[willamette-fcst] Forecast - Tuesday, December 23rd, 2008

Tue Dec 23 12:00:07 PST 2008

Daily Smoke Management Forecast

Oregon Department of Agriculture
Smoke Management Program
Weather Outlook and Field Burning Advisory for Willamette Valley Growers and Fire Districts.

NOON UPDATE

Issued: 
     Tuesday, December 23rd, 2008 at 12:00pm.

Burn Advisory:
     Agricultural burning is not recommended.
     Stack burning is not allowed.

Weather Discussion:
     Snow showers tapered off Monday afternoon, but not before
     dropping several more inches of snow over northwestern
     Oregon, adding to the most significant snowfall for the
     region since the winter of 1968-69.  The cold Arctic air was
     drawn a little further south, in the Willamette Valley,
     Monday, with Corvallis finally seeing about one inch of
     snow.  Eugene bounced in and out of the cold dome of
     low-level air Monday, with temperatures dropping into the
     mid 30s and then climbing to the mid 40s.

     A comprehensive summary of snow and ice totals, since
     Saturday morning, for western Oregon and southwestern
     Washington is available, from the Portland National Weather
     Service, at:

     http://www.wrh.noaa.gov/total_forecast/getprod.php?wfo=pqr&pil=PNS&sid=PQR

     A weather system offshore was headed mainly towards
     California late this morning, but it has dropped the surface
     pressures enough to create northeasterly gradients across
     western Oregon.  The surface analysis showed that the Arctic
     cold front was still south of Eugene late this morning, with
     near-freezing temperatures in the south valley and
     north-valley readings in the upper 20s.  The low-level cold
     air is very shallow over Eugene and progressively deeper
     down the valley, towards Portland, and deeper yet in the
     Columbia Gorge.  That will play in big role when the next
     Pacific storm makes it into the region Wednesday morning.

     Satellite imagery late this morning showed a large area of
     clouds off the Oregon and northern California coastlines,
     moving southeast towards California.  Some clouds and light
     snow flurries were making it as far north and east as the
     Willamette Valley.  Some sunbreaks were showing up along the
     northern Oregon Coast, but skies remained generally cloudy
     across the state at midday.

     Northwestern Oregon should get a brief break from the heavy
     snow this afternoon, although skies will stay mostly cloudy.
     Some snow flurries are likely along the north coast and in
     the Willamette Valley.  Some sprinkles are likely along the
     central and south coast.  Light northeasterly gradients will
     continued to keep cold air in place across the entire
     Willamette Valley today, with high temperatures only
     climbing into the mid 30s in the south valley and staying
     below freezing from Salem north to Portland.

     Many roadways acrosss the region remained snow-packed this
     morning, with some travel restrictions are in place.  Some
     minor melting of snow is possible during that afternoon,
     which could make snow-packed road surfaces even more slick. 
     Before traveling, please check the latest road conditions
     and requirements, from the Oregon Department of
     Transportation, at:

     http://167.131.0.179/Pages/RCMap.asp?curRegion=1&mainNav=RoadConditions

     Cloudy skies are expected tonight with one system headed
     into California and another moving southward, along the
     British Columbia Coastline, towards Washington and Oregon. 
     There is an increasing chance for light snow, in the
     pre-dawn hours Wednesday, along the north coast and in the
     northern Willamette Valley.  The National Weather Service
     has issued a Winter Storm Watch for Wednesday, in the
     greater Portand area, where the potential for more heavy
     snow exists.  More on that in the extended discussion...

     The latest weather warnings, advisories, and forecasts, from
     the Portland National Weather Service, are available at:

     http://www.wrh.noaa.gov/pqr/

Surface Winds:
     NE 3-6 this afternoon.
Transport Winds:
     NE 4 this afternoon.
Atmospheric Ventilation Conditions:
     Maximum mixing height today will be near 800 feet.  Ventilation index 3.
High Temperature:
     Salem\'s high temperature today will be near 31.
Humidities:
     Minimum relative humidity will be near 78%.
Sunrise/Sunset:
     Salem sunset tonight: 4:35pm; sunrise tomorrow: 7:49am.

Extended Outlook:
     The next significant weather event will involve a system
     dropping into the region, from the Gulf of Alaska on
     Wednesday.  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. 
     This system has the potential to turn the surface winds more
     southerly in the valley by Wednesday afternoon, which would
     eliminate the cold influence of the Columbia Gorge winds and
     warm the lowest levels of the atmosphere above freezing. 
     That would lift the snow level to about 500-1000 feet over
     most of the valley.  However, that transition may be very
     slow in the Portland area, where cold outflow from the
     Columbia Gorge could enable significant snow and ice to
     accumulate, before the warmer southerly winds lift surface
     temperatures abve freezing.

     The system coming in Wednesday does not have as much warm
     air aloft associated with it, as the one over the weekend,
     so the threat of significant freezing rain is not as great. 
     Most likely, precipitation will start as snow in the valley
     and transition mostly to plain rain, wher the warmer
     southerly winds send surface temperatures above freezing. 
     Snow levels are not expected to climb above 2000 feet,
     before dropping back to about 500-1000 feet by Wednesday
     night.  There could still be areas of briefsleet or freezing
     rain, but I would not expect significant ice accumulations
     from this storm.  On the other hand, if the transition to
     warmer air is slow, in the north valley, significant snow
     accumulations are a real possibility.

     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 levels will be around 3-4000 feet.
     Temperatures will return to near seasonal normals.

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 3000 Feet.  36/43

Tue (30 Dec):  Rain at Times.  Snow Level 4000 Feet.  38/47

ODA Meteorologist
weather at oda.state.or.us