Winter 2020-2021 was basically a no-show at the start. But when we got to Mid-January the northern gates opened, and true arctic air invaded The Plains, Midwest and East Coast. The cold brought by the Sudden Stratospheric Warming Event in Early January with the displaced Polar Vortex help setoff several major winter storms for the Middle Atlantic and a few blockbuster storms for the Northeast.
If you want to look at my 2021-2021 winter
outlook, you can find it here.
Why discuss winter when doing a spring outlook? Because, Spring is a transition between winter and summer. Spring can be greatly influenced by the winter that went before it.
If you don’t want to read all the meteorological analysis, you can skip to the bottom.
Spring 2021:
Meteorological Spring will start March
1, 2021; with astronomical Spring officially arriving at 5:37 a.m. EST on March
20, 2021.
Teleconnections:
Below is a look at Sea Surface
Temperatures
The El Nino-Southern Oscillation (ENSO):
it is the strongest year-to-year fluctuation pattern of the global climate
system. The ENSO has three phases: the warm extreme is El Nino, the cool
extreme is called La Nina, and neutral meaning, neither La Nina nor El Nino.
The ENSO is a combination of changes in the ocean and atmosphere that affect
weather in many areas of the world.
El Nino (Sea Surface
temperature anomaly, >0.5C)
La Nina (Sea Surface temperature
anomaly, <-0.5C)
Neutral (Sea Surface temperature
anomaly, within +/-0.5C of average)
During a La Nina event, SST in the equatorial Pacific water from off the coast
of South America to the central tropical Pacific cools to below average
temperatures. This cooling occurs because of stronger than normal easterly
trade winds lead to upwelling which brings cooler, deeper sea water up to the
ocean’s surface. Sea temperatures can warm above average in the far western
Pacific when this happens. The unusually cool water in the eastern Pacific
suppresses convection. Which also impacts weather patterns around the world,
but in a general reverse way than El Nino does.
For the entire winter we’ve had a
moderate to strong La Nina. ENSO is still in a La Nina
pattern. So, La Nina will be a major influence on the overall Spring
pattern.
The IRI plume seen below, has backed
off the idea for some kind of El Nino rebound. It is showing a slow weakening.
With the ENSO becoming neutral over spring heading into summer. It is possible that the ENSO will rebound back toward a La Nina later this coming fall..
The Arctic
Osculation (AO):
The AO is a climate index on the state
of the atmospheric circulation over the Arctic. Like all teleconnections it has
a positive and a negative state.
Positive phase:
During the positive phase the AO
features below average geopotential heights, which means the PV is strong. The
lower area of low pressure at the mid latitudes is higher (weaker), due to the
air travels from areas of high pressure to low pressure. This means the air flow is toward the arctic.
Because of this the westerlies and northeasterly trade winds are flowing much
stronger. The stronger winds keep the
cold corralled in the arctic. So, the
air over the lower 48 is warmer.
Negative phase:
In this phase there are above average
geopotential heights, which means the PV is weaker. The higher area of low
pressure at the mid latitudes is higher (weaker), due to the air travels from
areas of high pressure to low pressure.
This means the air flow is away from the arctic. The winds circling the arctic are weaker,
allowing for the PV meander north and south, allowing colder arctic air to
invade the mid and low latitudes. So
cold air overruns North America invading the lower 48.
Impacts on the Eastern U.S.
A positive AO is
associated with above-average temperatures
A negative AO is associated with below-average
temperatures and contributes to colder winters and an increase in nor’easters
(coastal storms) for New England states
North Atlantic
Oscillation (NAO):
The NAO is defined by the difference
in surface pressure between two atmospheric centers of action, the Icelandic
Low and the Azores High.
The NAO is one of the major players in
the climate variability in the Northeast and North Atlantic. It's these east
west oscillation motions, that represent a north/south swing in pressure across
the North Atlantic, that control the strength and direction of the westerly
winds and storm tracks.
When the NAO is in its positive phase,
we see upper-level ridging over the Eastern U.S. The ridge causes a predominant
south-westerly flow which brings warmer temperatures into the Eastern CONUS.
Impacts on the Eastern U.S.
Positive NAO:
it's associated with above-average
temperatures.
The NAO is also believed to have an
impact on the weather over much of eastern North America. During the winter,
when the index is high (NAO+), the Icelandic low draws a stronger south-westerly
circulation over the eastern half of the North American continent which
prevents Arctic air from plunging southward. This effect can produce
significantly warmer winters over the northeastern United States and
southeastern Canada.
Negative NAO
index:
Is associated with below-average
temperatures.
The Eastern Pacific Oscillation (EPO):
The EPO is a dipole pattern similar to
the NAO in the Atlantic, but located in the eastern Pacific. There is a
tendency for heights/pressures/temperatures to be higher to the north and lower
to the south in the negative phase and lower to the north and higher to the
south in the positive phase. The negative phase corresponds to widespread
cooling over central and eastern North America and the positive phase to
warming. During the positive phase we typically see negative geopotential
height anomalies, and a negative phase in which the opposite is true.
Negative EPO:
The EPO pattern opens the door to very
cold Arctic and Siberian air into the US. Here's an image that depicts the
negative phase of the EPO.
Positive EPO:
The weather pattern in the positive
phase is just the opposite of the negative, and so you have low pressure over
high pressure which gives the central and eastern US a different outcome.
For the entire winter we’ve had a
moderate to strong La Nina. The last 30 days have featured near and record cold
in the Central and Eastern CONUS. During that time, we had a negative North
Atlantic Osculation (NAO), negative Arctic Osculation (AO), positive Eastern
Pacific Osculation (EPO), and positive Pacific North American Pattern (PNA).
Pacific North-American ( PNA):
As for the PNA, it’s about the
variation of atmospheric circulation patterns over the Pacific Ocean and North
America. Like most of the other atmospheric circulations, the PNA has its
strongest impact on climate variables during the winter. These teleconnection
phases promote cold intrusions into the central and eastern CONUS. If they had
been the opposite then we could be talking about the warmest North American
winter on record.
Polar Vortex:
This is a strong wind flowing around a
low-pressure system normally present over the Arctic in winter. I think that
reads a little easier. The PV is
analyzed at 500 millibar level of the atmosphere. It can often be located over
Canada or Siberia since the coldest surface air is often found over these
regions.
Strong Polar
Vortex:
Strong is the more common state of the
polar vortex. When the polar vortex is strong, this creates strong low pressure
in the Arctic region. Because of the
pressure difference between the Arctic and mid-latitudes, air flows into low
pressure and this confines the cold air to high latitudes closer to the Arctic.
Therefore, it is often mild across the Eastern US, Europe and East Asia during
winters when the polar vortex is strong.
During strong polar vortex, the air
flow is fast and in a direction from west to east.
Low pressure in the Arctic region is
referred to as the positive phase of the Arctic Oscillation AO and the North
Atlantic Oscillation NAO.
Weak Polar Vortex:
When the polar vortex is weak, the
flow of air is weaker and meanders north and south (rather than west to east).
This allows a redistribution of air masses where cold air from the Arctic
spills into the mid-latitudes and warm air from the subtropics is carried into
the Arctic. This mixing of air masses also favors more storms and snow in the
mid-latitudes.
During a weak polar vortex, high pressure
occurs in the Arctic region and is referred to as the negative phase of the AO
and NAO. Air flows away from the high-pressure Arctic. The north to south
direction of the polar vortex carries cold Arctic air into the mid-latitudes of
Eastern US, Europe and East Asia. Therefore, it is cold across the Eastern US,
Europe and East Asia during winters when the polar vortex is weak.
Sudden
Stratospheric Warming (SSW):
what is an SSW? This is when the winds
in the PV slow down or reverse from the normally westerly direction. Frequently
when an SSW takes place, it displaces cold air in the mid and upper levels and
then build in the lowest layer of the atmosphere; once this happens it will
move southward into the continental U.S.
We had a stratospheric Warming Event
in Mid-December. Then we had a very strong SSW during the first week of
January. The result was the Arctic
Oscillation going strongly negative. The
cold shot during Mid-January into Mid-February was well telegraphed, which is
why I even mentioned that time period in my 2020-2021 winter outlook.
Current Teleconnection
setup:
During the first half of the 2020-2021
winter, it was very warm. A big reason for all that warmth was the Pacific jet
stream pushing mild air from the Pacific Ocean into north America. The positive EPO was one of the prime reasons
that happened.
The Madden–Julian
oscillation (MJO):
The MJO phase diagram illustrates the
progression of the MJO through different phases, which generally coincide with
locations along the equator around the globe. RMM1 and RMM2 are mathematical
methods that combine cloud amount and winds at upper and lower levels of the
atmosphere to provide a measure of the strength and location of the MJO. When
the index is within the center circle the MJO is considered weak, meaning it is
difficult to discern using the RMM methods. Outside of this circle the index is
stronger and will usually move in an anti-clockwise direction as the MJO moves
from west to east. For convenience, we define 8 different MJO phases in this
diagram.
Below are the GFS and EURO Ensembles phase
diagrams and the MJO Composite Maps for March April, and May.
The cold air is going to retreat into
northwest Canada. But don’t count on it staying there all the time. The pattern
does suggest we could see a cold snap during March. The flipping
teleconnections could be signaling a possible storm around that same time. We
will see!
If the MJO can get into phases 2 and 3. It would aid the negative NAO and negative AO, as those phases are very cold phases for this time of year.
The MJO is going to be important for March, if the MJO emerges into phase eight and then phase one; we would indeed see a return to some cold. This time of year, when the MJO is in phases eight and one we tend to see blocking across Northern Canada and troughing in the Eastern U.S. Arctic sea ice is below average but not as low as it has been in recent winters. Less sea ice favors a weaker PV. So, while the PV is strengthening it is also a bit elongated. It is also looking to drift toward Siberia. If this does happen, we could see the PV become displaced again, if it does it most likely wouldn't be as bad as we saw with the early January displacement, but that is no sure thing.
The Plains are going to see dynamic warming as we move forward. Here in the East the warming anomalies won’t be as vigorous, but it will still be very noticeable. When we compare analogs, we see a similar pattern affect as 1977 and 2011.
So, what does all
this mean?
We should continue to see an active
northern storm track. So, Precipitation across the northern half of the northeast
should be wetter than average.
The Climate Prediction Center’s new outlooks for spring (March-April-May) 2021, indicate warmer than normal temperatures and average to wetter conditions are likely to prevail over the Northeast and Middle Atlantic.
I expect to see the active stormy
pattern continue into March and April.
Typically, colder Februaries lead to
cooler Marches. But while March looks to
start out warm; I think as the atmosphere completely recovers from the SSW; The
second half of March should see a quick transition to well below average
temperatures for a short time as we close out March. But still, March will see more above 32°F days than
below average 32°F days. But due to the cold mid month. March
should see overall temperatures end up average to slightly below average. During April
we should see the La Nina starting a noticeable shift toward ENSO neutral. April should end up with overall temperatures
well above average. I expect to see
above average temperatures during May as well.
The best chance for any wintry weather
in March would favor the first half of the month.
We could see some cold air get into the pattern for the second half of March. But by April we will see overall temperatures becoming much more spring like.
Current Northeast Soil Moisture Anomaly
Below is a look at the latest Soil
Moisture Anomaly for the Northeast and Middle Atlantic. This is used by
NOAA/CPC for the purpose of drought monitoring and drought outlooks.
Flood Potential, isn’t really high for
the Northeast. But those along the New England Coast, back into eastern and
southern Pennsylvania, and the Middle Atlantic Region have a higher risk of experiencing
some flooding during the spring due to snow melt.
Spring severe
possibilities:
Winter 2020-2021 demonstrated the
power of analog forecasting techniques; they still have a place in modern
forecasting.
Typically, La Nina springs tend to see
more tornadoes than non-La Nina Springs. Here is a chart showing a correlation.
During Spring we see the clash of seasons, as cold dry air from the north collides with the warmer moist air from the south. March could see a few severe events. But April should become quite active. If that happens, this will be the third April in a row, the U.S has seen an above average severe month. May into June also could see well above average severe weather. I think the Mississippi Valley, Tennessee Valley and parts of the East will see an uptick in severe potential this spring due to dryness in parts of the Plains and points west. I think the Middle Atlantic Region will be at a heightened risk.
Most of us remember the spring of
2011. That year became one of the most active spring severe seasons in recorded
history. I’m not saying we will see a
spring severe season as terrible as 2011. But an above average April and June
tornado season could be a possibility.
I am using 1974 and 2011 as severe
season analogs.
If the pattern goes the way it looks, we could be heading for a hot summer; but too early to know for sure.
