[willamette-fcst] Forecast - Tuesday, September 16th, 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.

Issued: 
     Tuesday, September 16th, 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:
     The massive upper-level ridge of high pressure that has been
     responsible for this late-summer heat wave will bring one
     more day of near-record warmth to the interior of Western
     Oregon.  The ridge is beginning to expand eastward and
     encompasses most of the western half of the country.  The
     axis of the ridge extended from Colorado northwestward,
     through Northern Idaho, to SE Alaska.

     Increasing southerly flow aloft is transporting smoke,
     mostly from California wildfires, northward over
     Oregon...making for hazy conditions.  That will continue
     today, as the upper-level ridge slowly shifts to the east. 
     The ODA surface analysis showed a thermal trough stretching
     across the interior of Western Oregon with onshore flow onto
     the immediate coastline and offshore flow from the
     Willamette Valley to the Idaho border.

     Satellite imagery showed low clouds and fog along the coast
     and generally clear skies over the rest of Oregon.  Some
     high clouds were circulating around a weak upper-level
     disturbance, off the Northern California Coast, northward
     over Oregon.  It may circulate enough moisture into Southern
     Oregon for afternoon heating to set of a few mountain
     thunderstorms this afternoon.  Sunshine over the Willamette
     Valley will lift temperatures to near 90 again this
     afternoon.  Some filtering of the sun by the upper-level
     smoke will likely keep temperature below record levels.  The
     coastline will see temperatures in the realtively chilly
     upper 50s and 60s with low clouds and fog.

     The Salem sounding this morning showed even more warming
     aloft, so mixing heights will stay below 2000 feet for most,
     if not all, of the day.  However, the thermal trough may
     shift into Central Oregon late this afternoon, which would
     induce weak onshore flow into the Willamette Valley.  If
     that combines with a little cooling aloft, it could create a
     brief burning opportunity late this afternoon, but the
     latest computer guidance suggests that is unlikely.

     The California disturbance is forcast to move into Southern
     Oregon overnight with a chance of, mainly mountain, showers
     of thunderstorms moving northward across Oregon.  Weak
     onshore flow will likely bring some marine air into the
     Willamette Valley for the beginning of a cooling trend. 
     There may even be some low clouds to start the day
     Wednesday.

Surface Winds:
     Var 0-5 this morning, N 4-8...becoming SW 5-10 late this afternoon.
Transport Winds:
     NE 5 this morning, NW 5 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 2500 feet.  Ventilation index 13.
High Temperature:
     Salem\'s high temperature today will be near 91.
Humidities:
     Relative humidity drops to 50% by 11am.
     Minimum relative humidity will be near 21%.
Sunrise/Sunset:
     Salem sunset tonight: 7:20pm; sunrise tomorrow: 6:54am.

Extended Outlook:
     A weak upper-level trough will move northward, from Northern
     California, across Oregon Wednesday.  It will cool the air
     aloft and bring a chance of showers and thundershowers
     mainly to areas from the Cascades eastward.  The surface
     thermal trough is forecast to shift eastward, to over
     Eastern Oregon and Idaho, wit increasing onshore flow
     cooling temperatures west of the Cascades closer to normal. 
     This transition may create a burning opportunity Wednesday
     afternoon.  However, southerly flow aloft and the possible
     development of showers and thunderstorms over the Cascades
     makes that questionable.

     A stronger upper-level low pressure system is forecast to
     approach the Northern California Coastline by Thursday
     afternoon but stay south of Oregon, when it comes onshore
     Friday.  South-southwesterly flow aloft will increase over
     Oregon, but not much in the way of shower activity is
     forecast this far north.  This system also has the potential
     to produce a burning opportunities Thursday and Friday. 
     However, the transport winds may be too weak with southerly
     flow aloft.

     The computer models have considerable disagreement,
     beginning late this week, so my confidence in the extended
     forecast details is below average.  A split-flow jet stream
     pattern is forecast by Saturday, with one branch to our
     south and the other to our north, leaving Oregon in a dry
     weather pattern with weak onshore flow making for seasonal
     temperatures.  That could create burning opportunities over
     the weekend.  The next weather system is forecast to
     approach the coastline Monday, but it also looks to be
     fairly dry

Tomorrow (17 Sep):  Partly Cloudy.  52/83

Thu (18 Sep):  Partly Cloudy.  52/78

Fri (19 Sep):  Partly Cloudy.  52/76

Sat (20 Sep):  Partly Cloudy.  51/76

Sun (21 Sep):  Partly Cloudy.  49/76

Mon (22 Sep):  Partly Cloudy.  48/73

Tue (23 Sep):  Partly Cloudy.  47/73

ODA Meteorologist
weather at oda.state.or.us