As a lovely Friday afternoon is unfolding, meteorologists continue to look at a very anomalous blocking pattern which is forecast to develop over the next several days, which will bring vast changes to the weather pattern. A blocking pattern, when strong enough, can slow down the entire weather pattern, and force storm systems to cut off from the jet stream and stall. This will be the main culprit to our unsettled weather next week — particularly from Tuesday through Thursday. It will not be a total washout, but it will still be quite unsettled, so make sure to carry your umbrellas with you next week.
Let’s take a look at the weather pattern that is forecast to develop on Monday afternoon, on last night’s European Model. We are looking at the 500mb pressure level, which is right around the middle of the atmosphere. The image below has thin black contours, showing how high you have to go to drop to a pressure of 500mb (500mb heights) — the higher the height, the more expansive the column of air is, which is associated with warmth and ridges. The opposite is associated with cold air and troughs. When a ridge becomes strong enough, it can also cut off from the jet stream and become a block. The shaded colors are areas of vorticity, or general counterclockwise spin in the atmosphere, which is often associated with storms or storm formation.
Taking a look at the weather pattern shown above for Monday afternoon, the main features have been outlined in black — there are two Omega blocks — notice how they are both shaped like the Greek letter Omega. Normally, storm systems can progress throughout the country in a west-to-east fashion, or perhaps quickly turn to the northeast. However, when a storm runs into the influence of a strong Omega block, the block serves as a brick wall. It slows down all west-to-east movement. This forces any storm system to essentially stall — often underneath the Omega block on adjacent sides of it.
Off the east coast, we see a large closed off upper level low. This was initially the disturbance that will slide through tonight and tomorrow night. It slowed down and cut off, due to the presence of the Omega block south of Greenland. The atmosphere often seeks to balance itself out — so when there is a strong, closed off ridge (Omega block), any adjacent storm system or trough also tends to become strong and closed off — especially considering it stalls so long that it has time to do so. The close proximity of that strong storm system and associated energy — particularly the energy between the two blocks — will become important, and we’ll explain shortly.
Towards south-central Canada, we see an arguably even stronger Omega block. A ridge began to develop downstream of the large storm system in the West — the same storm system that may lead to an outbreak of tornadoes on Sunday and Monday in parts of the Plains and Southeast. Additionally, ridging was reinforced behind the large storm system off the East Coast (remember, the atmosphere’s balancing act). This leads to an extremely strong Omega block, which helps greatly to slow down the storm system and large trough that has now entered the Plains. Since it will be sitting there for days, it will have plenty of time to draw up lots of moisture from the Gulf of Mexico — leading to severe weather in the South, and eventually rain for our area.
Moving forward to Tuesday morning, we see that the stronger Omega block in south-central Canada has continued to strengthen and is now dominating the weather pattern. Additionally, the Omega block forced the leftover energy that was previously in between the two blocks to come due southward and strengthen. This allows it to interact with our already very strong storm system. What now goes on is that the energy phases with the large system, which strengthens the storm and “tugs” it northward a bit, but it still remains underneath the Omega Block, so it still has very little room to move. The tug northward allows rain to eventually reach our latitude. Furthermore, on the downstream side of the Omega block/ridge, a surface high pressure system will develop (essentially, just to our north and northeast), which, due to clockwise flow around a surface high pressure, will help to bring colder, marine air from the Atlantic Ocean, keeping our temperatures in the mid to upper 50s.
Moving forward to Tuesday night, it is not surprising to see that the center of the storm system has moved a bit further north, and it has become even stronger, as there are more closed off height contours around it than before. Also notice how it has still barely moved, and sits directly underneath the Omega block, which is still quite strong. As long as the Omega block remains strong, so will our storm system, since this is the atmosphere keeping itself in balance. This allows for a continuous feed of moisture from the Gulf of Mexico via counterclockwise flow, which will be directed northward, through the Southeast, Tennessee Valley, Great Lakes, and the Mid Atlantic & Northeast. Additionally, the storm system that was just off the coast on Monday has still barely moved from where it initially was — a further indication of how “slow” the weather pattern has become under this blocking regime.
On Thursday morning, the large storm system is still sitting there! The same storm system that will be bringing the Plains and Southeast severe weather from Saturday through Monday, and that will bring our area rain on Tuesday, will still be giving our area rain on Thursday. Fortunately, the Omega blocking finally weakened, as now we are only left with transient ridging, so the storm will gradually weaken from here on out. Still, however, the weakening is a slow process, and the storm will not rush out of here. Thus, clouds and potentially a few showers can still be expected on Thursday evening, Friday, and potentially even next Saturday — though with the storm system becoming weaker, widespread rainfall is unlikely, and even some peaks of sun may be possible on Friday and Saturday.
Ultimate impacts: Next week will not be a total washout, but it will generally be quite unsettled, so the umbrellas are a good idea. Monday looks like a pleasant day, as we will be in the downstream side of the ridging in between the two storm systems, where sun will still prevail, though high clouds will gradually move in throughout the afternoon. Tuesday will be cloudy with scattered showers, but it will not be raining all day. Occasional moderate rain may not be fall until Tuesday night; the more organized heavy rainfall should hold off until Wednesday through Thursday morning. Scattered showers will continue through Thursday afternoon. Total rainfall should generally be between 1.5 and 3″, with some locally higher amounts. The rainfall may not be heavy enough in a short period of time for widespread flooding concerns, but isolated flooding may be possible on Wednesday night, when the heaviest rain is expected to fall.
A surface high pressure will also develop on the eastern side of the Omega block (so just to our north and northeast), which will lead to an onshore flow of chilly, marine air from the Atlantic Ocean, which should help to keep temperatures in the mid to upper 50s.
Rainfall will not be as prevalent come next Friday and next weekend, but the general trough pattern may still remain, leading to chillier temperatures through the first couple/few days of May.