[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.

NOON UPDATE

Issued: 
     Friday, July 31st, 2009 at 12:00pm.

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

Weather Discussion:
     The early morning sounding over Salem this morning showed a
     few degrees of cooling below 6000 feet, compared to Thursday
     morning, due to a weak influx of marine air.  However, the
     impressive upper-level ridge, that has been parked over the
     Pacific Northwest all week, is forecast to strengthen
     slightly over Oregon today.  That will warm the air aloft
     and anchor the surface thermal trough near the crest of
     the Cascades...keeping mixing heights low and not allowing
     for eastward evacuation of air out of the Willamette Valley.

     Under sunny skies, midday temperatures were quite uniform
     across the Willamette Valley...mostly in the mid 70s. 
     Morning low clouds and fog were clearing from the coastline,
     where temperatures had warmed into the upper 50s and low
     60s.  The warmest conditions were over central, eastern, and
     southwestern Oregon, where temperatures were already well
     into the 80s.

     Daytime heating and mid-level instability will likely fuel
     at least a few thunderstorms near the thermal trough axis,
     along and just east of the Cascade crest, and southeastern
     Oregon.  Very weak onshore flow west of the Cascades will
     cap afternoon temperatures in the upper 80s in the southern
     Willamette Valley and low 90s in the north valley.  There is
     a risk of an evening thunderstorm sneaking into the southern
     Willamette Valley, with the flow aloft turning more southeasterly.

Surface Winds:
     NW 5-15 this afternoon.
Transport Winds:
     NNW 9 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 3500 feet.  Ventilation index 32.
High Temperature:
     Salem\'s high temperature today will be near 90.
Humidities:
     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 build even slightly
     more over Oregon Saturday, with increasing southeasterly
     flow aloft over the southern half of the state.  That will
     increase the mid-level moisture and instability over
     southern Oregon, for an increased threat of afternoon and
     evening thunderstorm development.  In the Willamette Valley,
     temperatures will likely warm a few degrees from Friday,
     with the thermal trough trying to build northwestward, into
     the Portland area.

     By Sunday, an upper-level low-pressure area will come close
     enough to the California Coast to shift the upper-level
     ridge axis is into eastern Oregon.  Increasing southeasterly
     flow aloft will further highten the thunderstorm potential
     across the entire state.  Due to the warmth of the low-level
     air mass, there is a chance that some of the storms could
     become quite strong.  Willamette Valley temperatures will
     likely cool slightly, as the thermal trough gets forced back
     east of the Cascades.

     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. Again,
     some of the storms could be quite strong.  Due to the dry
     conditions, this pattern has the potential to start many
     wildfires across the state.  The surface thermal trough will
     shift into eastern Oregon by Monday, and possibly as far
     east as Idaho by 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 low-pressure system onshore and others leaving
     it offshore.  My forecast reflects it coming onshore about
     Tuesday and Wednesday of next week, followed by a drier more
     westerly flow aloft late next week.

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