This winter has been tough.
In December we dealt with blocking high pressure along the northwest coast of North America. This drove arctic cold deep into the eastern United States. This cold outbreak lasted into the first week of the new year before a warm stretch arrived in mid-January. Our storm systems were dry and moisture deprived for the first half of winter.
We had a brief episode of western troughiness which lead to one or two storms that were able to interact with gulf moisture. It was warmer. The pattern shifted back towards cold by the end of January, albeit with a bit more westerly component. For late January into early February pattern was cold but not as cold as late December, arriving in shots. Over the last 12 days the pattern has been dynamic and interesting. Chicago observed snow on 9 consecutive days!
It was looking pretty certain that February would be very cold in the east with a return to ridging in the west. It was supported by all major teleconnections and the upcoming SSWE (Sudden Stratospheric Warming Event) only reinforced the cold message.
Western Trough Returning
I hinted last Monday that the models had changed their tune. It was unexpected and surprising, effectively coming out of nowhere. I was uncertain whether the trends would continue with consideration for prevailing thought and with the CPC ho-hum on the precipitation anomalies. Last Monday the PNA 15 day forecast started to show sharply opposite trends. The development of a western trough was the first sign the month of February was going down a more stormy path. The pattern signals have gotten much stronger. The next few days will be boring and quiet. The long range forecast is now filled with storms, plenty of -PNA, and a recurring western trough. It won't be 100% western trough all the time but certainly some episodes. Dare I say, some excitement?
Sudden Stratospheric Warming Event
SSWE's are situations where the warming occurs over the polar vortex and splits into pieces, or halves in this case. Minor events are common throughout the winter. Here is a wikipedia article for more information. We assumed at the first sign of a SSWE that it would send cold into the eastern half of the country. Evidently the direction and/or location of where the polar vortex splits off can vary. This has worked out well for us so far however, the biggest impact coming on the western end of the continent. Models are picking up on this and changing mid-late February progs accordingly, i.e the western trough.
MJO Looping in 7, then shooting to Phase 1-2
The MJO has been strong in phases 6 and 7. Part of the initial cold forecast for February was the prediction that MJO would continue along it's strong path into phases 1 and 2, the cold phases. Recent forecasts are much weaker with the MJO still taking it through 1 & 2 but the MJO signal is weak crossing through the unit circle (break down). Only weak signals from the MJO. For Wisconsin the more time we spend in phases 4-7, the stormier it will be. So the quicker we can get there the better.
We will see the biggest episode of -PNA this winter sometime over the next two weeks. Over the next several days, at first we will have a -PNA/+AO relationship which is supposed to be great for snow and precipitation. This transitions to a -PNA/-AO towards the end, assuming the forecast holds. The AO forecast turns to mush after Feb 16.
The Climate Prediction Center continues with dry anomalies to the west in the 6-10 period centered on Feb 17-20. This doesn't mean there won't be storminess. The CPC foreast seems delayed relative to what current GFS forecasts indicate, however the ensembles hold off the best western troughiness until late February. The southern stream is ready to go now but we are still waiting for the polar jet to descend. The present pattern is thus uneventful. The late Feb wetness corresponds historically with where we typically see the biggest snowstorms of the year, at least in my opinion. We'll see about March.
This forecast should give snow lovers hope that there will be major snow opportunities coming up. This winter has been tough for storm chasers like me waiting anxiously for the pattern to turn stormy. As always, nothing is guaranteed but this gives me several reasons to believe the western trough is coming. This would induce southwest flow, ridging over the southeast US, and allow gulf moisture to flow north. The challenge will be keeping storms along a track through the eastern great lakes. My concern with this forecast is possible model bias and reliance of long range models. We'll know if either of these are an issue if the mid-range models consistently push the western trough anomalies into the future.
When we had our weak western trough in January, the storm that formed drove straight northward across the western great lakes leaving the majority of Wisconsin on the warm side. Hopefully this time things will be different for southern/central Wisconsin.
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