[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 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 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 was centered over Colorado
     late this morning.  The ridge encompasses most of the western half of the country with
     the axis extending northwestward, through Northern Idaho, to SE Alaska.

     Increasing southerly flow aloft is coninuing to transport California wildfire smoke
     northward over Oregon...making for hazy conditions.  The late-morning ODA surface
     analysis showed a thermal trough extending from SW Oregon, through the Willamette
     Valley, to just off the Washington Coast.  Offshore flow was continuing over all of
     Oregon...east of the coast range.

     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 mainly SW Oregon.  
     Enough moisture may circulate into Southern Oregon for afternoon heating to generate
     a few mountain thunderstorms later this afternoon.

     Willamette valley temperatures had already warmed into the low to mid 70s by midday.
     Hazy Sunshine will further lift inland temperatures to near 90 this afternoon.  The
     warmth of the air aloft supports even warmer surface temperatures, but wildfire smoke
     is blocking some sunlight.  The coastline was still in the 50s late this morning and 
     will struggle to reach 60 degrees with onshore flow maintaining low clouds and fog.

     The Salem sounding this morning showed even more warming aloft, since Monday, so mixing
     heights were estimated to be only about 1000 feet late this morning and will stay below
     2000 feet for most, if not all, of the day.  There are some changes taking place that
     could improve ventilation conditions later this afternoon.  The upper-level ridge is
     slowly shifting east, so the the surface thermal trough may also shift into Central
     Oregon late this afternoon.  That would induce weak onshore flow and potentially turn
     transport winds more westerly.  The air aloft will still be very warm, so mixing
     heights will not likely exceed about 2500 feet.  We will monitor the transport winds,
     via pibals, starting at 2pm, but the outlook for open burning today remains doubtful.

     The California disturbance is forcast to move into Southern Oregon overnight with a
     chance of, mainly mountain, showers or thunderstorms.  Weak onshore flow will likely
     bring some cooler air into the Willamette Valley overnight but little or no marine clouds. 

Surface Winds:
     Var 0-5...possibly becoming SW 5-10 late this afternoon.
Transport Winds:
     Light...Becoming SW 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:
     Minimum relative humidity will be near 26%.
Sunrise/Sunset:
     Salem sunset tonight: 7:20pm; sunrise tomorrow: 6:54am.

Extended Outlook:
     A weak upper-level trough will move northward, across Oregon, Wednesday.  It will cool
     the air aloft and bring a slight chance of showers and thundershowers...mainly from the
     Cascades eastward.  The surface thermal trough is forecast to shift into Eastern Oregon
     and Idaho, with increasing onshore flow cooling temperatures west of the Cascades closer
     to normal.

     This transition has the highest potential of creating an open burning opportunity, Wednesday
     afternoon, that most of the valley has seen for a couple of weeks.  However, there are
     some potential limiting factors.  Southerly flow aloft may keep mixing heights fairly low
     (near 3000 feet), and potenital thundershower development may create poor gradient-stacking
     and not allow for good evacuation of smoke over the Cascades.  

     Another upper-level low pressure system is forecast to approach the Northern California
     Coastline Thursday afternoon with increasing south-southwesterly flow aloft and continued
     onshore flow at the surface.  This system should bring a little more cooling to Western
     Oregon but not much inthe way of precipitation, as it comes onshore late Thursday and
     Friday.  This system also has the potential to produce burning opportunities. 
     
     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 Sunday or Monday and may
     bring some light rain to the region.

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

Thu (18 Sep):  Mostly Sunny.  50/78

Fri (19 Sep):  Increasing Clouds.  52/78

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

Sun (21 Sep):  Increasing Clouds.  Chance of Light Rain Late.  50/74

Mon (22 Sep):  Chance of Showers and Cooler.  48/68

Tue (23 Sep):  Partly Cloudy.  45/68

ODA Meteorologist
weather at oda.state.or.us