[willamette-fcst] Forecast - Friday, July 31st, 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, July 31st, 2009 at 8:50am.

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

Weather Discussion:
     Slight weakening of the massive upper-level ridge of high
     pressure over the west coast of North America allowed for
     weak onshore flow to penetrate into the interior of western
     Oregon Thursday.  Temperatures in the Willamette Valley
     generally cooled 10-15 degrees from the record shattering
     heat on Wednesay but were still above normal.  Highs ranged
     from the upper 80s in the south valley to the mid 90s in the
     Portland area.  The surface thermal trough shifted only as
     far east as the Cascades, with the hottest condtions
     shifting into north-central Oregon.  Hood River (103
     degrees) and The Dalles (105 degrees) soared well over 100
     degrees.  The triple-digit heat continued in southwestern
     Oregon, with Medford tying The Dalles for the hot spot in
     the state at 105 degrees.

     Converging surface winds, into the thermal trough, and
     daytime heating helped fuel explosive thunderstorm
     development along the eastern slopes of the Cascades
     Thursday afternoon and over central and south-central
     Oregon.  These storms did not have a lot of low-level
     moisture to work with, so they produced very little rainfall.

     The upper-level ridge is forecast to strengthen slightly
     over Oregon today, which may decrease the thunderstorm
     development over central Oregon, somewhat, this afternoon. 
     Daytime heating and mid-level instability will still likely
     fuel at least a few storms.  West of the Cascades, weak
     onshore flow was resulting in morning marine clouds along
     the coast and in sections of the extreme northern and
     southern Willamette Valley.  Those should give way to
     sunshine this afternoon, even along most of the beaches. 
     Temperatures dropped generally into the 50s in the valley
     this morning...making it feel much more comfortable.  Highs
     this afternoon will be very similar to yesterday, except for
     a few additional degrees of cooling in the Portland area.

     The Salem sounding continued slight cooling aloft but still
     very warm temperatures in the mid-levels of the atmosphere. 
     As the upper-level ridge rebuilds over oregon today, the air
     aloft should warm slightly.  That will keep mixing heights
     at or below about 3500 feet this afternoon and maintain the
     fairly stagnant ventilation conditions across the region.

Surface Winds:
     W 0-5 this morning, NW 5-10 this afternoon.
Transport Winds:
     S 4 this morning, NNW 7 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 3500 feet.  Ventilation index 25.
High Temperature:
     Salem\'s high temperature today will be near 90.
Humidities:
     Relative humidity drops to 50% by 12pm.
     Minimum relative humidity will be near 33%.
Sunrise/Sunset:
     Salem sunset tonight: 8:39pm; sunrise tomorrow: 5:58am.

Extended Outlook:
     The upper-level ridge is forecast to hold firm over Oregon
     Saturday, with some increase in southerly flow over the
     southern half of the state.  That will introduce more
     mid-level moisture into the region for an increased threat
     of afternoon and evening thunderstorm development. 
     Temperatures will likely warm a few degrees from Friday,
     with the thermal trough trying to build back westward into
     the Willamette Valley.

     By Sunday, the upper-level ridge axis is forcast to finally
     shift into eastern Oregon with an approaching upper-level
     trough further increasing the southerly flow aloft over most
     of the state.  Willamette Valley temperatures will either
     hold steady of cool slightly, as the thermal trough gets
     forced back east of the Cascades.  Increasing southerly flow
     aloft will act to destabilize the very warm air mass over
     Oregon and could lead to impressive thunderstorm development
     Sunday afternoon and evening...especially over the southern
     half of the state.

     Progressively cooler temperatures are likely next week, as
     the offshore upper-level trough slowly approaches the
     coastline.  Increasing south-southeasterly flow aloft will
     lead to more widespread shower and thundershower activity
     across Oregon...including over the Willamette Valley. The
     surface thermal trough will shift into eastern Oregon by
     Monday, and possibly as far east as Idaho on
     Tuesday..allowing stronger onshore flow to initiate a
     cooling trend statewide.

     There are big differnces in the long-range computer models
     for the middle of next week, with some bringing the
     upper-level trough onshore and others leaving it offshore. 
     My forecast reflects the former solutions but may need to be
     updated, towards warmer and drier conditions, if the
     upper-level low-pressure area ends up staying offshore.

Tomorrow (01 Aug):  Sunny and Very warm.  T-storms over the Cascades in the Afternoon.  59/95

Sun (02 Aug):  Becoming Parly Cloudy. Chance of T-Storms...Mainly South.  60/91

Mon (03 Aug):  Partly to Mostly Cloudy.  Chance of Showers or T-Storms.  Muggy.  60/88

Tue (04 Aug):  Partly to Mostly Cloudy.  Chance of Showers and T-Storms.  58/85

Wed (05 Aug):  Mostly Cloudy.  Chance of Showers and T-Storms.  Cooler 56/78

Thu (06 Aug):  Mostly Cloudy.  Chance of Showers.  53/77

Fri (07 Aug):  Morning Clouds...Partly Sunny. 52/80

ODA Meteorologist
weather at oda.state.or.us