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Thundersnow

Portland, OR

January 10 to 11, 2017

Storm Discussion Outline

Storm highlights

Snow fell in Portland, OR, Vancouver, WA and the surrounding metro areas from late Tuesday afternoon on January 10 through Wednesday morning January 11. The snow increased rapidly in coverage and intensity from 3 PM to 6 PM PST. The heaviest snow fell from 6 PM to around 12 AM PST. Thunderstorms helped boost snowfall rates to 1 to 2 inches per hour during the mid to late evening as noted in the Area Forecast Discussion (AFD) issued by the National Weather Service Forecast Office in Portland at 10:04 PM PST. The F6 Preliminary Local Climatological Data Report shows that snow accumulated 6.5 inches at the Portland International Airport by 12 AM PST. Lighter snow fell through the overnight and daylight morning hours producing an additional 1.5 inches for a two day total of 8.0 inches. Snowfall measurements reported to the National Weather Service showed that 10 to 15 inches accumulated in many locations of Portland, Vancouver, and surrounding hills. A location in downtown Portland reported a snow total of 11.8 inches. That's impressive.

Radar images, surface analysis maps, and upper air maps give an overview of the storm's evolution and structure. Inspection of these various sources of weather data, plus weather observations at the Portland International Airport, reveal certain features of the storm critical to snow production.

Radar summary

The first four Pacific Northwest radar mosaics from the National Centers for Environmental Information show the rapid development and intensification of precipitation from 3 PM to 6 PM PST. The last two images show the snow dissipating late the following morning.

3 PM PST Tuesday, January 10, 2017


4 PM PST Tuesday, January 10, 2017


5 PM PST Tuesday, January 10, 2017


6 PM PST Tuesday, January 10, 2017


9 PM PST Tuesday, January 10, 2017


12 AM PST Wednesday, January 11, 2017


3 AM PST Wednesday, January 11, 2017


6 AM PST Wednesday, January 11, 2017


9 AM PST Wednesday, January 11, 2017


12 PM PST Wednesday, January 11, 2017

Surface analysis

A series of surface maps of the United States and Southern Canada from the Weather Prediction Center of the National Weather Service shows the storm moving onshore. At the same time, a ridge of high pressure to the northeast is pushing cold air into the Portland, OR and Vancouver, WA region.

1 PM PST Tuesday, January 10, 2017


4 PM PST Tuesday, January 10, 2017


7 PM PST Tuesday, January 10, 2017


10 PM PST Tuesday, January 10, 2017


4 AM PST Wednesday, January 11, 2017


10 AM PST Wednesday, January 11, 2017

Upper air storm structure at early stage of snowstorm

Upper air maps from the Storm Prediction Center for the 925 mb, 850 mb, 700 mb, 500mb, and 250 mb pressure levels are valid at 00 UTC January 11 (4 PM PST January 10). The storm is near vertically stacked with closed low pressure circulations at 925 mb, 850 mb, and 700 mb, and open troughs at 500 mb and 250 mb near the same geographic location of the surface low. The high pressure system at the surface is present at 925 mb and 850 mb, but then erodes into a trough of low pressure at 700 mb and above. The trough is the southwest extension of a large mid to upper level trough covering central and western Canada and extending into the adjacent part of the United States. At 250 mb a Pacific jet streak (wind maximum) is flowing across central and northern California. A stronger jet streak at the base of the large trough is located from Idaho and western Montana to western Nebraska.

925 mb, 4 PM PST Tuesday, January 10, 2017


850 mb, 4 PM PST Tuesday, January 10, 2017


700 mb, 4 PM PST Tuesday, January 10, 2017


500 mb, 4 PM PST Tuesday, January 10, 2017


250 mb, 4 PM PST Tuesday, January 10, 2017

Storm characteristics critical to the production of snow

The following storm features are discussed concerning their contribution to enabling the production of snow.

Atmospheric pattern conducive to cold air outbreaks in the Pacific Northwest

Cold air outbreaks in the Pacific Northwest of the United States are frequently characterized by the presence of a strong upper level low pressure trough covering southwest Canada, the Pacific Northwest of the United States, and the adjacent Pacific Ocean waters. If a strong, cold surface high pressure system covers the high plains and Rocky Mountains of Canada, extending into the northern United States, the trough allows the westward movement of cold air toward the coast. The cold air must usually wind its way west through the mountain valleys and intermountain basins to get into places like Portland, OR and Vancouver, WA. In the case of this particular storm, the upper trough appears farther east than an ideal location, although it does have a weaker southwest extension to the Pacific Coast. The high pressure system is not so strong at 1020 mb to 1025 mb over southwest Canada. Would like to see stronger than 1040 mb.

An important observation to note is the evolution of the high pressure system several days before the storm occurred. Another sequence of surface maps from the Weather Prediction Center, for 00 UTC January 7 (4 PM PST January 6) to 00 UTC Jan 11 (4 PM PST January 10), covering all of North America, shows that the high pressure system was initially much stronger in Alaska, the Yukon, and the Northwest Territories. Pressure center at 00 UTC January 7 was over 1050 mb. The high pressure system then weakened as it expanded south through British Columbia and Alberta and then into the United States. Cold air had several days prior to the onset of the snowstorm to work its way into a favorable position to support snow.

4 PM PST, January 6, 2017


4 PM PST, January 7, 2017


4 PM PST, January 8, 2017


4 PM PST, January 9, 2017


4 PM PST, January 10, 2017

East winds from a cold high pressure system located to the northeast

East winds around the periphery of the surface high pressure system pushed colder and drier air into Portland, Vancouver, and vicinity. Funneling through the Columbia River Gorge intensified the influx of cold air. Surface observations for the Portland International Airport, show that a brief shot of light rain fell early in the afternoon. After a break in precipitation, snow started between 5 PM PST and 6 PM PST. The temperature held fairly steady through the afternoon at 2 to 3 o C ( roughly around 37 oF) but dew points between 3 PM PST and 5 PM PST dropped from 0 to -3 oC (32 to 26 oF). The shot of drier air correlates with a wind shift from the southeast (around 120 deg) to the east (around 80 degrees). Once snow started to fall, the temperature fell to near freezing due to the dry air and melting snow.

The following is a list of the referenced surface observations. Bold text is used to emphasis the more important weather elements. For time clarity, the PST time for the observations was added in parentheses following the Zulu (Z) time. Zulu (Z) is the same as Universal Coordinated Time (UTC). The surface observations are available from the Aviation Weather Center of the National Weather Service.

KPDX 110553Z (9:53 PM PST) 09021G25KT 1/2SM R10R/3000V3500FT -SN VV005 M01/M01 A2941 RMK AO2 PK WND 10030/0501 SLP959 4/004 P0011 60035 T10061006 10033 21006 58000

KPDX 110453Z (8:53 PM PST) 10020G26KT 1/2SM R10R/4000V5000FT -SN VV006 00/M01 A2940 RMK AO2 PK WND 10030/0443 SLP955 P0017 T00001006

KPDX 110353Z (7:53 PM PST) 09013G22KT 1/4SM R10R/2800VP6000FT SN VV010 01/M01 A2944 RMK AO2 TWR VIS 1/2 SLP967 P0004 T00061011 $

KPDX 110253Z (6:53 PM PST) 06007KT 9SM -SN BKN019 OVC039 01/M01 A2941 RMK AO2 PK WND 08029/0207 SLP959 P0003 60003 T00111006 58000

KPDX 110153Z (5:53 PM PST) 08008KT 9SM -SN FEW025 OVC045 03/M03 A2942 RMK AO2 PK WND 07029/0105 SNB31 SLP963 P0000 T00281033

KPDX 110053Z (4:53 PM PST) 08019G25KT 10SM FEW030 BKN070 OVC090 03/M03 A2941 RMK AO2 PK WND 09029/0038 SLP959 T00281033

KPDX 102353Z (3:53 PM PST) 08015KT 10SM FEW015 BKN060 OVC085 03/M02 A2945 RMK AO2 SLP971 60000 T00331022 10033 20022 5

KPDX 102253Z (2:53 PM PST) 12011KT 10SM FEW029 BKN060 OVC090 02/00 A2947 RMK AO2 RAE17 SLP979 P0000 T00220000

KPDX 102241Z (2:41 PM PST) 12009KT 10SM FEW012 BKN039 BKN065 02/00 A2948 RMK AO2 RAE17 P0000 T00220000

KPDX 102153Z (1:53 PM PST) 11012KT 10SM -RA FEW012 OVC029 02/00 A2949 RMK AO2 RAB27 SLP987 P0000 T00220000

KPDX 102053Z (12:53 PM PST) 12010KT 10SM FEW012 BKN026 OVC040 03/00 A2952 RMK AO2 SLP994 T00280000 56036

KPDX 101953Z (11:53 AM PST) 12012KT 10SM SCT012 BKN020 OVC027 02/01 A2955 RMK AO2 SLP004 T00220006

KPDX 101853Z (10:53 AM PST) 13009KT 10SM OVC014 02/01 A2959 RMK AO2 SLP019 T00220011

KPDX 101753Z (9:53 AM PST) 12010KT 10SM OVC013 02/01 A2962 RMK AO2 RAE00 SLP030 P0000 60008 T00220011 10022 20017 51008

The 00 UTC (4 PM PST) upper air observation for McNary Field (Salem Municipal Airport) in Salem, OR, approximately 50 miles south-southwest of the Portland International Airport, showed a general east wind flow extending up in height through 850 mb. The upper air station plot for Salem, OR is located in the northwest part of the state on the upper air maps presented earlier showing the storm structure near the start of the storm. The wind barb at 925 mb is from the east-northeast at 20 knots. The wind barb at 850 mb is from the southeast at 15 knots.

Pacific air riding over the cold air to supply moisture for the snowfall

The same upper air observation but higher up at 700 mb shows winds from the southwest at 40 knots from over the Pacific Ocean, indicating some overrunning of the colder air that is in place closer to the surface.

Frontogenetic forcing to enhance vertical motion

The Area Forecast Discussion (AFD) issued by the National Weather Service Forecast Office in Portland at 10:04 PM PST emphasized that frontogenetic forcing was enhancing vertical motion. Frontogenetic forcing is a process where the air is forced to rise as two air masses, usually denser cold air and warmer less dense air, are pushed into each other. The sequence of surface maps, previous shown, covering the United States and Southern Canada shows an increase in the pressure gradient from 4 PM PST to 7 PM PST as evidenced by the tightening of the isobars. The pressure gradient tightened as the surface low pressure system approached western Oregon. The flow of cold air from the east increased in response to the tightening pressure gradient. Warm air being drawn north by the low pressure system converged on winds from the east. The pressure gradient maintained itself through 10 PM PST as the low pressure system moved onshore, then weakened through 4 AM PST.

Coupled jet streaks favorable for upper-level divergence and synoptic scale (broad scale) vertical motion

A jet streak is a region of stronger winds embedded within the overall jet stream flow. The left front exit region and right rear entrance region of jet streaks are favorable locations for divergence to occur. When the left front exit region of one jet streak is positioned near the right entrance region of another jet streak, divergence is enhanced. The divergence induces vertical motion as air lower in height rises to replace the air that is exiting the region. The 250 mb pressure map, shown earlier, shows that northern Oregon and southern Washington state are near the right-rear entrance region of a jet streak centered over Montana and northern Wyoming. The area is also near the left-front exit region of a weaker jet streak centered over northern California. Maybe not the best positioning of the jet stream winds but the plotted wind barbs show stronger winds downstream indicating air is moving away from the area faster than it is moving in.

References and data sources

Climatological Data - Portland, OR F6 Preliminary Local Climatological Data Report

Radar mosaics - NOAA's National Centers for Environmental Information

Snowfall totals - Public Information Statement from the National Weather Service Forecast Office in Portland, OR issued at 6:40 PM PST Wednesday January 11, 2017

Storm analysis - Area Forecast Discussion from the National Weather Service Forecast Office in Portland, OR issued at 10:04 PM PST, Tuesday January 10, 2017

Surface maps - Weather Prediction Center of the National Weather Service

Surface observations (METARS) for Portland International Airport (KPDX) - Aviation Weather Center of the National Weather Service

Upper air observation maps - Storm Prediction Center of the National Weather Service