As Autumn has arrived, the jet stream tends to get a bit stronger and sag to the south, helping make nor’easters more likely. Some computer model guidance had been hinting at a strong coastal storm approaching the area early next week, which would bring strong winds, heavy rain, and coastal flooding. However, the pattern does not support such an occurrence.
Looking at last night’s European model ensembles at the 500mb level, there do appear to be some features that support the potential for a strong nor’easter to head towards our area. Initially, there is a kink in the heights at 500mb just offshore of the Eastern Seaboard, which indicates our storm system. With strong storms like Sandy, we had a blocking ridge near and to the north of the storm, forcing it to stay to offshore of the southeast coast, instead of shooting out to sea. We also had a trough diving into the Central Plains, heading eastward, eventually capturing the storm and bringing it to the west. At the basic level, those two features certainly exist, which raise an eyebrow.
However, taking a deeper look at things, it is not prudent to say that a storm will trek towards us. For one thing, although there is a strong ridge in the east, extending up through SE Canada and New Foundland, it is merely a function of being downstream of the trough, and is not a block. Blocking typically occurs in higher latitudes — such as Greenland — and the height contours are typically closed. The true blocks are very slow to move and act to buckle the weather pattern, rather than being within the “flow” of the pattern. Since the ridge is merely downstream of the trough — it is within the “flow” of the pattern, and there is nothing to keep this ridge in place. It is thus a transient feature, and will eventually scoot out to sea and weaken. In fact, looking the higher latitudes, the heights are actually very low up there; more evidence that there is no true blocking.
Additionally, although that trough looks pretty potent — look at what is upstream to the trough. There is a huge, closed off upper level low in the Gulf of Alaska. This forces the heights out ahead of it to be very zonal, and also very close together. This leads to extremely strong west-to-east flow that is pushing into the western United States. This is a hostile environment to maintain downstream trough amplification; thus, the trough will quickly be able to scoot to the east and flatten as it does so. The fast zonal flow in the Pacific will tend to yield fast, zonal flow downstream. This is why the trough has to weaken and move eastward, which also means the downstream ridging that keeps the storm to the south in the first place also will weaken and move eastward. This will give the storm room to escape out to sea.
Taking a look at Monday morning (the timeframe for when the storm would have affected our region) on the European ensembles, we can see these changes take place.
For one thing, look at how flat the trough has become. If anything, there is some ridging in the eastern Plains, with a weak trough quickly scooting to the northeast in southeast Canada. That feature is way too weak and far-removed from our storm system to capture it and help it turn northward. The flow has become a lot more zonal across the country, because of that large storm system sitting near Alaska.
The ridging has also greatly weakened and scooted to the east. It thus only has minimal impacts in keeping the storm to the south and west. Based on the zonal flow across the United States and the trough being weak and to the northwest rather than stronger and to the west and southwest, this will gradually act to push the storm eastward, and the ridge is not nearly strong enough to overcome that. It cannot push the storm into the trough.
Instead, the storm should take a turn to the north, and then slide just east of due north, and head out to sea — perhaps affecting Nova Scotia.
As expected, today’s European model forecast shows the storm remaining to the east; keeping the area dry. It could come close enough to increase our northeast winds a bit for Monday, and perhaps bring some clouds into the mix as well. Also, surfers will be quite happy, as the swells look to be decent — just watch out for rip currents. However, Sunday night and Monday should remain dry with a mix of sun and clouds, with perhaps a bit more clouds on the east end of Long Island — with temperatures around 70. By Tuesday, our skies should go back to being predominately sunny, with temperatures in the mid 70s.
Can we expect a true blocking pattern anytime soon? Some modeling guidance is hinting at a bit of blocking developing in early October, particularly in the October 5-7 period, which may yield a storm threat. However, any storm threats then should be a run-of-the-mill nor’easter, and nothing like Sandy, nor Irene.