds.  Afternoon Clearing.  53/80

Mon (11 Aug):  Mostly Sunny.  52/83

ODA Meteorologist
weather at oda.state.or.us