Blocking episode may create forecast headache

Blocking, both over Eastern Canada and in the high latitudes, more often than not means a forecast headache for our forecast area. We’ve seen it on many occasions with winter storms — as the strong blocking highs create anomalous weather patterns. We saw it with Hurricane Sandy, and the Nor’Easter that followed. And it seems that we may see it again, as we head towards the first week of May.

Forecast models are coming into agreement on the potential for an anomalous blocking pattern to develop over Eastern Canada and the Northwest Atlantic towards Newfoundland. With a shortwave trough predicted to be over Central Canada at that time, the trough may be forced underneath the block into the Ohio Valley and the Mid-Atlantic/East Coast. This would create the opportunity for a “cutoff low” and possibly even a coastal storm — with widespread below normal temperature anomalies wherever the cutoff low and upper level trough tracks.

At this range, the question becomes: How strong will the blocking be, and where will it develop? The positioning of the block, as well as its strength, will have a great deal of impact in determining where the cutoff low will eventually track. This will, in turn, have major impacts on the sensible weather forecast for our area. Needless to say, the potential is there for a prolonged period of showers and unsettled weather — as well as a coastal storm — if the cutoff tracks near our area. If it remains farther south, we may be spared the worst effects.

Forecast models are still struggling with the positioning of the block and positioning of the cutoff low. The Euro (or ECMWF), for example, has the cutoff low staying well to our south while the GFS and Canadian models bring it closer to our area with a prolonged period of rain in the Mid-Atlantic states.

In situations like this one, forecasters can utilize ensembles to see the variance amongst forecast models. In this case, we can check out the GFS Ensembles — which show the average height anomalies at 500mb (mid levels of the atmoshpere) on the left, and the “spaghetti” plot of each ensemble on the right. It is easy to see that most ensembles agree on the strong block developing over Newfoundland and Eastern Canada, but the exact positioning of the upper level trough over the Central US is less certain.

Forecasters will certainly be monitoring the models over the next several days, so be sure to stay tuned for updates on the potential unsettled pattern as we head towards next week. For now, enjoy the pleasant weather through the weekend!