The Extreme Winter of 2015
two storms |
| comparison |
Lodovica Illari, March 15, 2015
Boston breaks seasonal snowfall
TOP 5 SNOW SEASONS in Boston
here (from The Weather Channel)
Lodovica Illari, March 3, 2015
Why were there so many snow storms in Boston
During January and February 2015 Boston experienced a
record number of winter storms with record snowfalls for the
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
examine the flow patterns associated with two of the major storms of
The Blizzard of January 27, 2015
The Valentine's Day Storm of February 15, 2015
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
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
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
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
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
January 27, 2015 at 00z
January 23, 2005 at 12z
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
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
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
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
Jennifer Francis says it is the
Arctic amplification that gives a wavy jet stream - see
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….
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
Less than a month later, Typhoon Higos
had become the easternmost forming Pacific
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
Typhoon Amang on January 19th
occurred around the same time as the blocking is forming….
My sequence of events is:
enhanced baroclinicity over the
more cyclones activity along the Pacific Jet -->
large mature cyclones at the end of the jet transport tropical air
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
Illari L. and Sugiyama, M.:
Cyclone development and Tropopause maps
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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