Analysis: Two Disturbances to Watch

Good evening! We had a lovely Spring day with sunny skies and temperatures in the 60s, but winds are picking up a bit along the coast as winds turn more onshore and a marine layer develops. For more on these short term changes including the chance for light rain on Wednesday night into Thursday, please be sure to check the dashboard and read our latest zone article updates! This article will be focused on the two upcoming disturbances: one for Thursday night into Friday, and another one for Sunday into Monday. Both could bring periods of heavy rain, but uncertainty is still high as it’s also still possible that little rain falls from either system.

After the periods of light rain on late Wednesday night into Thursday, a stronger storm system will be traversing the Great Lakes. This will allow a warm front to surge northward towards the area, which may place us in the warm sector. There is still uncertainty with how far north this warm front gets — they often can get a bit hung up this time of year and not progress as far northward as model guidance indicates. This is important because if the warm front stays further south, we may not enter the warm sector and thus may remain too stable for heavy showers and thunderstorms.

On Thursday night, we do believe the warm front will make more progress northward, which will increase lift in the atmosphere. We also have a strong shortwave passing to our northwest with a large closed reflection at 500mb, which should provide a decent amount of dynamics for lift. Also notice the strength of the jet streak and how it’s at its peak strength at our latitude — an indicator that dynamics may in fact be southward enough to get our area some heavy rain. Often times this time of the year, the jet retracts northward, so it’s crucial for heavy rain that the jet gets pushed southward.

Today's NAM model valid for Friday morning shows a complex of showers and thunderstorms moving into the area, thanks to a rigorous shortwave to our northwest.

Today’s NAM model valid for Friday morning shows a complex of showers and thunderstorms moving into the area, thanks to a rigorous shortwave to our northwest.

Notice the surge in 850mb temperatures as the warm front passes, and also how the lower pressures bend southeast towards our area well out ahead of the actual low pressure itself. There’s also a decent amount of upper-level diffluence, as the height gradient goes from tighter to looser over our area, and the wind barbs spread out somewhat. This “spreading out” is a weak substitute for true divergence aloft, but can still help provide some lift since when wind barbs are spreading out, there is a bit of inherent diverging in doing so. We are not in the right-entrance or left exit region of this jet, which may act as a hindrance to true large-scale vertical motion.

Still, though, with precipitable water values surging over 1″ with a bit of a tropical connection, the warm front providing lift and also warming the low-levels enough to provide relative instability aloft with the temperatures aloft still cold, as well as height falls from the strong shortwave further cooling the temperatures aloft and providing lift — this may be just enough to offset the lack of true upper-level divergence and it may be able to trigger some showers and thunderstorms with heavy downpours during the overnight.

Today's GFS model valid for Friday at 8:00am shows a good amount of moisture convergence, which could trigger heavy showers and thunderstorms.

Today’s GFS model valid for Friday at 8:00am shows a good amount of moisture convergence, which could trigger heavy showers and thunderstorms.

The initial onshore flow from the marine layer that we will be having via higher pressures well to our northeast may actually help trigger some lift as well. Notice in the above image how all of the onshore streamlines are converging with the southwesterly streamlines associated with the warm sector — the moisture from the Atlantic source with the onshore streamlines and the moisture with the southwesterlies will both act to generate a good amount of moisture convergence as well, leading to good moist lift, which is more buoyant than regular lift. In an airmass with precipitable water values well over 1″, this supports heavy downpours and rumbles of thunder. The low-levels will be too stable for strong to severe thunderstorms, however, because of the marine layer.

Notice though how the best moisture convergence is to our north — this is closer to the strongest dynamics with the shortwave and the low pressure, so the best lift for widespread heavy rain will miss our area. Thus, coverage over our area may still be somewhat scattered and not everyone will receive heavy rain.

The trailing cold front will approach the area during the day on Friday, which may mix out some low-level moisture. This could prevent the “cold front round” on late Friday morning from having true widespread convection, but there may be enough lift with this front for additional showers with a quick burst of heavy rain and perhaps a rumble of thunder.

Today's GFS model valid for Sunday afternoon shows a large fetch of tropical moisture being sent our way.

Today’s GFS model valid for Sunday afternoon shows a large fetch of tropical moisture being sent our way.

The next storm threat is more of a true synoptic storm evolution which could result in a low pressure deepening to our south and sending heavy rain northward into the area. The storm will be digging towards the Gulf and slowing down, allowing for a very efficient moisture fetch to gather over time and be directly pipe-lined towards our area. Notice the high precipitable water values that are being directly sent from the Gulf and up the Eastern Seaboard.

It all starts with a very active and fast Pacific Jet pattern, quickly ushering in many systems into the US. But there will also be a large West-Atlantic Ridge (WAR) developing, which acts as a traffic jam in the Atlantic. So once all of these systems from the jet head eastward, they eventually run into “traffic” and thus are forced to come together into one large system.

Today's GFS model valid for Sunday afternoon shows several ingredients coming together for a large rain-maker for the Eastern third of the US.

Today’s GFS model valid for Sunday afternoon shows several ingredients coming together for a large rain-maker for the Eastern third of the US.

There are a lot of good ingredients that are coming together. We already have the aforementioned moisture surge, but also notice the large temperature gradient at 850mb in the Mid Atlantic, with all the warm air surging northward with strong winds, and cooler temperatures to the north with weaker winds. There is a strong trough digging in the Tennessee and Mississippi Valley with plenty of vorticity, and notice the SW to NE flow aloft which can transfer moisture ahead of it. This trough helps to trigger a surface low in the Southern Mid-Atlantic, but because of the large area of SW to NE flow aloft, there is a very wide moisture shield of moderate to heavy rain.

But also notice how the trough is bending back somewhat to the southwest, and the very fast flow in the Northern Plains. This is an indicator that the trough is losing connection with the long-wave flow and a lot of energy may end up ultimately suppressed. The fast flow comes over the top of our storm and ruins the trough’s alignment and shunts the height rises in the NW Atlantic and essentially “pushes” our storm southward — shutting the door on a northward progression. This may ultimately prevent a long-duration heavy rain event and instead may make this just a Sunday event.

Today's GFS valid for Sunday afternoon shows a favorable jet orientation for rain on Sunday despite the main storm being well to our southwest.

Today’s GFS valid for Sunday afternoon shows a favorable jet orientation for rain on Sunday despite the main storm being well to our southwest.

Still, however, the jet has a favorable orientation for rain on Sunday. We are on the downstream side of the trough, which provides lift via warm air advection and moisture is propelled up the coast, like we said before. But what’s particularly interesting is the confluent jet region in New England — we are in the right-entrance region of this jet, which is very favorable for upward vertical motion and thus rain. When combined with the high precipitable water values and other aforementioned factors, this can lead to periods of moderate to heavy rain on Sunday.

But with the storm itself remaining to our south, we do not enter the warm sector for strong thunderstorms or true convection. Instead, we get a chilly steady rain — if current guidance is correct, of course. However, if the WAR can trend stronger like it often has done over the past few years, it’s possible the downstream side of the trough could then become amplified enough to send the storm further up the coast. Given the relative separation between the hanging back trough and the fast flow to the north, this would mean the storm would take a very slow path up the coast and bring a long-duration heavy rainfall event through Monday and into Tuesday. Some individual GEFS have showed this scenario, so it bears watching. When combined with Sunday’s rain, the long-duration event could theoretically have 2-4″+ rainfall totals. Instead, as of now, we are looking at a 0.5 to 1″ rain event for just Sunday with locally higher amounts, as of now.