2014 – Page 35 – New York Metro Weather

Gradually increasing clouds will lead to an unsettled week

April 28, 2014/0 Comments/by Doug Simonian

The same storm system that gave Arkansas a powerful tornado last night, and will very likely be giving parts of the Mississippi Valley more powerful tornadoes, will be very slow to drift northeastward, thanks to an Omega blocking pattern. This will eventually usher in a multi-day period of rain, starting tomorrow afternoon, and lasting through Thursday.

What is currently a large, cutoff trough will cut off even more from the general jet stream flow over the next day or so, as it very slowly drifts northeastward. This allows it to remain to the west enough to be able to generate Gulf of Mexico moisture, but far enough to the northeast for it to reach our latitude. Had the storm been smaller in scale, the moisture would not have been as expansive in nature.

Today’s NAM at 500mb, valid for 2:00am Wednesday morning, shows a large cutoff low dominating the United States, which will be bringing plenty of clouds and rain during this work-week (weather.cod.edu).

Notice the large storm system that will be in the Upper Midwest, and all of the closed, circular 500mb height contours wrapping around it. This indicates a large cutoff low that is slow to move, which is mainly triggered by the large Omega blocking to the north of the system. For more on Omega blocking, check out our article from Friday. The most efficient lifting for precipitation tends to occur on the east side of a trough, which is exactly where we will be. Also notice how to the east of the trough, the height contours are oriented from the Gulf of Mexico, straight northeastward towards our area. That will be the general track of the rain.

Let’s move forward with more details on the rain itself and its timing.

Omega blocking to bring ominous weather

April 25, 2014/1 Comment/by Doug Simonian

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.

Last night’s European Model shows a duel Omega blocking pattern next week (WSI Model Lab).

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.

Pleasant start, but heavy rain likely Friday Night

April 25, 2014/0 Comments/by John Homenuk

It is not often, this time of year, that a relatively progressive passing disturbance will force the development of a low pressure system around 995mb off the Northeast US Coast. Exactly that will occur late Friday Night into early Saturday, as a mid and upper level disturbance passing through the Northeast US with some vigor moves toward New England. Forecast models indicate a high likelihood of increasing moisture as well as strengthening lift for precipitation by Friday Night — even some weak instability which could force thunderstorms — along an axis which will shift from west to east. This raises confidence in a period of heavy rain Friday Night into Saturday morning, as the low pressure system passes nearby and eventually into the Gulf of Maine.

Not all will be lost, however. Much of Friday will turn out pleasant. This morning, visible satellite imagery showed only some filtered high clouds moving into the area. High temperatures on most forecast models are expected to reach into the lower 60’s. Most notably, however, the winds which were blustery over the past few days have settled down. As a result, the 60 degree temperatures will feel much more comfortable in the warm sun as opposed to the brisk and blustery winds. Rain is forecast to move into the area after the PM Rush, and will be out of the area by Saturday’s sunrise.

Simulated radar for tonight shows an area of moderate to heavy rain with embedded thunderstorms.

Pleasant, but fire danger through Friday

April 24, 2014/0 Comments/by John Homenuk

Calm weather has settled into the area during the middle part of this week in the wake of a cold front, but gusty winds have ramped up over the last 12 to 24 hours with a tightening pressure gradient over the area. Blustery northwest winds led in a surface high pressure on Wednesday Night, as a low pressure system deepened well off to our north and east. The northwest winds and very low humidity are leading to concern for brush fire spread throughout the area. Subsequently, the National Weather Service has issued a Red Flag Warning for much of the area — save for Long Island, where higher humidity values are present near the coasts.

A Red Flag Warning is issued by the National Weather Service when high winds and low humidity are expected to make conditions favorable for dangerous and rapid spread of brush fires if ignition occurs. Despite the concern for fire spread, pleasant weather will continue — albeit a bit cool of this time of year. Highs in the lower 60’s will be common on Thursday and Friday before a disturbance approaches with a period of rain expected Friday Night.

Very low dew points and gusty winds could cause rapid fire spread today.

Archive for year: 2014