Cuddle Alert! Coldest Air of the Season Likely This Valentine's Weekend! And, What About Longer Term into March?

The more mixed pattern forecast and discussed recently has come to fruition the past month or so with a "sometimes mild, sometimes cold" pattern pattern taking hold. The biased however, continues to be on the mild side as El Nino continues to greatly influence our winter. However; the second player discussed for the winter, the Polar Vortex has been intermittently sticking its cold beak into our weather regime since January and thus, has been tempering somewhat our as an extraordinary mild winter! This resulted in January actually averaging just slightly above normal (as compared to December's double digit above normal departures) with colder air masses making an appearance. Actually; this month's pattern looks to be very similar to January's through mid month with a very mild week opening the month; only to be replaced by a cold one by mid month - and this is about to commence again.

A deepening low pressure system in formation over the Eastern Great Lakes into mid week will begin to draw down colder, Arctic air into the region by mid week with the coldest of the air arriving this Valentine's Weekend. Temperatures this weekend are anticipated to plummet from around the 50 degrees we felt this past weekend /Sunday/ to morning lows of single digits; above and below zero by Valentine's morning, this upcoming Sunday. Indeed, this would be the coldest air of this mild winter's run thus far. The colder air will be accompanied by light snow and snowshowers as moisture is fed into the system from the east and Great Lakes. The cold, cyclonic flow will aid in pulling down the much colder air that is in place over eastern Canada. The best snowfall amounts will naturally be downwind of Lakes Superior, Michigan and Huron where several inches could accumulate. In and around Southeast Lower Michigan away from Lake Huron and the Thumb Region; I'd look for generally one to three inches depending location through mid week. This expected snow cover will have more of an impact on overnight lows come this weekend, with the coldest mornings occurring in conjunction with snow cover, clearing skies and lighter winds.

Note the low temperatures projected for the coming weekend; granted these could change some but our past cold weather in mid January could see some competition this weekend. After; look for milder weather to again return to the region.

Rest of February into March 

Of interest to meteorological buffs; a very strong stratospheric warming has taken place over the Arctic as unseasonably warm air surged north into the highest of altitudes of the stratosphere. This pattern is generally associated with a dislodgement of the Polar Vortex normally over or close to that region. Latest indications are the Polar Vortex in in the process of splitting (as the GFS has picked up on) with one heading toward southeast Canada while the other heads over to Europe or further east.

"Major Sudden Stratospheric Warming"

These occurs when the westerly winds at 60N and 10hPa (geopotential height) reverse, i.e. become easterly. A complete disruption of the polar vortex is observed and the vortex will either be split into daughter vortices, or displaced from its normal location over the pole.

Longer Term
 
For a longer term through the rest of February and into March; note the following discussion by Atmosphere and Environmental Research /AER/.

AER scientists provide researchers and enthusiasts real-time insights on one of North America’s and Europe’s leading drivers for extreme and persistent temperature patterns. Author Judah Cohen, Ph.D., works at Atmospheric and Environmental Research (AER), a division of Verisk Analytics.

"Longer term we continue to expect that variability in the polar vortex (PV) to strongly influence Northern Hemisphere (NH) weather patterns.  Our confidence continues that a troposphere-stratosphere-troposphere coupling event (T-S-T) is underway (Cohen et al. J. Clim., 20, 5335, 2007), which is resulting in a sudden stratospheric warming (SSW) and is reversing the AO from positive to negative in the stratosphere.   The T-S-T should culminate with the negative phase of the AO favored in the troposphere from after the SSW peak through the end of winter.

General temperature trends across the NH should mirror trends in the AO.  As we have discussed many times previously, while the vertical energy transfer is active, the positive phase of the AO is favored in the troposphere.  The atmospheric energy transfer is now predicted by the weather models to be active through mid-February and even to achieve record values next week.  The record strong energy transfer is also predicted to bring record warm temperatures to the polar stratosphere next week making for an impressive SSW.  This SSW has been evolving for an unusually long time but is likely to peak with the record warm polar stratospheric temperatures next week.  The strengthening SSW should force a negative AO in the stratosphere. 

Our research and that of other scientists shows that the negative phase of the AO in the troposphere is favored, with a ten to fourteen-day delay, for the following four to six weeks after a SSW and a negative stratospheric AO. The evolution of the SSW or polar vortex weakening is likely to modulate which regions are favored for colder temperatures across the NH continents for the remainder of winter. The models are predicting for the PV to slide from its current position over northern Eurasia into the North Atlantic and then stretch out towards eastern North America. As the PV pinballs towards the North Atlantic, the models also predict geopotential heights to strongly build across the North Pacific side of the Arctic in the stratosphere.  This should result in a tropospheric reflection of strong positive geopotential heights across the North Pacific of the Arctic.  Strong geopotential height rises across part or all of the Arctic basin favors lower geopotential heights across Eastern Siberia, eastern North America and Northern Europe.  Lowering geopotential heights in eastern North America and East Asia coupled with ridging to the west will promote a northerly flow of cold air for those regions".


This forecasted pattern fits very well with the Winter Outlook where analogues indicated colder, but still variable temperature conditions likely to dominate the remainder of the cold season through March. The key here is how El Nino plays into this Polar pattern. Will it continue to be strong enough to influence the Pacific jet and allow mild, Pacific air to surge well east across the country; at least at intervals? While I believe the Polar Vortex has now become a viable contender to the El Nino influenced Pacific jet; both should remain in contention as spring approaches. That being said; this battle (as first mentioned back last fall in the Outlook) could very well intensify into a more stormy pattern not seen this winter in our neck of the woods. Again; WHERE these atmospheric battles take off is largely dependent on these two dominant patterns and if, when and where phasing takes place. It has been our observance this winter that notable winter storms have been basically all around the Lower Great Lakes - west, south and east - therefore it will be extremely interesting to see if that established pattern hangs tough the rest of the cold season or breaks down.


Making weather fun while we all learn,
Bill Deedler -SEMI_WeatherHistorian