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Synoptic Overview
Based upon SPC objective analysis from the 12z upper air observations this morning, there is a 300 mb ridge established deeply over the southwestern United States, as denoted by the anticyclonic stream lines across that region. This has shifted its axis westward with time as a deeper and more intense shortwave trough is propagating east-southeastward across the northern Plains and into the Great Lakes region. This is accompanied by a 95 knot jet streak across Wisconsin, extending southeastward into the Mid Atlantic region. Upstream from that is another jet streak, with an associated max of 100 knots propagating eastward out of the northwestern United States. This is the feature of interest, as the jet maximum is what is influencing out weather today.
At 500 mb, a similar pattern is noted. A deep and strong trough is becoming established across the Great Lakes region, northern Ohio Valley and into the northeastern United States. Strong northwesterly flow is associated with a mid-level jet streak/flow of around 55 knots, centered over southern Minnesota and extending eastward into the lower portions of Michigan. The deep trough is fairly neutral to negative in terms of axis orientation. Looking at the 500 mb flow for North America as a whole, the deep trough should continue to dig into the northeastern United States, as the height contours on the western side of the trough axis are tighter than on the eastern side, indicating a digging pattern associated with the trough. This should set up additional northwesterly flow on the western side of the trough, allowing the shortwave over the northwestern US to quickly eject into the northern Plains and northern Ohio Valley, setting up tomorrow.
On the 700 mb level, the overall flow is similar, with a deep trough center over southeastern Canada, with stronger northwesterly flow. The trough axis is similar to the one at 500 mb, as it is much more neutral than anything else. This is digging over the northeastern US, creating a tighter height gradient between the ridge across the southwestern US and the digging trough into the northeastern US, and this is noted at the levels above 700 mb. The shortwave across the northwestern US was beginning to become phased into the 700 mb flow, and looks to propagate southeastward within the general northwesterly flow. Also of note: a tight moisture gradient was noted across the northern Ohio Valley at 12z this morning, indicating the boundary sinking southward that will impact our region this afternoon.
At 850 mb, the flow is a bit more relaxed across the region, with the strongest northwesterly flow being 30 knots in southern Ohio. Similarly to 700, 500 and 300 mb, there is a deep trough across southeastern Canada, digging into the northeastern US. The trough axis is a bit more positive in nature than at the other few levels, but over all it is difficult to discern it from a neutrally tilted trough. The shortwave over the northwestern US is less discernible and is weaker than at the other levels, but is being drawn into the overall flow at 850 mb. What is notable about the 850 mb analysis is the depth of the moisture across the southern Ohio Valley, extending back into the central plains. 850 mb Td of > 12°C are noted across much of that region, indicating a deep moisture pool to draw from to the south of the forecast region.
Surface analysis this morning lends difference stories. A surface pressure kink is noted extending into the Ohio Valley associated with the southward moving cold frontal boundary. To the south, a warm and moist boundary layer was present (supported by 850 mb dewpoint temps), with surface dewpoints analyzed in the upper 60s and lower 70s across a large area of the Ohio and Tennessee boundaries. Though the cold front is moving through, the depth and expanse of this airmass provides an air mass that can be easily advected northward. Over the northwestern US and on the front side of the rockies, a closed surface low is noted across east-central Montana. This is in association with the shortwave in the lower, mid and upper levels of the troposphere.
The forecast models initialized these synoptic features fairly well, with lends confidence to their evolution and breakdown of the event as a whole.
Expectations
Based on North American 12z analysis, I anticipate that the 500 mb trough across eastern Canada will continue to dig into southeastern Canada, and parts of the northeastern United States. This will establish further northwesterly flow across much of the Ohio Valley and Mid Atlantic states, allowing for the shortwave across the northwestern US to propagate into the northern/central Ohio Valley back into the. This should help to strengthen the surface low as it moves into the central US, setting up the event tomorrow afternoon into the evening time.
Forecast models
As has been mentioned and broken down, a shortwave trough is forecast to eject out of the northwestern United States, and move southeastward within northwesterly flow. The model consensus is fairly good regarding overall strength of the general flow across the region, with a 50-70 knot 500 mb jet streak remaining across the region for the majority of the timeframe. The shortwave trough is likely to stay mainly positively tilted in nature as it ejects southeastward across the northern plains and into the Great Lakes region. A vort max will be associated with the main 500 mb shortwave itself, but vorticity largely won’t be insanely high in association with the surface feature. Even so, PVA is expected across the region, which will be a factor in surface low development and convective sustainment (not as much as WAA at 850 mb).
The shortwave trough at all levels will be propagating to the east-southeast across the northern plains and into the upper Ohio Valley/Great lakes region by 06z Thursday. The shortwave as a whole will be propagating within the right entrance region of the upper level jet, creating plenty of divergence aloft across the upper Midwest and into the upper Ohio Valley. This will draw 50-70 knots of west-northwesterly flow across the region, with west-southwesterly 40-50 knot flow at the 700 mb level (by 00z Thursday). At 850 mb, southwesterly flow will develop, upwards of 40 knots, and this will increase in strength as the evening progresses and typical diurnal LLJ strengthening occurs. This will create a very volatile shear environment across eastern Iowa, through northern Illinois and into northern Indiana and Ohio. Additionally, large scale 850 mb warm air advection will be present across the warm sector during the evening hours, creating a region favorable for large scale upward motion.
In conjunction with this, guidance is suggesting a northwest-southeast oriented warm frontal boundary moving northeastward during the day, advecting in a warm, moist and unstable environment across the region. First of all, south of this warm front (as I mentioned in the initial synoptic analysis) is a very moisture rich boundary layer. Guidance has been consistently showing 850 mb dewpoints >14°C, indicating a moisture rich environment. Additionally, that depth continues to the surface. Widespread dewpoints of >70°F are present across the Midwest and Ohio Valley, and this will be combined with excellent surface heating south of that boundary. This should create a very unstable environment. The low developed at the surface by the upper air features will propagate into eastern Iowa by 00z, and will create even more shear across the region, and will be one of the primary forcing mechanisms going into the evening.
Additionally, Theta-e values at the surface are forecast to be greater than 360 K just along and south of that warm frontal boundary, with values of >350 K being widespread across much of the Midwest. This is an unusual level of instability, and will play a key role in the development and sustainment of the severe thunderstorms as they traverse the Ohio Valley tomorrow afternoon and evening.
Forecast and Forecast target
Some issues arise with this scenario. First of all, the depth and orientation of the warm frontal boundary are key. The warm frontal boundary placement is largely going to be determined by the depth at which today’s cold front gets to, as this will turn northward as that warm front tomorrow, and will likely define the path that the severe thunderstorms and eventual MCS follow.
Secondly, this warm front is likely to initiate convection even before the main forcing does to its west. Guidance is indicating a fairly uncapped environment along it, and this should force some elevated convection. This could be significant later in the evening, as this may force additional supercells along it to the east, which will enhance the tornado threat along it, but take instability from the likely MCS following afterwards.
Lastly, the further north this front gets, the more likely an eastward path of the MCS is. The SPC has the highest convective probabilities getting into southern Michigan, but I am not totally sure the warm front gets that far north. The warm front will represent a key boundary between a very moisture rich environment to its south, and a drier, less unstable one to the north of it. The eventual MCS is likely to take a path more towards the moisture rich environment than the less stable, drier one, and that is one reason I think the MCS will remain south of where the SPC has the outlooks currently. Also, the dewpoint and Theta-e gradient associated with this will be the driving factor for track (in my opinion), and those are more of a Chicago-Indy-Cincy type track, which I buy more so that the N IL-N IN-N OH track.
I think that by mid afternoon, the boundary layer in eastern Iowa will have destabilize substantially, and that the cold front will likely initiate severe thunderstorms in a well shear, highly unstable environment in eastern Iowa or northwestern Illinois. I think that these have a tornado threat associated with them as they move east-southeastward, and will definitely have one once they interact with the warm frontal boundary.
Once the cells congeal into an MCS, then it is game on. This is a difficult forecast to make because MCS meteorology isn’t easy, and MCS’ aren’t the easiest thing to predict or forecast for a day out. However, MCS’ love moisture rich environment, and to the south of that warm frontal boundary, there will be a very moisture rich environment. In conjunction with larger scale northwesterly flow, and the northwest-southeast orientation of the warm frontal boundary, I think the MCS likely takes a southeasterly track through Indiana and into southern Ohio and northern KY. Additionally, this system will likely be fed by an intense low-level jet getting into the evening and overnight hours, and this will likely enhance the wind damage potential associated with the system.
My main target: Ottawa, Illinois
As always, be sure to check in on the blog consistently for the latest on the forecast, and be sure to follow @WxOrNotBG on Twitter for any updates!