After the heat, blocking and wetter weather return

Good evening, everyone! We currently have a lovely stretch of warm weather coming up that already started today, so get out and enjoy it! While we will not necessarily go back to the consistently rainy and very cool weather from early this month, there are plenty of signs that we will go back to a relatively cooler and rainier regime starting this weekend.

For much of this month, we have had a large blocking pattern near Greenland that gradually retrograded into SE Canada and then the NW Atlantic. It has since dissipated, which allowed for a large ridge to finally move into the East and warm things up. But these blocking patterns often can be persistent and return in a relatively short period of time. We actually had a large blocking pattern in March, too, so this blocking pattern has already repeated itself, which gives more credence to the idea of continued returns of the blocking.

And indeed, that is where we are headed.

The problem is, we never completely reversed the stable atmospheric regime we had been in — this heat ridge is just a temporary blip.  Notice the upper-level low northeast of NewFoundland. This is somewhat of an extension of the previous Nor’Easter on Saturday — some energy from a large ULL in Central Canada will phase with the leftover ULL from that Nor’Easter and form yet another giant ULL northeast of NewFoundland. This pumps up the heights downstream and thus can send yet another blocking ridge into Greenland. This will also time itself with a large storm system that will be traversing the Plains and into the Great Lakes, which will pump up the heights out ahead of that, too. Those higher heights help to reinforce the ridging/blocking in Greenland — an indicator that this block will once again stick around.

The true implications of this begin this weekend, as that ULL will force confluent flow in SE Canada, allowing high pressure to slide offshore, bringing onshore flow and cooler weather for the weekend. But it does appear that Saturday and Sunday will be dry. However, as we head into Monday, the blocking takes influence as far as getting stormier weather in the East. The blocking will halt the progress of a ULL associated with the disturbance initially in the Plains, forcing it to stall just north of the Great Lakes. This will allow a more prolonged SW fetch out ahead of it, which means that plenty of moisture will be able to interact with a cold front that gets dragged out ahead of the ULL. If the blocking weren’t there, the ULL would’ve rapidly shot northward and we wouldn’t get the true moisture return, and all the forcing would stay well away from the region. But instead, it appears that showers and thunderstorms with heavy rain are possible on Monday evening. Now having said that, a more widespread significant threat will be averted due to the ULL being a bit too far NW of us — but it’s still close enough to us for some showers and thunderstorms.

Notice how in the above 4-panel plot, there is plenty of backside energy behind the ULL that is dropping southward. This will be forced to drop further southward than the last disturbance, due to the blocking being more established now, with this ULL reinforcing the block.

This backside energy will rotate beneath the ULL and actually phase with it once it drops into the Central Plains. When combined with a lot of upstream ridging developing in the Western US, this forces a full-latitude trough to develop in the Central US. It has very little room to move much, due to the blocking pattern in the Atlantic. This large, rounded trough in the Central US is a classic precursor to a heavy rain event, as it signals lowering pressures to our southwest and a very large moisture feed up and down the East Coast. Shortening wavelengths may prevent this from resulting in a large Nor’Easter type of storm, but the above image actually shows almost winter-like longer wavelengths, which is very unusual for this time of year — really owing to the blocking pattern that is developing. This means that it is becoming increasingly likely that a large low pressure system will originate somewhere in the South-Central US and slide northeast towards the Appalachians, giving us a threat for heavy rain sometime around next Thursday.

With the pattern in the Atlantic becoming very blocked up and continually getting reinforced by these storm systems, it appears that this blocking pattern may last well beyond next week. This could mean continued areas of deepening low pressure being forced near our latitude, and more threats of rain with these storms for the rest of the month. This blocking may even last into the first week of June.

All in all, it appears likely that we will again be tracking rain threats before we know it.