[willamette-fcst] Forecast - Tuesday, August 19th, 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, August 19th, 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:
     A moist and unstable southerly flow aloft brought numerous
     showers and thundershowers to Western Oregon Monday. 
     Rainfall amounts were greatest in the north valley with
     several loactions receiving up to one-quarter of an inch. 
     Most of the south valley recorded less than one-tenth of an
     inch.  The upper-level trough came onshore last night with
     the flow aloft turning southwesterly.  That stabilized the
     air mass west of the Cascades, but a few showers continued
     through early this morning.

     The upper-level trough was shifting east of the region at
     mid-morning with the flow aloft drying out and becoming more
     westerly.  Doppler radar showed a few very light showers
     over the valley, but they were weakening.  Satellite imagery
     showed a few breaks in the low clouds over the valley.  The
     ODA surface analysis showed weak high pressure over Western
     Oregon, ahead of a very impressive system offshore. 
     Gradients across the valley were weak southerly with south
     winds in the 5-10 mph range common. Temperatures were in the
     low 60s.

     The Salem sounding this morning showed considerable cooling
     aloft since Monday morning, so mixing heights should climb
     over 3000 feet as soon as surface temperatures reach the
     upper 60s.  Transport winds were southwesterly.  With a few
     sunbreaks today, surface temperatures may reach the low to
     mid 70s.  That would yield mixing heights above 4000 feet. 
     That may create an opportunity for open burning of dry
     fields when humidity levels drop below 65 percent.

     A very impressive weather system for mid-August was about
     300 miles offshore at mid-morning.  The high clouds ahead of
     it were already approaching the coastline.  It will likely
     warm the air mass aloft slighly this afternoon, which will
     tend to shut down the shower activity and help warm the air
     mass.  Rain is forecast to reach the coastline by late this
     afternoon and move into the Willamette Valley this evening.

Surface Winds:
     S 5-12 this morning, SW 8-15 this afternoon.
Transport Winds:
     SSW 12 this morning, SW 20 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 4800 feet.  Ventilation index 96.
High Temperature:
     Salem\'s high temperature today will be near 73.
Humidities:
     Minimum relative humidity will be near 58%.
Sunrise/Sunset:
     Salem sunset tonight: 8:11pm; sunrise tomorrow: 6:21am.

Extended Outlook:
     An unseasonably strong weather system is forecast to move
     onshore tonight with blustery winds along the coast and a
     soaking rainfall across the Willamette Valley.  The steady
     rain will turn to showers Wednesday afternoon with a strong
     upper-level trough keeping the showers going through
     Thursday.  Total rainfall amounts will generally exceed
     one-half inch in the north valley and one-quarter inch in
     the south valley.  Some computer models have total rainfall
     amounts in the north valley exceeding an inch, so this
     system appears as if it will be unusually moist.

     The flow aloft is forecast to turn northwesterly and begin
     drying out Thursday night with a return of more summer-like
     weather as we head into the weekend.  That will give fields
     at least 2-3 days to dry, before the next weather system
     comes onshore early next week.  There is disagreement among
     the computer models as to how long this warm dry period will
     last.  It is possible that another fairly potent weather
     system could bring more rain to the valley as early as
     Sunday night.

Tomorrow (20 Aug):  Rain Turning to Showers.  59/70

Thu (21 Aug):  Mostly Cloudy.  Showers Likely...Mainly North.  56/74

Fri (22 Aug):  Mostly Sunny & Warmer.  50/80

Sat (23 Aug):  Sunny and Warm.  52/85

Sun (24 Aug):  Mostly Sunny.  Increasing Clouds Late.  54/85

Mon (25 Aug):  Mostly Cloudy and Cooler with a Chance of Rain.  58/74

Tue (26 Aug):  Mostly Cloudy.  Chance of Showers.  55/74

ODA Meteorologist
weather at oda.state.or.us