Heavy Rainfall Threat Around the Corner

Synopsis

 A certain situation is beginning to arise this weekend which will set up the possibility of heavy showers and thunderstorms from Thursday night through Sunday afternoon. There are several factors that will come into play:

1.) A closed low over the Central Plains and Midwest will begin to open-up in response to a digging shortwave in the Hudson Bay vortex. This will cause the ejection of a series of shortwaves around a positively-tilted trough, and allow for periods of thunderstorms.

2.)  In response to this deepening trough, the low-level "Gulf stream" or low-level jet will begin to strengthen, providing direct transport of a very moist and tropical air mass. Dew points will approach the upper 60's and lower 70's, with mid-level lapse rates approaching the moist adiabatic lapse rate (with at times a nearly saturated mid-level profile).

3.) The slow-moving nature of the trough, and frontal zone, as well as fairly weak upper level winds will allow for the possibility of slow-moving and training thunderstorm complexes. MCS development will also be likely Thursday night and Friday night as the low-level jet strengthens. 

Thus, these factors will come together to produce the possibility for a multi-day heavy rain event, culminating on Saturday night, and likely leading to the possibility of severe weather at times. 

 

 Rainfall threat/probability

As we can see from the above "factors and ingredients".. The possibility remains for flooding and heavy rainfall this weekend. Let's take a look at what the models are saying.


1.) A very rich and deep GOM tropical air mass will be entrenched across the area Friday through Saturday. PWAT (Precipitable Water) values will exceed 1.5-2 inches, indicating a rather incredibly moist atmosphere, and the potential for heavy rainfall, if deep-layer moisture convergence and efficency is also involved. 

2.) A positively-tilted, slow-moving upper level trough will be the main "controller" of this weather situation. The slow-moving trough will only allow for weaker low-level convergence along the stalling surface front, while upper level divergence from the upper level jet core could allow for multiple thunderstorm complexes to develop parallel to the front in a moderately-unstable and moist air mass. Weaker winds aloft will allow for training thunderstorms, especially in tandem with strong storm-relative inflow from the low-level jet.

3.) These periods of thunderstorms will not likely be continuous, given the lack of strong upper level forcing and only modest instability at times. They will develop at peak heating times, and organize into complexes in regions of stronger wind-shear. And with this, comes the possibility of highly localized flash-flooding.. Some areas will see lower rainfall amounts near a half inch, while other areas could see as much as four inches of rain (depending on how thunderstorms train).


Models currently vary on how much rainfall will fall across the region, and given the isolated nature of thunderstorms, its going to be hard to pinpoint a general range of rainfall amounts, but at this time, I will have to agree with the WPC (former HPC), who are forecasting a general 2-3 inches of rainfall areawide. While I do not see such rainfall amounts being "widespread", I do see a wide variety of rainfall amounts, ranging from a half an inch to over 4 inches possible. Heavy rainfall on this order could cause regions of minor to moderate flooding.

 

Severe Weather

Although I do not expect the severe weather threat to be nearly as substantial as the threat over the Central Plains today or tomorrow, there is still the possibility for hail-wielding or thunderstorms with damaging winds from Thursday night through Saturday. This will all hinge upon the moisture-content of the lower atmosphere, solar insolation, and the depth of the PBL on both Thursday, Friday and Saturday.

1.) The #1 factor in this event will be instability. Dew points are indicated to approach the upper 60's and lower 70's, with a very moist profile throughout the entire atmosphere by Friday. Although mid-level lapse rates will be lower due to the tropical influence, surface heating and mixing will allow for MLCAPE to approach 2000 J/kg both tomorrow and possibly Friday. This will be heavily dependent on surface heating, as mentioned before. Models continue to develop scattered numerous showers and thunderstorms Thursday afternoon through Saturday, but I disagree with their assessment. The models, the NAM especially, seem to be overdeveloping convection, and developing convection too far away from the front itself. Thus, it seems to me that models are too cool with surface temperatures. I believe that if cloud cover is not widespread tomorrow or Friday, temperatures could reach the upper 80's on both days. If this is the case, expect substantial instability.


2.) 0-6 km bulk shear will be lacking through most of the event (unless, like I described before, a jet streak develops along the eastern flank of the trough), and thus storm organization will be limited to localized shear along boundaries, and the development of shear due to storm-scale processes. Thus, any kind of organized storm development will also be limited to multi-cellular storms and bow-echoes.


3.) With the above factors, I think the severe weather threat in itself will be limited, especially if we see a longer duration of cloud cover tomorrow, Friday, and Saturday. But even so, we need to be on the look out for strong to severe thunderstorms.