On November 10th, 2021, at 17Z, I issued a forecast for 2 low pressure systems in the Midwest. As shown in the Weather Prediction Center’s 15Z surface analysis map, one of the systems was 1007 millibars and was centered over North Dakota at the time the forecast was issued, and the other was 1009 millibars and centered over Kansas.
Figure 1: The Weather Prediction Center’s surface analysis map issued at 15Z. The centers of the two low pressure systems are labeled with “L” and are located over North Dakota and Kansas. The 1007 millibar low pressure system over North Dakota has an occluded front, or a warm air mass caught between two cold air masses, branching off to the north and a warm front (a region where warm air is replacing cold air) and a cold front (a region where a cold air mass is overtaking a warm air mass) branching off the occluded front to the south. The 1009 millibar low pressure system over Kansas has a warm front and a cold front branching off the center of the low to the south.
The forecast calls for the low-pressure systems to combine and deepen because of upper-level support and the movement of the surface systems. The warm front and the cold front associated with the combined low are not forecasted to increase in strength significantly, as shown by the frontogenesis forecast in figure 4, however the cold front will provide the lifting mechanism, or method by which air parcels can be lifted vertically in the atmosphere, that will release instability and allow for the creation of thunderstorms. The convergence of the surface winds associated with the cold front, shown with the North Dakota cold front in figure 1 demonstrates not only the wind shift associated with the cold front, but also the directional convergence of winds at the surface that could lead to upward vertical motions. As shown in figure 5, the HRRR model is forecasting a squall line feature, and depending on the strength potentially a narrow cold frontal rainband feature, ahead of where the cold front is predicted to be at 10Z on Thursday. On the northern side of the low in Northern Michigan into Canada, there is forecasted to be snow.
Figures 2-3: Figure 2: GFS Model forecast initialized at 12Z on Wednesday for Thursday at 12Z of 700 millibar geopotential height in decameters and frontogenesis (fill pattern) in degrees Celsius/100km/3 hr. Figure 3: HRRR Model forecast initialized at 02Z on Thursday of 1000-500 millibar thickness in decameters, and reflectivity in the fill patterns (blue-purple spectrum being snow, green-warm spectrum being rain).
There is forecasted to be an intensification of the trough-ridge structure at 500 millibars prior to the surface cyclone’s peak in strength. This can be attributed to the precipitation associated with the system. With precipitation comes latent heat release (heat released when water vapor changes phase to liquid water), and with that comes higher geopotential heights. This will strengthen ridge feature downstream and by extension the trough upstream. In the strengthening of the trough and ridge, the divergence between the ridge downstream and the trough upstream increases in strength. This situation is termed cyclone self-development as the cyclone itself is intensifying the features that in turn strengthen the cyclone. The cyclone will reach its peak in strength approximately 24-36 hours from when the forecast was issued. This is due to dynamical mechanisms such as the westward tilt with height of the system, supporting the strengthening and combination of the two low-pressure systems. Subsequently, the dynamics will weaken and prevent optimal alignment for maintenance of the system in the upper levels.