Memorial Day Severe Weather Threat

This is a convoluted severe weather threat developing across the state this afternoon with several things working toward a major, high-end outbreak and several things working against.

There’s no question the atmosphere will be primed for big severe weather later today. As I’ve written about quite a bit on this blog we will have an elevated mixed layer over Connecticut this afternoon with northwesterly winds in the mid levels of the atmosphere. This is an immediate red flag for high end severe weather potential!! The elevated mixed layer is a region of very steep lapse rates (temperatures decreasing quickly with height) 2 to 3 miles up in the atmosphere. When coupled with warmth and humidity near the ground a “loaded gun” sounding develops. If you can tap into that instability – watch out!

The 12z sounding from Pittsburgh shows an elevated mixed layer firmly in place. The OKX sounding shows an EML as well with a lifted index of -4.5C. Not bad for 12z.

So we have the instability – but do we have enough wind shear? Wind shear is important in promoting storm organization and developing rotating updrafts. Without a rotating updraft you’re not getting big hail. Today it appears we have the shear.

While winds at 500mb (or about 3.5 mile up) are only modestly strong there is significant directional shear. Winds near the ground will be out of the south while winds upstairs will be out of the northwest! With enough speed this will develop long and curved hodographs as seen above off the 6z NAM. This indicates supercells are possible. The low level shear especially is quite impressive indicating the potential for tornadoes. Both deep-layer and low level shear is not to the level it was last June 1st, however.

So what’s the negative? What’s working against today’s storm potential is the lack of a solid trigger and large scale subsidence. You’d like to see some type of shortwave or disturbance in the atmosphere with falling 500mb heights. That’s absent today. What this may do is reduce the areal coverage of storms but no necessarily decrease the severity of storms.

While storms may be isolated they have the potential to be quite severe. High-end severe reports and even tornadoes are possible in western New England and New York today. What makes the threat even more difficult to ascertain is the mesoscale convective system that has developed over Ontario and Lake Ontario. Not sure how that will impact downstream development.

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13 thoughts on “Memorial Day Severe Weather Threat

  1. Yup, a total fail. The atmosphere was absolutely primed to go yesterday. Heights built in during the day and the cap held. Good news. Looks like more storms are possible today.

  2. Dear Mr Ranrahan,

    The day after you posted this, I wrote you a longish inquiry about the concept of an “elevated mixing level”. My curiosity continues to burn a hole in my mental pocket. Did you get the comment? If not, I will try to post it again
    thanks,.
    nicholas thompson

      • Dear Mr. Hanrahan,

        My hope is that now that we have all been sitting under the same d—-d maritime inversion for the last three days, you might have a moment to explain to me about cold pools and elevated mixing levels. What holds them up? remember, my thesis is that an “EML” is actually a “hot pool aloft” and that a “cold pool” is actually a “moist pool aloft”, relative to the layers below. .

        One point I have never been able to satisfy myself on is the trade-off between the buoyancy lost to condensation (less water vapor) and the buoyancy gained from the release of latent heat. So, as saturated air is forced upward we lose buoyancy to condensation but gain it because of the release of latent heat. How exactly does that work? We must gain more than we lose, else cumulus clouds wouldn’t cumulate, right?

        I promise you that if you let the sun come out, I will go back to my garden and stop bothering you with all these questions.

        Nick Thompson

  3. I am fascinated by weather jargon, it’s coming and going. I assume that weather people attend conferences, and come home with new jargon, because it appears, simultaneously, in blogs and forecaster discussions all over the country. Today’s new jargon is ‘EML”. As in,

    VERY WARM AND HUMID CONDITIONS WILL DEVELOP IN THE WARM SECTOR WITH COOLER AIRMASS N OF THE BOUNDARY. LEADING EDGE OF EML WILL BEGIN TO ADVECT E INTO REGION RESULTING IN STEEP MID LEVEL LAPSE RATES. THIS COMBINED WITH SFC HEATING WILL RESULT IN SBCAPES INCREASING TO 1000-2000 J/KG BY LATER TODAY WEST OF THE BOUNDARY ACROSS WESTERN
    NEW ENG. BUT SOUNDINGS INDICATE THERE MAY BE A UFFICIENT CAP FOR MUCH OF THE DAY WHICH COULD DELAY CONVECTION UNTIL VERY LATE TODAY OR MORE LIKELY THIS EVENING. WE WILL CARRY CHC POPS FOR MA AND N CT WITH SLIGHT CHC ELSEWHERE. [From a recent BOX forecast discussion]

    Now the thing about jargon is that people want to use it whether they understand it or not. My understanding of a Elevated Mixing Layer is that it is a chunk of dessert boundary layer that has been moved up the lee side of the dry line and now sits above cooler, moister air below it. From one point of view, an EML is a place where elevated convection can develop, because its lapse rates are so steep; from another point of view, it might be seen as a cap. Or it could be both: a temporary cap that restrains convection until it breaks through and can be REALLY BAD. So, the correct formulation is a Hot dry air (cT) airmass over running a cooler, moister (mT) air mass. This situation is stable in the mornings, but can become violently unstable when the boundary layer is heated. Have I got this right?

    This is what explains the “cap” and the violence of what happens when the cap is broken. As the sunlight heats the earth, it moistens and warms the layer below the elevated dry line until the buoyancy of the capped airmass approaches that of the layer above it. As soon as the EML begins to be moistened, the whole atmosphere turns over. If you add a high level jet to evacuate the column, then you get violent thunderstorms.

    I have a similar problem with the notion of an elevated cold pool of air (which is about to dominate our weather in SNE for the next three days. The mystery to me is how does a COLD pool stay elevated, given the relative weight of cold air. There are no sky hooks, here. If the air is heavier, it will SINK. My imagining, here is that, while cold, it is MOIST, and so lies on top of the dryer milder air below it. But, once the sun starts to act on the boundary layer, it starts to mix with the elevated cold pool and produces the cloudy showery afternoon weather that is predicted for the next few days. Thus, we really ought to be calling it a “moist” pool, rather than a “cold” pool, because it is its moisture that is making it ambiguous.

    I am sure you will say this is all nuts, but then I hope you will explain why dry air does not sink in the one case and cold air does not sink in the other.

    Thanks for your work,

    Nick Thompson.

    • Hi Nick, One of the reasons the EML remains in tact is that the EML resides above a sharp inversion which prevents vertical turbulent mixing from mixing out the moist layer in the boundary layer. The strong static stability of the cap results in a lid on the convection which takes some type of perturbation to overcome. The dry line in the Plains is in place at the origin of the EML… where the deep mixed boundary layer over the arid western Plains lies on one side of the dry line while a cooler and more moist boundary layer resides to the east. The drier air is advected over the cooler/moist air to the east and you have the EML. Occasionally the EML will advect north and east far from its source region and approach New England. Does that make sense?

  4. Dear Mr. Hanrahan,

    Everything makes SO much sense EXCEPT the idea that dry air is riding over wet air. Other things being equal, dry air should sink and mix with moist air below it. I think it doesn’t because the air “above” the dry line is [relatively speaking] MUCH WARMER than moist air below it, overcompensating for its greater density due to its dryness. We don’t call it the “hot line” because its [absolute] temperature is actually cooler, due to adiabatic cooling. But its temperature, relative to altitude is greater. It might be called a “dry inversion”.

    A similar problem exists, does it not, concerning a “cold pool”? Other things being equal, cold air should sink and mix with the warmer air below it. I think it doesnt because the air in the cold pool is much MOISTER than the warmer air below it, overcompensating for its greater density due to its coldness. It’s a wet inversion.

    In both cases, the reason you don’t get mixing early in the day is because the higher layer is actually LIGHTER than the lower layer, and remains so until diurnal warming makes the boundary layer more buoyant than the layer above it.

    Yesterday afternoon was a wonderful example of the “wet pool” phenomenon. Plaques of flat strato cumulus every where, but every once in a while, one of them would get a nudge from below and start to pile up.

    I wish I could read skew-T’s with greater confidence, but I think they should show that a dry inversion is accompanied by a sharp “zag” of the temperature line to the right as the dewpoint line zigs to the left, and a wet inversion (cold pool?) is accompanied by a rightward zag of the dewpoint line as the temperature line zigs to the left.

    No sky hooks or membranes, in meteorology. Less bouyant air NEVER overlays more bouyant air, right?

    Nick

  5. Much to my surprise, the NWS is predicting severe weather for this afternoon. A quick check of the Albany SkewT seems to indicate an elevated mixing level (“hot dry pool”) at about 500mb. Here in central Mass the only clouds are a vague cirrus haze to the W. If ever I saw a stable looking sky, this would be it.

    It would be a wonderful thing for all us weather nerds out here if you would talk through your thinking about this situation as it develops, today. How are we going to get from a cloudless sky to Hail and strong winds by nightfall?

    I would paste in the Albany 12Z skew T but , it won’t paste.

    Nick

  6. Ok, so despite the dire forecast, not even a shower at my central mass location, all day. Now, there is a line of showers on the radar approaching obliquely from the North. Not sure it will make it into MA. The late afternoon skewT’s show that the “dry pool” (EML?) is no longer over albany but has moved eastward to ME. However, Albany has developed some healthy vertical sheer. So, was today a loss for the EML theory, or do I not understand?

    Nick

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