Premium: Omega block returns, with different results

An anomalous blocking pattern has returned for the second time this Spring, but the sensible weather results will be quite different this time around. Instead of falling underneath a large blocking ridge of high pressure, as we did in early April, this blocking pattern has brought (and will continue to bring) persistent troughiness and unsettled weather to our area, as well as below normal temperatures.

The blocking pattern responsible for the troughiness is quite impressive. Higher than normal height anomalies are present throughout much of the Arctic, including Northern Canada and much of the Northwest Atlantic. This blocking is something that seemingly hid itself during the winter months — which some are certainly thankful for, as its presence would lead to extreme cold and snow. Unfortunately, this time of year, it has resulted in clouds, showers, and temperatures in the 50s for this entire week.

The cutoff low and Omega blocking pattern will come to a head during the latter part of this week, as ridging builds from the Pacific into the Northwestern United States. Two large cutoff lows will form, one over the Southwestern United States and another over the Eastern United States. A mid level ridge will form through the Central United States between the two — a textbook Omega block.

In our area, the sensible weather results will be quite dreary. As a cutoff low meanders near the region toward the latter half of this week, multiple bouts of unsettled weather with rain and cooler temperatures are likely. Highs in the 50s will be commonplace, and multiple mid level atmospheric disturbances rotating around the upper level low are likely to provide increased moisture and lift for precipitation.

The hemispheric pattern undergoes some changes by late this weekend, however, with the upper level low and blocking pattern receding a bit. The omega block will disintegrate and push eastward, with mid level heights rising throughout the Eastern US allowing for moderating temperatures and improving conditions by early next week.

The degree of warmth is still somewhat in question, given that we’ll still be having influence from a trough in SE Canada. That will help to push down cold fronts from time to time and prevent a true “torch”. That being said, with some ridging over our immediate area, we can still generate SW flow ahead of that trough and thus still be warm in the means. By the time Monday and Tuesday comes around, temperatures should rise into the upper 60s to perhaps low to mid 70s.

Further confidence in this eventual turn to warmth is supported by May analogs that had a declining El Nino and a +PDO. Their composite shows a trough in the SW, and warmth in the NE. It also shows the -NAO over Greenland that we’ve been dealing with that helped to cause the Omega blocking pattern in the first place. This, along with the fact that the pattern in the medium range ensembles has a similar look, gives more credence to the analogs.

Now the one caveat is that these analogs do not have the large trough in SE Canada that is forecast to persist next week and perhaps even a bit longer. But as previously mentioned, while the trough in SE Canada can hinder significant warmth at times, we can still have warmth in the means with that kind of pattern.

This article was written and edited by John Homenuk and Doug Simonian