Significant Coastal Storm Possible End of the Week

Computer forecast models are in very strong agreement for a substantial storm system to impact Connecticut at the end of this week. We are looking at the likelihood for heavy rains, strong to damaging wind gusts (ESPECIALLY along the coast), the potential for significant coastal flooding, and there is even the possibility of snow as well. What is leading to the potential for such a potent event? Substantial changes are occurring within the atmosphere and these changes will be tied to a big sudden stratospheric warming event (SSWE) which just occurred. First off, what is a sudden stratospheric warming event? A sudden stratospheric warming event is defined by a substantial increase in temperatures in the lower stratosphere which can be up to 50°C in just a couple of days! These events can play substantial roles in the evolution of the weather patterns down in the troposphere. 

Using the period from February 18th to February 24th, we see above-average temperature anomalies at 10mb (~100,000 ft above the ground!) along with above-average height anomalies (indicating ridging):

This event is rather significant in that it will lead to something we have not seen since late November...the negative phase of the North Atlantic Oscillation (NAO). For us here in New England, the negative phase of the NAO during the winter and early spring months can be a dominant player in our weather. When the NAO goes from the positive to negative phase it can coincide with shots of very cold weather and it can also be associated with stormy weather, particularly an increased chance for snow. In the end, however, there is much more to than whether the NAO is just positive or negative. The structure of the NAO is extremely important as well but we will not get into that here. For the past few weeks, long-range forecast models have been very aggressive and consistent with the negative phase of the NAO developing. In fact, forecasts are for the NAO to dip to -2 standard deviations below-average:

The negative phase of the NAO is correlated with above-average height anomalies (ridging) over Greenland. We can see this by looking at the forecast 500mb height anomalies for Thursday morning. The red's and purple indicate above-average heights and ridging. The purple shadings indicate much above-average height anomalies which is why the NAO is forecasted to become so negative:

Since about mid-February we have also been in a negative phase of the Pacific-North American teleconnection pattern (PNA). This, along with a pretty positive NAO were big players in the record warmth we saw last week. The PNA is expected to remain fairly negative through the week:

A negative phase of the PNA is typically associated with a trough across the western United States with ridging into the eastern United States. This would usually correlate to below-average temperatures in the west and above-average temperatures in the east. Given the extreme negative development of the NAO, however, this will really help to suppress any ridging come Friday and establish a trough across the northeastern United States on Friday which will be enhanced by ridging across the central portion of the country:

A combination of a negative NAO and negative NAO holds a fairly high correlation to a rather active storm track and has even been associated with some rather potent storms and that looks to be the case once again come the end of the week. So now that we sort of took a quick look into how the evolution of the atmosphere over the past week will lead up to the potential of this event let's looks at how things will evolve this week. 

Currently there is shortwave energy entering the Pacific Northwest. This shortwave energy will continue digging into the western United States and phase with the sub-tropical jet stream. As it does so, the energy will continue progressing northeast through the central Plains before phasing with northern stream energy on Thursday:

The result will be rapid cyclogenesis (strengthening) of a surface low across the Ohio Valley during the day on Thursday:

As this system continues to rapidly strengthen we will see low pressures develop at 925mb (~2500 ft off the ground), 850mb (~5000 ft off the ground), 700mb (~10000 ft off the ground), and 500mb (~18000 ft off the ground). The system is also expected to become vertically stacked rather quickly. A system is vertically stacked when these series of low pressures throughout the troposphere are vertically aligned. This is when a storm system is at it's strongest. In the below image, come Thursday evening the placement of all these low pressure centers is roughly over NE Ohio which indicates the vertically stacked nature of the system:

During this process, a secondary area of low pressure will begin to develop just south of New England as the main energy begins to shift towards the southeast from the Ohio Valley. Typically when we see storms track to our south this time of year we have to keep an eye on the possibility for snows. At this time it appears there will be just too much of an easterly component to the winds and not much of a cold air mass in place ahead of the system for a major snow event. The easterly component to the winds will advect in warmer air in the lowest few thousand feet of the troposphere from the Atlantic Ocean. This isn't to say, however, we won't see snow with this system. But this aspect of the forecast is a bit too complex now. As the system strengthens and the winds aloft begin to strengthen a process called dynamic cooling will occur. Dynamic cooling occurs when very rapid upward vertical motion occurs along with heavy precipitation falling. This will work to cool the atmosphere about several thousand feet up. At this juncture, it does not appear snow will be a big threat but snow could be possible within the hills and we could see rain perhaps end as snow but this potential will continue to be tracked as the week progresses. 

What we will focus on right now is the closed off circulation at 500mb as this will add some complex challenges to the forecast. When systems close off at 500mb soon after systems will occlude and this process begins to shut off the influx of warmer and moist air into the system which is an ingredient necessary for both strengthening and precipitation production. Forecast models indicate that the system will close off at 500mb well west of Connecticut. This could mean less in the way of heavier precipitation here but this is something to watch as the week progresses. 

Regardless, there is the potential for a quite a bit of rainfall as forecast models suggest a very anomalous low-level jet on the order of -3 to -4 standard deviations indicating a rather strong easterly component to the winds. This would supply a great deal of moisture into the state:

At this juncture this is what we are tracking for the state:

• Potential for heavy rainfall (but again how much rain we see may be tied to the evolution of the 500mb low and whether it occludes too early and shuts off inflow of warm/moist air into the system.
• Potential for strong to damaging wind gusts, especially along the immediate shoreline. Very strong winds aloft and a very strong pressure gradient will yield the possibility for strong to damaging wind gusts statewide, however, the biggest threat will be along the shoreline. 
• Coastal flooding could be a fairly big concern. A prolonged period of easterly/northeasterly winds coincident with astronomical high tides and strong winds could make for a moderate-impact coastal flooding event, especially along the southwestern shoreline. 

As the week progresses details will begin to become much more clear but for now the idea of a rather significant storm is certainly in the cards.