[willamette-fcst] Forecast - Thursday, September 3rd, 2009

[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.

MIDDAY UPDATE

Issued: 
     Thursday, September 3rd, 2009 at 11:35am.

Burn Advisory:
     Agricultural burning is not recommended.
     Preparatory burning is allowed from now until 2:00pm with a 100 acre limit.
     Propane flaming is allowed from now until 2:00pm.
     Stack burning is not allowed.

Weather Discussion:
     A vigorous upper-level trough of low pressure brought some
     showers and thundershowers to the extreme northern
     Willamette Valley overnight.  The Portland area picked up
     about one-tenth of an inch of rain, with thunderstorms, but
     no rain was reported south of a line from about McMinnville
     to Aurora. Skies were partly to mostly sunny across western
     Oregon, late this morning, with the upper-level trough
     moving northeastward across northwestern Washington.

     The Salem sounding this morning showed brisk southwesterly
     flow aloft and sigificant cooling since Wednesday afternoon.
     Late-morning temperature were in the mid 60s along the
     coast and upper 60s to low 70s across the Willamette Valley.
     Surface heating was already leading to cumulus cloud
     developement, with cloud-bases between 3000 and 4000 feet. 
     That is a good indicator of the current mixing height.

     The ODA surface analysis showed high pressure and weak
     southerly gradients over western Oregon.  As the inland
     areas warm up this afternoon, the center of the high
     pressure area is forecast to move over the coastal waters,
     which will increase the onshore gradients across western
     Oregon.  It will be cooler today, with Willamette Valleys
     highs only climbing into the mid to upper 70s.  That should
     be enough heating to lift mixing heights to around 6000 feet
     by mid to late afternoon.

     If onshore gradients increase, as forecast, and transport winds
     are not too southerly, then there should be an opportunity for
     more open burning this afternoon.  Like yesterday, we plan to
     begin pibal readings at 12:30pm.

Surface Winds:
     S 5-10 mph...becoming W 5-12 mph this afternoon.
Transport Winds:
     SSW 12 mph this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 6000 feet.  Ventilation index 90.
High Temperature:
     Salem\'s high temperature today will be near 77.
Humidities:
     Minimum relative humidity will be near 32%.
Sunrise/Sunset:
     Salem sunset tonight: 7:45pm; sunrise tomorrow: 6:38am.

Extended Outlook:
     The next weather system is forecast to stall near the
     coastline Friday, before moving onshore Saturday.  It is
     unlikely that there will be a burning opportunity, ahead of
     that system, as the flow aloft becomes more southerly.  When
     the cold front finally swings onshore saturday, it should be
     strong enough to bring rain to all of western Oregon
     Saturday.  Computer models have not been consistent with
     forecast rainfall amounts, with the latest guidance
     suggesting that the Willamette Valley will pick up around
     one-quarter of an inch.  The heaviest amounts sould be in
     the north valley.  

     A cool upper-level trough is forecast to follow the cold
     front onshore late Saturday through Monday...maintaining
     some showers across the region with well below normal
     temperatures.  A return more summer-like weather is expected
     after the holiday weekend.  Temperatures should recover to
     near-normal on Tuesday.  Highs may approach 90 degrees by
     next Thursday, as a stong ridge of high pressure builds over
     the Pacific Northwest.  Transport winds are forecast to turn
     offshore next week, which may allow for burning of fields on
     the extreme west side of the valley.

Tomorrow (04 Sep):  Increasing Clouds.  51/80

Sat (05 Sep):  Rain Turning to Showers.  Much Cooler.  54/67

Sun (06 Sep):  Mostly Cloudy and Cool.  Showers Likely.  52/65

Mon (07 Sep):  Showers Ending...Afternoon Clearing.  49/74

Tue (08 Sep):  Mostly Sunny...Warming Back to Normal.  48/80

Wed (09 Sep):  Sunny and Warm.  52/85

Thu (10 Sep):  Sunny and Very Warm.  53/88

ODA Meteorologist
weather at oda.state.or.us