Warmth, Humidity, and Thunderstorms On the Way

For the past several days, I have been monitoring the potential for much warmer weather, and along with it, the chance for strong to severe thunderstorms.

Although it doesn't look like the area will see any widespread severe thunderstorms, instability on Monday evening and Tuesday afternoon may be sufficient for the development of a few severe thunderstorms, especially Tuesday. This will be highly dependent on the coverage of rainfall Tuesday morning and early afternoon, as well as the proximity of the low-pressure system itself.

So here's the scoop:

Let's take a look at current observations and satellite trends before we go any further.

 

This image is the current surface observation chart across the lower Great Lakes and the Ohio Valley region. If you are more experienced in meteorology, you've probably noted the strong wind-shift and convergence zone which is draped across Northern Indiana and Ohio, as well as the "actual" warm front, which is further south. The actual warm front is demarcating the arrival of tropical, moist air. Although there is very little temperature contrast across the front as of right now, the warm front should strengthen later this evening and night. This will occur as the EML, or Elevated Mixed-Layer, is advected from the Southern Plains into parts of the Ohio Valley in advance of a major cyclone.

There are a number of reasons why I do not expect this front to advance much today:

1.) East-northeasterly winds north of the warm frontal boundary (where the wind-shift and convergence zone is located) will keep advecting drier air from a weaker Canadian high into far Northern Indiana and Ohio.

2.) Although I do expect some progress northward of the front, it doesn't look likely, and thus the reason why instability and moisture will be limited. With this in mind, do not expect anything more than a few showers this morning and afternoon. Other than that, I would just expect partly cloudy skies.

3.) The low-pressure system in the Central Plains has not yet consolidated, and has yet to develop significantly, and thus why the warm front is moving so slowly. Once the cyclone develops, the warm front will begin to shift northward quite rapidly.

The latest GOES sounding also gives a clear indication as to the strength of this warm front. We can clearly see the warm frontal inversion by looking at the moisture and temperature profile. Based upon the current height of the frontal inversion, this suggests that the actual front is not too far away, so it could very well pass through the area today.

 

The above image is current infrared satellite imagery. Based upon this, it doesn't look like there will be much rainfall or even shower activity later today. With instability on the lower side, low-level moisture scarce, and no upper-level forcing mechanism, precipitation is not likely today.

 

One of the more prominent features on the analysis charts this morning is the building upper level ridge across the United States, in advance of a strong Pacific jet streak, which is causing a major shortwave trough to dig. Although this is happening slowly right now, accelerated height falls later this evening will lead to further development of the upper level disturbance, which will cause the cyclone to strengthen considerably.

So.. Let's summarize the main features on the weather charts this morning, and I will elaborate on what this means for the next few days.

1.) A large synoptic scale ridge is building across the Central Plains and Midwest, and into the Great Lakes and Ohio Valley. This will lead to further northward movement of the warm front, and thus well-above normal temperatures by tomorrow and Monday. Ridging should cause weak subsidence to develop over the area tomorrow, and thus, little or no thunderstorm activity, even though instability will be in the 1500-2500 J/kg range.

2.) Current satellite imagery suggests that precipitation will be scarce or non-existent today, so expect a nice, dry and warm Saturday! Looking at other parts of the Midwest, we can see that thunderstorm complexes are developing north of the warm front and north of the low pressure system. These thunderstorm complexes will lead to the development of outflow boundaries, and the possibility of thunderstorm development south of the current clusters of thunderstorms. But due to the building ridge, subsidence should limit thunderstorm development into Monday. 

3.) Surface observations and the GOES sounding suggests that the warm front is approaching, but is being hindered by dry northeasterly winds on its northern side. This suggests that the warm front will make little progress today. Even so.. If diurnal heating is more than expected, I would expect that the warm front pushes north of the area for at least a short while, which could allow for thunderstorm development in the humid air mass.

Now.. For the forecast through Monday.

As of right now, there isn't much model agreement on the timing and strength of the cyclone and upper level disturbance that will impact the Midwest from Sunday through Wednesday. Model agreement is especially lacking between global and mesoscale models. This kind of disagreement between global and mesoscale models is always expected, but it certainly hinders the forecast process.

1.) Given the NAM's track record with forecasts beyond 48 hours, I would expect that the ECMWF and GFS are more correct with their portrayal of a slower track of the low-pressure system on Monday. It is certainly likely that the NAM begins to trend toward this solution over the next three runs.

2.) The overall pattern suggests that the GFS and ECMWF solution is much more likely. This is likely for several reasons as well:

A.) The upper level wave that is digging in the West due to the Pacific jet streak is building very slowly. Upper-level height change analysis suggests this as well, with only 30-60 decameter changes in 500 millibar height in the last 24 hours. This doesn't appear likely to change at least until later Sunday afternoon when the surface cyclone begins to intensify.

B.) The jet stream itself is very weak, and thus not able to move storm systems very quickly. And given the shallow nature of the Pacific cold front behind the system, its also not likely to move very quickly. This gives credence to the global model solution.

This is the GFS model forecast for late Monday evening of surface dew point temperatures.

With upper level ridging in place, the warm EML will be advected from the Southern Plains into the Ohio Valley and Great Lakes region by Monday. The warm EML will allow 850 millibar temperatures to climb to near 18-19 degrees Celsius across parts of the area. With deep-mixing of the boundary layer, this could lead to high temperatures close to or over 90 on Monday. This in combination with dew points in the upper 60's to the lower 70's will lead to high instability; possibly in the range of 2500-4000 J/kg. This will likely lead to at least some thunderstorm development Monday night, but I will have to monitor the situation, as it is extremely uncertain. 

By Tuesday, we can definitely expect the arrival of more widespread and numerous thunderstorms across the area as the Pacific cold front approaches the area.

The main factor in whether we see severe thunderstorms or not on Tuesday is instability. On the one hand.. We have the NAM, which predicts a faster arrival of the Pacific cold front, and severely limits instability. On the otherhand, we have the GFS showing instability across the area in the range of 2000-3000 J/kg, which would definitely lead to the significant possibility of severe thunderstorms. At this time, it is uncertain, given the GFS's "moist" bias, and the NAM's bias towards faster solutions.

By Wednesday, the Pacific cold front will pass through the area slowly, dropping temperatures into the upper 70's. You can also expect scattered showers and thunderstorms throughout the day, but I don't expect it to be entirely a washout.

From Thursday through Memorial Day, much more quiet weather will settle in to the United States as a 1030 millibar Canadian high descends into the Midwest. This will keep temperatures in the lower to mid 70's. After this.. It is uncertain of what exactly will happen, but its likely that another surge of warmth will come this way. I do not agree with the long-range model solutions of a more "blocked" and amplified upper level pattern, as it does not fit with the pattern that has been observed for the past few weeks. Even if that were to happen, I would continue expecting high temperatures in the mid-70's.