[willamette-fcst] Forecast - Friday, August 14th, 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.

Issued: 
     Friday, August 14th, 2009 at 8:50am.

Burn Advisory:
     Agricultural burning is not recommended.
     Preparatory burning is allowed from 11:00am until 2:00pm with a 100 acre limit.
     Propane flaming is allowed from 11:00am until 2:00pm.
     Stack burning is not allowed.

Weather Discussion:
     An unseasonably cool upper-level trough is over the Pacific
     Northwest this morning.  The upper-level trough axis moved
     across western Oregon Thursday evening but did not produce
     much in the way of shower activity.  Most of the moisture
     from this system is well north and east of western Oregon,
     but the air aloft is quite cool for mid-August.  The
     freezing level over Salem was measured at just 7500 feet,
     compared with a mid-August average of 12,400 feet.

     The cool and unstable atmosphere helped to keep skies at
     least partially clear overnight, across western Oregon, with
     early morning temperatures locally dropping into the mid to
     upper 40s.  Eugene and Corvallis both dipped down to 47
     degrees.  Mid-morning readings were mostly in the low to mid
     50s with numerous sunbreaks and light winds.  Skies should
     be partly to mostly sunny this afternoon, with a drier
     northwesterly flow aloft.  Willamette Valley highs will only
     climb into the low 70s, with increasing northwesterly winds
     this afternoon.

     The air mass is cool and moist enough for convective cloud
     development, once surface temperatures reach about 62
     degrees, which should occur late this morning.  I do not
     expect the convection to be strong enough to develop
     showers, but the development of \\\"fair weather\\\" cumulus
     clouds will be a good indicator of mixing height (they shoud
     form between 2500 and 3000 feet late this morning).

     Cold air aloft will likely combine with daytime heating to
     lift mixing heights to over 5000 feet this afternoon. High
     mixing heights, combined with northwesterly transport winds,
     should provide adequate ventilation to allow for open
     burning in the north valley this afternoon. Winds may be too
     northerly to allow for burning in the south valley today.
     Winds are expected to turn more northerly overnight.

Surface Winds:
     NW 5-10 this morning, NW 5-15 this afternoon.
Transport Winds:
     NW 5 this morning, NW 10 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 5500 feet.  Ventilation index 55.
High Temperature:
     Salem\'s high temperature today will be near 72.
Humidities:
     Relative humidity drops to 50% around 2pm.
     Minimum relative humidity will be near 43%.
Sunrise/Sunset:
     Salem sunset tonight: 8:19pm; sunrise tomorrow: 6:14am.

Extended Outlook:
     The flow aloft is forecast to become northerly Saturday and
     Sunday, as a strong upper-level ridge builds along the west
     coast.  Temperatures will rapidly warm across western Oregon
     through Tuesday, as a surface thermal trough builds
     northward, into western Oregon.  Low-level winds will turn
     offshore early next week, even along the northern and
     central coast.

     It appears as if a slow cooling trend will begin around the
     middle of next week, as the upper-level ridge, and
     associated surface thermal trough, both shift east of the
     Cascades.  That may create a burning opportunity, as soon as
     Wednesday afternoon, with the low-level winds turning back onshore.

Tomorrow (15 Aug):  Mostly Sunny.  48/78

Sun (16 Aug):  Sunny and Warmer.  50/84

Mon (17 Aug):  Sunny and Very Warm.  52/88

Tue (18 Aug):  Sunny and Very Warm.  54/92

Wed (19 Aug):  Mostly Sunny and Warm.  Possible Onshore Flow Late.  56/89

Thu (20 Aug):  Mostly Sunny and a Little Cooler.  Onshore Flow in the Afternoon.  55/86

Fri (21 Aug):  Patchy AM Clouds.  Afternoon Sunshine. 55/84

ODA Meteorologist
weather at oda.state.or.us