3:30pm Update: So far, everything is on track with respect to downplaying the event relative to SPCs and other outlooks’ expectations. As expected, there is still a lingering strong capping inversion in the warm sector, so there is not enough lift to generate strong thunderstorms to take advantage of the instability. Additionally, the low-level jet (LLJ) still continues to lag significantly behind the daytime heating, so the low-level winds are not strong enough for additional lift, superceullar organization, nor for transporting severe wind gusts down to the surface. There are only a few isolated wind reports so far, which is far below the threshold for an “enhanced” risk.
We still do think that later this evening, when the storm system strengthens a bit and frontal forcing increases, we’ll get a decent low-level jet increase late this afternoon and through this evening, which may produce a strong to marginally severe line of storms along the cold front from eastern Indiana into Ohio. We still think Ohio may be more likely to see strong to severe winds since they are in the best location for an increase in the low-level jet. Some initially isolated discrete cells out ahead of the line may be able to produce some hail in Indiana and perhaps parts of Ohio, but the linear wind profiles indicate a transition to a line with moist mid-levels, so we do not expect many severe hail reports.
Additionally, the low-level jet is way too linear and weak right now to generate the necessary updraft helicity for tornadoes — which is why we never bought into a true tornado threat. There still may be some isolated tornado reports later this evening if some discrete cells can form ahead of the line of thunderstorms when the low-level jet does increase in strength. Fortunately, any tornado threat will be isolated and any tornado that does form should be weak.
Previous Content: During this early-spring…I mean late-winter day, the ingredients are beginning to come together for an impressive severe weather episode on Friday in the Ohio Valley. A strengthening storm system which will provide blizzard conditions for parts of the Northern Plains will have its warm sector line up perfectly with Gulf of Mexico moisture transport and instability. Furthermore, this strong storm system will be dragging a strong cold front to its east, helping to increase low-level moisture return via strong low-level winds from the Gulf of Mexico, provide plenty of lift via the frontal forcing, and also provide very cold mid-level temperatures. This is because behind the cold front, mid-level temperatures are very cold since there is actual winter-cold behind the front, but mid-level winds often blow so fast that they advect the mid-level cold air to places on the warm side/ahead of the cold front, helping to juxtapose a warm, moist airmass with cold air aloft and a lifting mechanism — perfect ingredients for thunderstorm formation. This cold air aloft and thus very strong thermodynamic and dynamic lifting with these types of systems this time of year often make up for not having a true summer-like low-level airmass and can provide lift equal to that of spring and summer-like outbreaks.
From here, the devil lies within the details as far as taking this severe weather episode from minor to major. At this point, we think this event will be somewhere in between. While the above details portend a significant outbreak, there are more nuances to this event, such as jet-streak behavior, as an alignment of strong jet streaks can turn lift from merely sufficient for thunderstorms, to majorly supportive. And what we see currently is a not ideal alignment of jet streaks in the low-levels and the upper-levels with when the storms actually initiate.
The first problem we see when it comes to having a big severe weather outbreak is the fact that when the atmospheric instability and mid-level lapse rates are highest, there is a “cap” in the mid-levels of the atmosphere. This means there is a small inversion that prevents convective clouds from lifting high enough to generate rain and thunderstorms. While sometimes a cap can prevent what we call “crapvection” — which is weak convective showers that stabilize the boundary layer — this cap lasts long enough so that thunderstorms do not initiate until after peak daytime heating, and also after the mid-level lapse rates are their strongest. This means that the thunderstorms will not be taking advantage of the most dangerous atmospheric conditions. These conditions are mainly true in Ohio, which is a bit further east of the front.
Further west in Indiana, closer to the front, the cap will be broken a bit earlier — probably at around 4:00pm, which is closer to daytime heating. However, the low-level winds are still pretty weak at this point, as the low-level jet streak is still in its infant stages of development. Thus, while lift may be strong enough for a squall of rain and some thunder, the wind fields do not support severe wind gusts mixing down to the surface. Also the airmass aloft gets pretty moist when the storms roll through, so that means it’s less dense there. This means there won’t be as much tendency for the high winds aloft (above the LLJ) to sink southward to the surface — aka low downdraft CAPE. This is not to say that some wind gusts between 40-50mph cannot be had, but we do not see widespread severe wind reports.
Now going back further east into Ohio, the low-level jet does strengthen significantly at night — after 8pm. Thus, the frontal forcing and the cap breaking does coincide with stronger low-level winds. There is still the aforementioned issue of not juxtaposing ideal daytime heating and instability with the storms, as well as the fact that the airmass aloft is too moist for good downdrafts. But with the increased low-level jet in Ohio, there may be enough lift, speed shear for linear organization of thunderstorms, and strong enough winds at the low-levels of the atmosphere where severe wind gusts can still be had despite the lack of good downdraft CAPE.
Notice how in the above image, the downdraft CAPE is somewhat high, though not tremendously high — but it’s highest in areas that are still capped. Thus, the downdraft CAPE is inflated by the cap causing dry air aloft, and it’s not even that high to begin with. Thus, when the cap breaks, the downdraft CAPE lowers a bit more, limiting the overall potential for severe weather.
Now as far as a hail and tornado threat is concerned, it’s pretty unlikely. If a rogue discrete cell can develop ahead of the line and break the cap, there could be some isolated hail reports while the air aloft is still dry. Otherwise, when the actual line of storms hit, the airmass aloft looks a bit too moist and updrafts are a bit too weak to support hail. However, hail below severe criteria size (1″ in diameter) is more likely. Additionally, there is very little directional wind shear — the winds in the entire atmospheric profile are basically in the same direction, which means it will be very difficult to obtain the necessary rotation for tornadoes. However, a very isolated tornado may be possible in Ohio where the low-level jet is at its strongest and thus low-level wind shear can still be somewhat sufficient — but probably still below typical tornado thresholds.
So overall, we are looking at a few isolated cells to develop in Indiana around 4:00pm, which could have some isolated severe hail reports, but probably just below severe criteria for wind gusts. Later in the evening, a line of storms may form in Indiana with the cold front at around 8:00pm, and then strengthen as it moves east towards Ohio from 9:00pm through midnight as the low-level jet increases. Some severe wind gusts are possible in Ohio, but severe hail and tornado threats are isolated — though small hail may be somewhat prevalent.