[willamette-fcst] Forecast - Monday, December 22nd, 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: 
     Monday, December 22nd, 2008 at 9:00am.

Burn Advisory:
     Agricultural burning is allowed.  Suggested burn times are from now until 2:30pm.
     Straw stack burning is allowed from now until 2:30pm.

Weather Discussion:
     A moist Pacifc storm system approached the coastline
     Saturday, inducing strong easterly winds through the
     Columbia Gorge, gusting over 60 mph at the west end of the
     Gorge and to about 40 mph in the Portland area.  That pulled
     low-level Arctic air, from the Columbia basin, into
     northwestern Oregon.  Temperatures dropped into the 20s,
     from about Salem northward in the Willamette Valley, as
     abundant warm and moist air circulated onshore over the top
     of the cold dome.  That brought a substantial amount of snow
     and ice to northwestern Oregon.  There was enough warming
     aloft, by Saturday night, for the snow to turn to sleet and
     freezing rain across much of the northern Willamette Valley,
     with many areas receiving about one-quarter of an inch of
     ice over the top of 6-10 inches of snow.  Meanwhile, the
     cold Arcitc air was not able to penetrate southward in the
     Willamette Valley much beyond Salem, where just plain rain
     fell from this system.

     The storm was not able to scour out the low-level cold air
     from the Columbia Basin nor from over northwestern Oregon. 
     As the associated upper-level trough approached the
     coastline Sunday, cooling aloft wiped out the above freezing
     layer of air over the Arctic dome and turned precipitation
     mostly back to snow over northwestern Oregon.  Most areas
     received another 2-3 inches of snow accumulation, with the
     snow showers Sunday afternoon and evening.  Once again,
     areas south of about Salem received plain rain showers with
     temperatures just above freezing.

     A summary of snow and ice totals for western Oregon is
     available, from the Portland National Weather Service, at:
     http://www.wrh.noaa.gov/total_forecast/getprod.php?wfo=pqr&pil=PNS&sid=PQR

     The cold upper-level trough will move over western Oregon
     this morning.  Shower acticity increased eaarly this
     morning, and satellite and radar imagery showed an organized
     band of showers rotating into western Oregon...mainly just
     south of Portland to Eugene.  The surface analysis showed a
     low-pressure center dropping south, along the coastline, to
     near Newport.  That was helping to draw the Arctic air
     further south, into the Willamette Valley, than it has been
     during this entire episode.  Temperatures were nearing the
     freezing mark in Corvallis and had dropped into the upper
     30s in Eugene.  Rain could mix with or turn to freezing
     rain, sleet, and/or snow in the Albany and Corvallis areas
     later this morning.  It is less likely, but mot impossible,
     that some wintry precipitation could also make it as far
     south as Eugene.

     The bottom line is that it will stay well below freezing in
     the north valley, with snow showers likely today.  In
     addition, cold air could spill far enough south today to
     bring wintry precipitation to areas that have not yet seen
     much, if any, from this event.  Mid-morning Willamette
     Valley temperatures were in the mid to upper 20s, from Salem
     north to Portland, in the mid 30s in the Albany/Corvallis
     areas, and in the upper 30s in Eugene.  Most valley
     temperatures will hold steady or fall during the day today,
     as the upper-level trough moves over the region.

     The north coast will also likely see snow showers today with
     snow showers possibly extending as far south as Lincoln
     City.  Rain showers will fall along the central and south
     coast.  Locally heavy snow showers will continue over the
     coast range and the Cascade passes today.  Showers will
     taper off across all of western Oregon tonight, as the
     upper-level trough pushes into eastern Oregon.  Overnight
     temperatures should range from the low 20s near Portland to
     the low 30s near Eugene.

     The latest warnings, advisories, and forecasts, from the
     Portland National Weather Service, are available at: 
     http://www.wrh.noaa.gov/pqr/

Surface Winds:
     This Morning...N 5-10 north valley; S 5-10 south valley
     This Afternoon...SW 5-10 south valley; NW 5-10 north valley
Transport Winds:
     S 15 this morning, NNW 10 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 1800 feet.  Ventilation index 27.
High Temperature:
     Salem\'s high temperature today will be near 36.
Humidities:
     Minimum relative humidity will be near 82%.
Sunrise/Sunset:
     Salem sunset tonight: 4:34pm; sunrise tomorrow: 7:48am.

Extended Outlook:
     Tuesday should bring a small break in the weather, with a
     transitory ridge of high pressure bringing some partial
     clearing to western Oregon.  The cold air mass will remain
     in place over the region with high temperatures not likely
     goin above freezing in the Portland area and struggling into
     the mid to upper 30s in the central and southern Willamette Valley.

     The next significant weather event will involve a system
     dropping into the region, from the Gulf of Alaska, Tuesday
     night and Wednesday morning.  Enough cold air will be in
     place, ahead of this storm, for precipitation to begin in
     the form of snow for most of the Willamette Valley,
     especially from Salem north.  However, this system had the
     potential to turn the surface winds southerly in the valley
     by Wednesday afternoon...warming the lowest levels above
     freezing and lifting the snow level to about 500-1000 feet. 
     That transition will be slowest in the Portland area, where
     significant snow and ice may first accumulate.

     A cold upper-level trough will bring showers to the region
     for Christmas, with the snow level between 500 and 1000
     feet. It will likely be a White Christmas for the north
     valley, because of the snow already on the ground.  Locally
     heavy new snow is possible in the coast range and in the
     Cascades.  Another system will move onshore Friday.  It will
     pull some easterly winds, from the Gorge, ahead of it.  If
     enough cold air is still trapped there, then precipitation
     may begin in the form of snow in the Portland area and in
     the Gorge.  This system appears strong enough to fairly
     quickly lift the snow level to about 3000 feet by Friday
     afternoon.

     A strong westerly jet stream is forecast for next weekend
     and early next week with rain in the valleys and snow for
     the mountains.  Snow level will progressively drop Sunday
     and Monday to as low as 1000 feet in the north. 
     Temperatures will return to near seasonal normals.

Tomorrow (23 Dec):  Mostly Cloudy.  Slight Chance of Snow Showers. 26/35

Wed (24 Dec):  AM Snow North; Rain or Snow South. PM Rain South...Wintry Mix North. 28/37

Thu (25 Dec):  Showers.  Snow Level 500-1000 Feet...near the Surface in the Gorge.  32/38

Fri (26 Dec):  Rain or Snow North...Turning to Rain.  Rain South.  Snow levels Rising. 32/39

Sat (27 Dec):  Rain at Times.  Snow Levels near 4000 Feet. 39/46

Sun (28 Dec):  Rain at Times.  Snow Levels 3-4000 Feet. 37/44

Mon (29 Dec):  Rain at Times.  Snow levels 1-3000 Feet.  36/43

ODA Meteorologist
weather at oda.state.or.us