The Contrasting Winters of the Past Two Years
The first half of the winter season is now behind us. Thus far it is apparent that this winter
has been both colder and snowier than the previous winter season for southwestern Ohio, northern
Kentucky, and southeastern Indiana. There are a couple of particular synoptic pattern differences
noted so far this winter, that when compared to the same time period last year, give us a hint as
to why these two time periods have been so different.
The 500 hPa mean flow across the Northern Hemisphere in the first half of the 2011-2012 winter
featured a more northern position of a Hudson Bay polar vortex, which includes a trough extending
southward toward the Great Lakes Region. This resulted in more zonal flow upstream. This type of
pattern would have resulted in bringing storm systems to the area with Pacific maritime origins.
The same time period of 2012-2013 depicts a farther south centering of a Hudson Bay polar vortex
and accompanied by a more amplified trough across our region. Upstream flow at the middle levels
of the troposphere has therefore been
more northwesterly comparatively to last winter thus far. This has helped to bring storm tracks of a
more polar origin to our region this winter.
Side by side images of 500 hPa mean geopotential heights for December 21st through February 7th,
2011-2012 (left), and 2012-2013 (right). These images depict the differences in the mid level flow pattern
during the two time periods.
Surface pressure anomalies from the 2011-2012 winter season indicate that the low level flow favored
frequent intrusions of warmer air into Ohio Valley. In contrast, the surface pressure anomalies thus far
this winter in our region are essentially non-existent. A lack of surface pressure anomalies could be
indicating that an equal blend of both northerly and southerly flow has been occurring since December 21st.
Pressure reduced to sea level departure from normal for December 21 through February 7,
2011-2012 (left) and 2012-2013 (right). The pressure gradient resulting from such an anomaly during the
start of the 2011-2012 winter suggests low level southerly flow was dominating the region.
The comparisons of these two patterns just from these images would suggest a warmer start for the
2011-2012 winter compared to the 2012-2013 winter. The result of the above patterns becomes much more
apparent when we look at the mean temperatures, precipitation, and snowfall across the region that
occurred during these two time periods.
Temperatures across the Wilmington forecast area were averaging up to 6
degrees F above normal during the first half of the 2011-2012 winter season.
As expected, the low level southerly flow that appeared to persist for the majority of the time period
to start off the 2011 to 2012 winter season resulted in well above average temperatures. What some may
forget is that the region actually received near to above normal precipitation during this same time period.
The fact of the matter was that most precipitation fell in the form of rain rather than snow due to
the presence of a warmer air mass. The beginning weeks of the 2011-2012 winter was the 12th warmest start
on record for the Cincinnati area, and the 9th warmest for the Columbus area. The 2011-2012
meteorological winter ended up being one of the warmest on record for our area. To no surprise then,
our snowpack was lacking during this same time period (see images below).
This winter season has been fairly close to normal with regards to precipitation and temperature, and
above average thus far for snowfall. The result of more snowfall has been a healthier snow pack going
into the month of February. What to expect from here on out? The signals over the next 10 days indicate
that not much warming is on the way, as the low over the Hudson Bay and mean trough across the Great Lakes
will continue to keep the door open for cold air to move into our region over the next several days.
Calculated snowpack map from February 1, 2012 depicting a snow free Ohio Valley.
Calculated snowpack map from February 1, 2013 showing a stark contrast to the previous winter.
The chart below shows a side by side comparison by the numbers of last winter's start compared
to this winter's start (December 21 - February 7):
Pressure and temperature images courtesy of National Centers for Environmental Prediction, and
the Midwestern Regional Climate Center.
Snow map images courtesy of the National Operational Hydrologic Remote Sensing Center
Page created by Andy Latto