The Extreme Winter of 2015

two storms  |  blocking  |  comparison  | anomalies
 

Lodovica Illari, March 15, 2015

UPDATE: Boston breaks seasonal snowfall record

TOP 5 SNOW SEASONS in Boston

#1 2014-15 108.6"
#2 1995-96 107.6"
#3 1993-94 96.3"
#4 1947-48 92.5"
#5 2004-05 86.6

Read more here (from The Weather Channel)

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Lodovica Illari, March 3, 2015

Why were there so many snow storms in Boston this year?

During January and February 2015 Boston experienced a record number of winter storms with record snowfalls for the season.

The large-scale flow over the US has been very unusual this winter and in an ideal configuration to favor winter storms over the East Coast.


                                                                                              
(Graphics by Bill McKenna, using IDV-Unidata)
Fig. 1:  Typical flow patterns over the US during February 2015. The 250 mb streamlines (white lines) show the location of the jet at a height of 12km or so, with a split flow over the west coast and a trough over the continental US. The 850mb temperature (color shading) shows the warm west and the cold east at lower levels in the atmosphere, at a height of order 1.5km.  In the eastern US, upper level anomalies (marked with the L) travel southward and then eastward along the jet. They interact with the warm surface waters over the Gulf Stream (marked by the W) and develop into surface lows along the east coast. A succession of these lows was responsible for the very high snowfall accumulation over the North Eastern US.

 
Two winter storms:
The Blizzard of Jan 27th and the Valentine's Snow Storm of Feb 15th, 2015

Here we examine the flow patterns associated with two of the major storms of the season.

The Blizzard of January 27, 2015                     The Valentine's Day Storm of February 15, 2015

   
a)

   
b)

   
c)

   
d)
Fig. 2: a) 250 mb wind, b) 500 mb height, c) 850 mb T and height, d) MSLP

There are marked similarities between the synoptic evolution in the two storms.

The jet stream (Fig. 2a) is zonal over the Pacific sector but splits in to two branches near the west coast and becomes very wavy over the continental US. This is due to the presence of a "blocking" dipole over the west coast - see the 500 mb maps in Fig. 2b. In both cases the jet is in an ideal configuration to bring warm air north over the west and cold air south over the continental US  - see Fig. 2c). Downstream of the Blocking High any disturbances are forced to move south towards the Gulf region. Here they interact with the warm ocean and develop in to strong storms over the east coast - see Fig. 2d, which shows a low pressure system approaching Boston both on Jan 27 and Feb 15. This repeatedly happened during the season leading to severe winter storms on the east coast and anomalously warm weather on the west coast.


The set up of the blocking pattern over the west coast

The Blocking dipole became established in the middle of January, the result of a very active Pacific storm track. A sequence of low pressure systems moved along the Pacific jet, growing along the way and maturing near the west coast. The following maps show storms in the Pacific on January 15th and January 20th approaching the west coast.

January 15, 2015                                             January 20, 2015

a)

    
b)

    
c)
Fig. 3: a) IR satellite images, b) MSLP, c) tropopause temperature maps on Jan 15, 2015 at 00z (lhs) and on Jan 20, 2015 at 00z (rhs)

Fig 3a present IR satellite images showing strong low pressure systems at the end of the Pacific storm track which advect warm moist air towards the west coast. This coincides with the establishment of the Blocking High - see the high pressure building up over the west coast in the MSLP, Fig. 3b. The process is even clearer in the tropopause maps, Fig, 3c, showing a pool of warm tropopause air (green/yellow contours) been advected towards the west coast. In response, the jet is displaced northward around the High. Storms are forced to move north around the high and then move south along the jet which is now beginning to orientate itself north-south. This is the ideal configuration to encourage storm development over the east, as shown in the following schematic.

Schematic diagram showing the ingredients required for a 'big storm'.


For more on cyclone development- see  here.


Was the winter of 2015 unusual? Comparison with previous winters.

Often in the winter time we observe pronounced ridge over the west coast and a trough downstream. The term RRR (Ridiculously Recurrent Ridge) was coined in 2013 - by Daniel Swain, a Ph.D. candidate at Stanford University - to label a ridge over the west coast that 'does not want to go away'. It was blamed for the persistent drought over the west coast. This year the ridge has been particularly pronounced and was positioned further downstream of the Rockies. Utah, Nebraska and even Colorado were the affected by the high pressure system with record high temperatures for winter 2015.

Note the very warm air over the southwestern US reaching up to Utah and Nebraska. Some cities in the west reached record high temperatures this winter. For example Salt Lake City, UT, Boise, ID and even Seattle. See also mean and anomaly for February 2015.

The warmth over the western US is the precursor to a stormy east coast. The persistent high pressure over the west forces the jet stream to flow south and then curve back north over the east coast, setting the scene for severe weather. Low pressure systems along the northward flowing jet gain energy from the warm ocean and matures over our region. Here in Boston very cold arctic air was already in place leading to the precipitation falling as snow. See, for example, the flow on Jan 27, when Boston experienced the first major snow storm of the season.

The charts below compare the synoptic situation on January 27th of this year with Jan 23rd of 2005.

January 27, 2015 at 00z                                    January 23, 2005 at 12z
   
a)

   
b)

   
c)
Fig. 4: a) tropopause temperature, b) 850 mb temperature, c) MSLP

On January 27th of this year we see a blocked flow with a warm tropopause over the west coast, accompanied by a pronounced cold tropopause over the east coast. This results in a low pressure system developing over the east coast. A warm west and a cold snowy east go together. In fact severe Winter Blizzards over New England are often associated with blocking over the west coast and a wavy meridional jet over the rest of the continental US.

For example let's go back to January 2005, another very cold and snowy month. On January 23, 2005 Boston was hit by a major blizzard - the rhs of Fig. 3. The pattern looks strikingly similar to the January 27th, 2015 blizzard shown on the lhs.

Is this year's extreme weather related to global warming?

We know from the work of Paul O’Gorman that as the world warms we would expect to observe a decrease in average snowfall but extreme snowfalls are likely to increase. So we should not be surprised to experience increased heavy snowfall. His argument is based on thermodynamic considerations and a statistical analysis of IPCC models outputs.

What about changes in the circulation patterns and dynamics? Has the jet position changed to favor blocking patterns and has the occurrence of blocking events increased? 

Looking over the past 30 years of NCEP reanalysis it does not seem the case …Martin Singh paper for 12.804 did not see any increase in blocking events over the past 30 years. But did we look properly? Elizabeth Barnes found the same -see Barnes (2013).

Jennifer Francis says it is the Arctic amplification that gives a wavy jet stream - see http://news.rutgers.edu/news/climate-change-driving-brutal-winter/20150217#.VOqWSHzF_E0

This is an interesting hypothesis, but this winter we have noticed a very strong zonal Pacific jet and a wavy meridional jet down stream of the blocking pattern…..It is not wavy everywhere, it looks more  local.

What about the warming of the tropical pacific? We are entering an El Nino phase....

This winter the Pacific was unusually warm – we even had the most eastern typhoon ever….

The first two months of the season were unusually active and intense. Mekkhala became an early-forming storm of the season and affected the Philippines. The typhoon had affected the Pope's recent visit to the Philippines. Less than a month later, Typhoon Higos had become the easternmost forming Pacific typhoon as well as being among the strongest February typhoons of record. Despite its intensity, Higos did not cause any significant effects over the landmasses and islands on the West - see http://en.wikipedia.org/wiki/2015_Pacific_typhoon_season#Typhoon_.28Amang.29

Typhoon Amang on January 19th occurred around the same time as the blocking is forming….

My sequence of events is:

enhanced baroclinicity over the Pacific -->
more cyclones activity along the Pacific Jet -->
large mature cyclones at the end of the jet transport tropical air northward -->
they create the Blocking High -->
the jet is forced to split and re-curves south over the US -->
anomalies along the jet move southward over the warm surface over the Gulf region -->
surface cyclones forms over the warm surface anomaly -->
upper and surface cyclones interact (mutually amplify?) and superimpose -->
explosive cyclogenesis or "bombogenesis" occurs

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Links:

Illari L. and Sugiyama, M.: Cyclone development and Tropopause maps  poster)

Tropopause maps: MIT_loop

Dr Jeff Masters Wunder Blog  - See Snow-Gripped in New England, Wet in the West, and Toasty in Between

Barnes, E.2013: Revisiting the evidence linking Arctic amplification to extreme weather in midlatitudes..

Francis, J.A. and S.J. Vavrus, 2015: Evidence for a wavier jet stream in response to rapid Arctic warming

O'Gorman, P., 2014: Snowfall in a warmer world. MIT News
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two storms  |  blocking  |  comparison  | anomalies
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