[willamette-fcst] Forecast - Thursday, September 24th, 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: 
     Thursday, September 24th, 2009 at 9:00am.

Burn Advisory:
     Agricultural burning is allowed.  Suggested burn times are from 2:00pm until 6:00pm.
     Preparatory burning is not allowed.
     Propane flaming is not allowed.
     Stack burning is not allowed.

Weather Discussion:
     Southeasterly flow aloft spread considerable smoke, from
     southern Oregon Cascade wildfires, over the Willamette
     Valley Wednesday. The strong upper-level ridge over the
     region shifted eastward to over Idaho with onshore flow
     developing Wednesday evening.  That capped Willamette Valley
     highs in the mid to upper 80s and helped push the wildfire
     smoke out of the region overnight.

     Mid-morning visible satellite imagery showed the smoke
     plumes, from the southern Oregon Cascade wildfires, being
     directed north-northeastward across the central Cascades,
     north-central Oregon, and much of eastern Washington. Marine
     low clouds had penetrated into much of the northern and
     southern Willamette Valley and were in the process of
     filling in over the central valley. Skies were mostly clear
     over the northern Oregon and Washington Cascades and across
     eastern Oregon.

     The ODA surface analysis showed the surface thermal trough
     stretching from eastern Washington through central Oregon,
     with a much cooler onshore flow across the Willamette
     Valley.  The morning sounding over Salem showed considerable
     cooling, below 10,000 feet, but the air mass over western
     Oregon is still quite warm for late September. The freezing
     level was measured at more than 14,000 feet.

     A dissipating cold front will move onshore today and
     maintain the onshore flow across western Oregon.  It could
     be just strong enough to produce some localized drizzle
     along the north coast late this morning, but otherwise dry
     conditions will persist.  Morning low clouds and fog will
     retard surface heating today...keeping mixing heights below
     3000 feet until mid to late afternoon. Ventilation
     conditions will improve later this afternoon, with
     increasing northerly surface and transport winds. Valley
     highs will only climb into the mid to upper 70s today, due
     to the strong influx of low-level cool marine air.

Surface Winds:
     N 5-10 this morning, N 10-15 G20 this afternoon.
Transport Winds:
     NE 10 this morning, N 15 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 3300 feet.  Ventilation index 50.
High Temperature:
     Salem\'s high temperature today will be near 78.
Humidities:
     Relative humidity drops to 50% by 2pm.
     Minimum relative humidity will be near 40%.
Sunrise/Sunset:
     Salem sunset tonight: 7:05pm; sunrise tomorrow: 7:03am.

Extended Outlook:
     The upper-level ridge is forecast to rebuild over the region
     Friday but not as strongly as earlier this week.  Low-level
     winds should turn more northerly with a full day of sunshine
     warming valley temperatures a few degrees from Thursday.  A
     weak weather system is forecast to move across southern
     British Columbia and flatten the upper-level ridge a bit on
     Saturday.  That will incrase the onshore flow and cool
     temperatures slightly.

     The ridge is forecast to rebuild over the region Sunday with
     a thermal trough building northward along the Oregon Coast. 
     That will turn low-level winds offshore and warm
     temperatures well above normal.  The warm-up will be
     short-lived, however, with a fairly strong upper-level
     trough forecast by all of the long-range computer models to
     approach the coastline Monday. Increasing southwesterly flow
     aloft will force the surface thermal trough east of the
     Cascades with increasing onshore flow cooling temperatures
     back to near-normal.  That could be a burning opportunity,
     if any fields become available for burning.

     An autumn-like cold front is forecast to come onshore Monday
     night with light rain likely by Tuesday morning along with
     much cooler temperatures.  A drier but cool northwesterly
     flow aloft is forecast for next Wednesday and Thursday.

Tomorrow (25 Sep):  Sunny.  North Winds.  46/82

Sat (26 Sep):  Sunny.  A Touch Cooler.  NW Winds.  46/78

Sun (27 Sep):  Sunny and Warmer.  NE Winds.  45/85

Mon (28 Sep):  Increasing Clouds and Onshore Flow.  Chance of Rain Late.  47/74

Tue (29 Sep):  Light Rain Likely Early...Then a Chance of Showers.  Cool.  50/65

Wed (30 Sep):  Mostly Cloudy AM.  Afternoon Clearing.  44/66

Thu (01 Oct):  Partly Cloudy.  44/68

ODA Meteorologist
weather at oda.state.or.us