"Natural" Fireworks for Independence Day?
By Hunter Anderson
July 4, 2018 - 9:30 AM CST | 124 0
Residents across the Upper Midwest could be greeted with some natural fireworks displays as we move along through Independence Day 2018. The areas at highest risk of experiencing severe weather will be in southeast South Dakota, southern Minnesota, and northern Iowa. Forecast discussion is further below, but first I want to mention how well/poor the forecasts went from a verification standpoint this past weekend...
First thing I notice is a lot of blank space across southern Minnesota and northwest Wisconsin. The atmosphere in that area would get washed-out by the overnight outflow and associated strong convection (look at all those reports in central Minnesota!) The residual boundary would move along into southeast Minnesota, west-central Wisconsin, and northern Iowa before afternoon initiation occurred. The cluster of reports circled in red are mainly wind damage reports (some really minor damage too), as well as wind gust measurements less than 58mph (51kts.) We've decided from here-on-out that wind gusts measured at or greater than 58mph and reasonable wind damage will be kept under the "severe only" heading under "reports" regarding map layers. Prior to this change, only measured gusts of 58mph or greater would fall in that category. This will give us and our followers/visitors a better idea on the true impact any strong/damaging winds will have from a verification perspective. The tennis ball hail report (eastern-most reports in Upper Michigan) fell outside even the general t'storm/iso strong outline and I'm kicking myself for that.
I won't be mentioning much about the July 1 forecast, as that performed very well to my standards. In hindsight I should've extended the "strong/iso severe" outline further southeast to include southwest lower Michigan but the majority of those wind reports are under 58mph so it looks like I made the right decision after all. Although it's always my goal to create consistently accurate and precise forecasts, don't expect all of my forecasts to execute this well!
Sunday (July 1, 2018) T'storm Outlook
A strong mid-level ridge is expected to retrograde slightly along the Ohio River Valley throughout the period. A digging negative-tilt shortwave will round this ridge along its northern periphery across the U.S.-Canada international border throughout the day Wednesday. An increase in large scale ascent associated with multiple impulses, diffluent flow aloft, and mesoscale features as well as a strong belt of mid-upper level flow will result although amplitude appears to be suppressed due to the stubborn ridging. At the surface a cold front will sweep southeast across the northern plains and into the upper Mississippi River Valley as the attendant surface low lifts northeastward into Manitoba. By the end of the period (05Z Thursday) the cold front should extend from the Minnesota arrowhead southwestward into Nebraska.
Siouxland and into Wisconsin...
Complicated forecast scenario with a couple question marks: 1) will overnight convection remain given strong low-level jet placement, warm air advection regime, and elevated CAPE? and 2) if it does how much further does this offset residual outflow boundaries as opposed to if convection completely dissipated by late morning? It currently appears that the once quite impressive quasi-linear convective system (QLCS) that marched across North Dakota last night is still somewhat unblemished over northeast and eastern Minnesota by 14Z. This activity is expected to dwindle as it heads into a more capped and stable environment in western Wisconsin by late morning, but can't guarantee your parade will/won't be rained out.
Current thinking is that an outflow boundary and cold front intersection somewhere around the SD-IA-MN tri-state region will initiate robust convection with brief supercells possible. A moist warm sector with somewhat steep lapse rates aloft will yield ample MLCAPE values upwards to 4000 j/kg or more. Enhanced storm-scale boundaries could provide as an additional focus for robust updrafts owing to some very large hail and damaging winds. A brief tornado can't be ruled out either especially if a storm can ingest old outflow. Overall wind shear is much more favorable with regards to updraft organization further north where the better upper dynamics lie. Some opportunistic CAM runs show redevelopment near the old decaying cold pool around 19-20Z across northwest Wisconsin but confidence on this is low attm as they're poorly handling current observations. Hence why I refrained from placing a "SEVERE" outline in this region. Main hazards with any activity that redevelops across Wisconsin would initially be large hail before transitioning to damaging straight-line winds. Precipitable water values in the upper 10th percentiles across Iowa/Minnesota/Wisconsin (1.75"-1.9") could produce flooding reports especially once storms move right of the mean flow and train along the nose of any instability gradients or low level jets.
Enjoy the sunshine and heat today (you know, if you're into that kind of thing) ahead of the cold front--tomorrow the front moves through and provides some relief before the next rain producer arrives towards the latter half of the weekend. The 07/04 00Z GFS hints at some impressive shear embedded in the northwest flow with Sunday's wave.