Jan 23: The Return of Winter

Jonathan Carr
By Jonathan Carr January 23, 2018 18:48

Jan 23: The Return of Winter

Discussion: This is a long range article about how the overall pattern is expected to return to colder and snowier conditions for February. This is not a detailed forecast for your back yard for a specific event in early February. Therefore if I see “How much snow for [INSERT LOCATION] on [INSERT DATE]?” questions in the comments, I might do a permanent eye-roll. I will then have to face down at my keyboard to see my monitor for the rest of my life. Let’s please try to avoid this and just accept the general nature of pattern discussion for now.

We’re currently wrapping up a warm and wet period that has been the most prolonged period of January thaw yet this month. From now until the first week of February I would expect back-and-forth alternating shots of cold and mild. I suspect that the warmer periods will be shorter and the colder periods will be longer. Overall however we are likely to return to the really cold stuff by the second week of February.

The first week of February should therefore act as the kick-off for a prolonged cold and wintry pattern. I’m looking at a few teleconnections and global oscillations which suggest such:

1) Arctic Oscillation (AO) – This is a measure of pressure near the polar regions. When pressure is low in said area, the polar jet stream is traditionally bottled-up to our N representing the positive phase of the AO. When pressure is high in said area, representing a negative phase AO, the polar jet can relax into a meridional waveform allowing the coldest air of the northern hemisphere to spill southwards into the US via troughing. This is required to have a guaranteed cold air source tap for a winter storm. The AO is expected to fall into a negative phase for the start of February and last into at least mid-February.

2) North Atlantic Oscillation (NAO) – This is a measure of blocking (high pressure) over Greenland. Blocking slows the speed of a winter storm down allowing for more snowfall. Without blocking (positive NAO phase), most systems are progressive and only produce up to 12 hours or so of snowfall. When blocking is in place (negative NAO phase), there is a much better chance of a major snow storm rather than just a significant snow storm. The NAO is expected to fall into a negative phase for the start of February and last for a while just like the AO.

3) Pacific North Atlantic (PNA) – This is a measure of pressure for the US west coast and parts of British Columbia. When the PNA is negative, there is traditionally low pressure producing a trough for the W US and high pressure producing a ridge for the E US (cold for W US and mild for E US). When the PNA is positive, there is traditionally high pressure producing a ridge for the W US and low pressure producing a trough for the E US (mild for W US and cold for E US). The PNA is indicating a transition to the positive phase for the start of February which would mean a cold trough of air for the E US including New Jersey.

4) Eastern Pacific Oscillation (EPO) – This is a phase indicator just upstream of the PNA region (W US) and usually coincides with the opposite of the PNA phase. For example a -EPO traditionally feeds a +PNA and vice-versa. This is important for establishing cross-polar flow from the Siberian region to the US. The EPO is indicating a negative phase for the start of February.

5) Madden-Julian Oscillation (MJO) – This is a measure of where the greatest tropical forcing is occurring along the equator, typically measured in the western equatorial Pacific. There are 8 phases which traditionally result in pattern-based temperature fluctuations for various parts of the US. The MJO is expected to go into phases 8 into 1 by the second week of February. This coupled with the -AO, -NAO, +PNA and -EPO pretty much scream a favorable pattern for winter storm development.

Essentially, this should set up a ridge over the W US, a deep trough over the E US and a ridge over Greenland. This extends the jet stream length for a surface low to form and track ideally for a snow storm…a storm that would not be able to progressively exit the east coast.  When everything aligns like this, a surface storm is almost guaranteed to form under the greatest area of divergence and difluence on the front side of the trough (under the jet stream). It’s all a matter of exact positioning once the system is captured by model consensus and live observations.

Again, no specific winter storm system is currently showing in the first half of February. But the overall pattern tells me we’ll probably be focusing on a few winter storm signals once the surface catches up to the upper-level atmospheric changes. For lottery/dice-rolling sake I like Feb 4-6 as the first period to watch. Until we’re within 7 days of a system however, there will be no details. Just pattern observation and analysis.

In English: A cold front has pushed the remaining rain offshore. We’ll now get colder for a few days followed by a milder weekend. We’ll then bounce back and forth again next week. I don’t see any major wintry systems during this time. Therefore January should finish uneventful. February however has multiple long-range pattern indicators that winter will likely be returning…as early as the first week of February. To say winter is over at this point in time is a complacent and foolish statement IMO. Be safe! JC

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Jonathan Carr
By Jonathan Carr January 23, 2018 18:48

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