February Review & March Preview
February Recap
Arctic & Atlantic
The supposition going into the season was that any major disruption of the polar vortex would occur in latter February or March, after a very mild February. However, said disruption occurred in early January, which significantly altered the February landscape from what was forecast. What was forecast to be a rather benign and very mild month
Eastern Mass Weather February 2021 Forecast Temp Departures
began with the capstone event of the winter on the first of the month, which set the tone for what was an average month temperature wise with above average snowfall. Thus the residual impact of the January Sudden Stratospheric Warming (SSW) continued the theme of both delaying and muting the anticipated warm up during the second half of the winter season. The anticipated positive AO during the month of February obviously did not work out, although it has finally since recovered. The steep decline centered around mid month coincides with the arctic plunge into the deep south.
Accordingly, the NAO was also slower to recover than anticipated, but did manage to average slightly positive for the month. In fact, the NAO was actually neutral at the time of the mid month arctic outbreak, which combined with the arctic and pacific pattern to focus the severe mid month arctic outbreak in the nation's mid section. While the impact of the polar vortex disruption significantly impacted the forecast for the arctic region, the Pacific was much better forecast, which is a reversal from the first half of the season.
Pacific
The month overall in the mean featured a modest positive PNA, which was actually well forecast. And as can be noted in the annotation below, the peak of the monthly positive PNA spike coincided with the arctic plunge mid month.
Thus the higher heights over the western US worked in conjunction with the relative dearth of a vehicle to drive the cold east, given the neutral NAO, to focus the wrath of this outbreak over the nation's mid section. This is very evident in the monthly temperature departure chart, which is obviously quite a bit different from the forecast from last fall, posted above. This is due to the misdiagnosis of the arctic region attributable to the major mid season SSW.
Actual February 2021 Monthly Temp Departures
There was also an active Pacific forecast during the month of February, as evidenced by the positive precipitation anomaly over the pacific northwest in the forecast anomaly chart from last fall, pictured below.
This active Pacific jet was well forecast:
Eastern Mass Weather Forecast February 2021 Monthly Precipitation Anomaly Chart
However the high latitude blocking in conjunction with the well forecast modest + PNA resulted in a storm track that was more suppressed than anticipated, which focused snowfall more across southern New England and the northern mid atlantic, rather than northern New England, as expected.
Actual February 2021 Monthly Precipitation Departure
Conclusions
The implications of the disparity between the how this season has evolved and the outlook issued in November will be discussed more extensively in the post season analysis issued this spring. In the meantime, what has become apparent this February is that the hemispheric forcing regime accompanying this particular cold ENSO event has been more redolent of the EP la nina seasons included in the mixed-type la nina composite, as opposed to the modoki type. Nothing illustrates this more than the vertical velocity chart for the month of February:
February 2021 Vertical Velocities (Forcing)
This depicts the vast majority of the atmospheric forcing pinned to the west of the dateline, as was the case with respect to the EP composite examined last fall.
EP La Nina Vertical Velocity (Forcing) Composite
Note that the warmer anomalies near the dateline ensure that forcing remains over the central and western Pacific, similar to a modoki el nino, which creates this DM H5 composite. Also evident is that the Aleutian ridge focuses more to the northwest relative to the modoki, cp event, which will be evident when that particular composite is reviewed. This often entails a protrusion of said ridge into the polar region at times, which likely contributes to the lower heights over the mid latitudes:
EP La Nina Composite
Here is the H5 composite for February 2021, with the high latitude blocking and accompanying aleutian ridge clearly displaced to the northwest, as cited in the above EP composite.
Actual February 2021 H5 Composite
Versus the forecast composite for the month, which was biased more towards the center of the EP/CP continuum and thus a relative death of high latitude blocking and an Aleutian ridge displaced more to the southeast.
Forecast February 2021 H5 Composite
More this in May, but March comes first.
March Preview
Last fall, the prevailing though for March was that a degree of high latitude blocking (lesser magnitude than what is indicated in the composite) may become reestablished at some point in the month, as the Pacific jet remained active and heights on the west coast elevated, somewhat.
March 2021 Forecast H5 Composite Form Last Fall
This was though to result in a near average monthly composite for much of the region:
March Forecast Monthly Temperature Departures Issued Last Fall
Normal to below average snowfall was implied, which still looks to the case, as the primary difference relative to the forecast from last fall is that more of an RNA looks to persist in conjunction with the modest high latitude blocking, mainly in the NAO region.
Forecast NAO
The notion of at least modest blocking is supported by the passage of the MJO through phases 7 and 8 this month.
Forecast AO
This is a change from February in that the bulk of the blocking was in the AO region, however, should the RNA develop and prominent Pacific jet persist as expected, then the bulk of cold shots would continue to be focused to the west and any wintery events should be marginal and favor higher terrain.
Forecast RNA
Forecast EPO
Currently, the only major storm opportunity, which coincides with the encircled area of modularity on each respective index chart above, looks destined for failure late next weekend. It looks to pass harmlessly put to sea, as the extension of a polar vortex lobe to the north acts to ensure its fate.
The risk should this overall monthly pattern materialize is slightly warmer than originally expected, which was near normal. Here is the forecast monthly mean pattern form the European guidance:
Canadien:
AND CFS:
Consensus is fairly strong on the guidance above, however, time will ultimately tell, as dynamical guidance is but half the equation.
