A long period of active weather characterized by amplification in the upper-levels will continue today as a negatively-tilted trough pushes into the region. At the surface, a low pressure system is expected to set up across South Dakota, with an adjacent cold front to the south and a warm front to the east. The cold front aforementioned is expected to set the stage for storms later today.

Sunday

Moisture will not be an issue with warm air advection occurring ahead of the cold front entering the area. Temperatures across southwest Iowa will be in the low 70s for many locations with accompanied dew points in the upper 50s to low 60s. This is where the best instability will be in place (west of Interstate 35).One area of concern in regards to severe parameters would be shear – 40-50 MPH unidirectional shear aloft will lead to an enhanced large hail threat, and perhaps a better tornado threat. This development should first occur across eastern Nebraska and western Iowa in the mid-afternoon and move through western Iowa in the evening, though the severe threat should diminish as the sun goes down.

Primary threat with these storms will be large hail over 2″ in diameter in some areas – maximized in eastern Nebraska. Damaging winds will also be possible with any storms that develop. The tornado threat should be isolated for the most part, but can’t be ruled out. The Storm Prediction Center has outlined an enhanced risk for far southwest Iowa – essentially the far western edges of Harrison, Mills, Fremont, and Pottawattamie counties. An elevated risk exists west of Interstate 35 with most other areas in marginal risk.

Monday

The low pressure system and accompanying fronts in the Plains on Sunday will shift into the upper Mississippi Valley region (near the Iowa/Minnesota border) on Monday morning as a weakening trough moves east. Dew points will be high once again with dew points in the 50s and 60s, conductive for severe weather development.

Uncertainty is a key thing to note with this outlook and any outlooks beyond day one. Confidence is low in regards to how long rain and cloudiness will hang around on Monday morning, and may impact peak heating – which influences instability, and therefore, severe weather. It’s also notable that several synoptic boundaries may be left behind from the storms the day before, again also impacting the setup of things.

As it currently stands, a marginal risk is in place across far eastern Iowa, northern Illinois, far southern Wisconsin, northern Indiana, northwestern Indiana, and southern Michigan. This may be upgraded to an elevated risk in later outlooks as confidence increases in placement, though this will probably be east of our immediate area. Nonetheless, with the parameters expected to be in place, thunderstorms will likely re-fire near the southward-extending cold front. Shear will be modest, leading to the threat of an isolated supercell or two. This risk will be refined later tonight after our D1 event is done.

Tuesday

Severe weather is again possible across the far southwest again on Tuesday, though the greater threat will be to the south of the state. Any storm that does skirt our area will be strong, perhaps with some small hail and gusty winds.

Wednesday

Another, more powerful upper-level trough will spurn out an upper-level low across the Rockies Monday night and shift into our area by Tuesday night. Very little predictability exists with this, and therefore I cannot inject a lot of confidence into this period at this time. For one, before this event, several rain chances exist, causing concern on whether we will be able to get enough heating to produce severe thunderstorms. There’s also the several severe events beforehand that may leave behind synoptic features, which can influence things greatly. There’s just many variables involved this far out.

If things work out and the warm sector reaches the southern part of the state (and storms are surface-based), all modes of severe weather will be possible. Plenty of instability will be available, leading to a large hail threat, with shear supporting the potential of tornadoes and damaging winds.

Obviously, more will be known about this as we go down the road. Make sure to check back for updates.

Stay tuned to the Iowa Weather Network for the latest information!