A rather abundant amount of remnant moisture, Hurrican Alex of yore, is centered over the Rio Grand valley in south central Texas, and it continues to progress steadily northward. The northern extent of this deep tropical moisture plume is currently advecting northward into the Missouri River Valley. As this moisture plume continues to advance northward, conditions appear to be coalescing for a rather substantial heavy rainfall threat across the Middle Mississippi River Valley, namely northern Missouri, eastern Nebraska, northeast Kansas, large parts of Iowa and perhaps to a lesser extent northwest Illinois and far southern Wisconsin.
Traditionally speaking, once a tropical system comes ashore, the numerical models often have difficulty with the remaining progression of extratropical remnants of these systems, at least as far as mesoscale features are concerned. Latent heat feedback and convective issues often plague the veracity of model output with these systems. When compiling extended range forecasts for these types of systems, it’s best to view the overall synoptic-scale of the event as opposed to relying too heavily on numerical guidance to resolve mesoscale details (e.g., placement of heavy rainfall axis, duration of rainfall, timing of surface features, etc.).
There is also a high pressure system that resides over the eastern CONUS, which has dry air entrenched across the eastern Great Lakes. Typically these ridges of high pressure are slow to progress, which creates a somewhat archetypal blocking pattern. The triggering mechanism for further enhanced convection in the form of multiple rounds of showers and thunderstorms will be a slow moving cold front, with the slow movement of this frontal boundary owed to the aformentioned blocking pattern, along with ongoing convection acting to achromatize this frontal boundary.
In addition to the slow moving frontal system and deep moisture fields, there are a number of additional ingredients amalgamating for a heavy rainfall event across portions of the eastern Midwest/western Corn Belt region. Enhanced lift at 250 mb (see below) will aid in the development of thunderstorms, though deep convection (severe weather) is unlikely due to the absence of additional thermodynamic and kinematic features. Nevertheless, strong upper air support will only act to aid in the development of widespread convection that will at least result in respectable rainfall totals.
Looking at mid-level height profiles, a discernible pattern is evident in that mid-level winds across the Midwest indicate that convective profiles will remain nearly parallel to the frontal boundary. While this will act to promulgate training rainfall patterns, the flow is also perpendicular to many river basins across the region, which may limit the amount of rainfall that any one given river basin might potentially expect to see materialize.
It should be noted that while our forecast isn’t indubitably preferential towards the NAM/WRF per se, because essentially all models are congruous in regards to the evolution of the synoptic-scale features of this event, it does depict what we feel is a rather accurate projection of the evolution of this forthcoming convective scenario.
Due to recent model trends, there is a potential that this situation could become a significant flooding event over portions of Iowa. Looking at the forecast SKEW-T diagram for KUIN (Quincy, IL) per the WRF (see below), note the extent of saturation within the column layer, and the indication of a PW value sitting at 5.25 cm. For a refresher on how to discern various features of SKEW-T diagrams, please visit Ryan’s previous entry here.
River levels in south central and southeast Iowa are already running rather high, an indicated by this hydrograph released by the National Weather Service for the Des Moines River at Ottumwa, Iowa (see below). Forecast rainfall totals over the next several days could seriously aggravate an already austere hydrological situation across that region.
Interested parties may wish to closely monitor this developing situation, as flash flooding is a very real possibility in areas that are forecast to see significant rainfall amounts in excess of 3.00 inches. In areas where soils are already saturated and river levels are running quite high, even 2.00 inches or more of additional rainfall will enhance the flash flooding and river flooding threat across those areas, though likely to a far lesser extent due to the dry conditions that have been present over the area for the past 7+ days.
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