Oregon Chapter
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Past Weather

Surprise Portland Metro Area Snowstorm - December 2009

Steve Pierce
Oregon AMS
stevejpierce@comcast.net

Headlines:
Quick hitting storm blankets NW Oregon and SW Washington with as much as 7" of snow in 8 hours, catching many by surprise.

Dec 29, 2009 Sounding

Forest Grove, OR

On the morning of December 29, 2009 a frontal system approached the Pacific Northwest from the southwest after cold, dry air funneled through the Columbia River Gorge for several days prior. With plenty of radiational cooling, overnight lows on the morning of the 29th dipped into the low to mid 20's around Portland. By 12z on the 29th, 4km mesoscale model data was insistent that cold air from the eastern end of the gorge would be transported westward into the Portland area by mid afternoon. In fact, about 6 hours prior to the onset of the precipitation, 4km model sounding data at the time showed a very sharp evaporative cooling episode would take place, pushing temperatures from the upper 30's to around freezing in a matter of about 2 hours. Precipitation first started in the form of a mix of rain, snow and sleet near Salem and points to the south and west about 12 noon. By 2pm the precipitation had reached Portland in the form of snow. By 4pm, the 4km model sounding data from 12z as well as the previous nights 0z run had correctly forecasted the 5-7 degree evaporative cooling that would take place over Portland as the precipitation began. This fact, coupled with low level easterly flow continuing through the gorge all but assured that precipitation would remain as snow across most of the Portland area.

The evening commute around Portland was the worst some officials had ever experienced, dating back more than 20+ years. The evening commute was still underway as late as 11pm in many locations, especially on the west side of Portland which was hit the hardest by the storm due to a deeper layer of cold air that typically banks up against the east side of the coast range mountains in situations like this. By 10pm the system had passed to the east of the metro Portland area and temperatures slowly began to moderate at upper levels, changing the remaining moisture to rain at lower elevations. This storm was just another example of the fine line that Portland dances on every time there is the possibility of snow. In the words of one area weather forecaster, "it is rarely a slam dunk when it comes to snow in Portland." Although this storm will likely be remembered by many as the "Portland Nightmare Commute 2009" it will also be a great meteorological case study in an attempt to better forecast these events in the future. 

 

Here are some of the area's snowfall totals via the National Weather Service's weather spotter network ---

GALES CREEK           
7.0"
FOREST GROVE          
5.5"
CORNELIUS                
5.0"
PARKDALE               
5.0"
DOWNTOWN HILLSBORO      
5.0"
SW PORTLAND (HILLSDALE)  
4.0"
LAFAYETTE        
4.0"
GASTON         
4.0"
SW PORTLAND (GARDEN HOME)   
3.2"
PORTLAND DOWNTOWN       
3.0"
DAMASCUS                    
3.0"
GRESHAM                     
2.8"
VERNONIA 5W                 
2.5"
STEVENSON, WA                
2.5"
VANCOUVER, WA                
2.5"
SHERWOOD                    
2.5"
HAPPY VALLEY                
2.5"
NE PORTLAND (NWS OFFICE)    
2.5"
SALEM 5NW                   
2.0"
NE PORTLAND (LAURELHURST)   
2.0"
OREGON CITY                 
2.0"
PORTLAND AIRPORT            
1.8"
KALAMA, WA                   
1.5"
VERNONIA                    
1.5"
KEIZER                      
1.5"
CLACKAMAS                   
1.3"
BATTLE GROUND, WA            
1.2"
CAMAS, WA                    
1.0"
LEBANON                     
1.0"
HOOD RIVER                  
1.0"
CASCADE LOCKS               
1.0"
BRUSH PRAIRIE, WA            
0.5"
CORVALLIS 0.25"
HOCKINSON, WA                
0.2"