A BURNER AND A BANGER...
Your Saturday starts as a burner and if things go the way it looks, could end as a banger with strong thunderstorms and MUCH NEEDED RAIN. There are still some mesoscale details that are unknown but that's typical in convective set ups. Let's get to it.
WHAT WE KNOW
What's clearly evident is that Saturday is going to be a hot day. The pre-draw warmth ahead of a deepening storm will send temperatures into the upper 80s to low 90s, perhaps 94 or 95 in the far south. While the day starts with meager moisture, a surge of higher dew points arrives late in the day meaning we end Saturday on a muggier note. The combination of warmth and moisture will create significant CAPE (instability) in central Iowa by late afternoon.
The CAPE acts as fuel for thunderstorm development in western Iowa ahead of an upper level trough and a 50-60 kt mid level jet max. Storms are likely to initiate on the northern edge of the CAPE plume and advance into central Iowa by evening. A key ingredient in the strength of updrafts will be shear in the low to mid levels which at 0-6 km reaches 50+ kts. That is more than sufficient for supercells and severe storms. Low level shear is healthy with 0-3 km helicity values on NAM soundings reaching 250 m2/s2. Any rotating discrete storms through evening would support a tornado threat out west if that environment evolves. The greatest tornado threat currently looks to be west of a Waterloo to Cedar Rapids line. If storms eventually grow upscale into a line or bowing segments high wind would be the greatest concern for my area, especially for my counties in eastern Iowa. SPC has pushed the slight risk assessment substantially further east and it now includes all of my area.
This is the simulated radar of the 3K NAM around 6:00PM showing the storms just beginning to enter some of my western counties. If this trend holds Saturday should generally be dry and hot with storms not entering the picture until the evening or overnight.
As I mentioned earlier, there is still uncertainty regarding the strength of the storms as they enter my area. The big caveat is the amount of low level moisture. The higher the late day moisture return, the greater the instability and the stronger the updrafts. The worst case scenario would be a few heavy rain producing supercells and perhaps a tornado in my counties west of the Mississippi.
Aside from the severe weather threat, the other significant factor of this system is its ability to generate some much needed rain. With uncertainty in moisture fields and potentially rapid movement of cells, rainfall is not likely to be overly heavy, although a few of the stronger updrafts could dump more than an inch of rain in some localized spots. Overall, amounts of 1/4 to 1/2 inch seems like a good starting point for the northern 1/2 of my area. Totals could be less in the south further away from the better forcing. We should have a little better idea on rainfall amounts and placement by noon Saturday. At least for now, here's what guidance is suggesting for rainfall totals Saturday night through Monday.
The 3k NAM
The 12K NAM
Cooler weather with the threat of scattered showers will exist Sunday and Monday as moisture and vorticity wraps around the upper air low passing slowly through the Midwest. After that warmer readings return later in the week with the possibility of scattered thunderstorms. Plenty of time to figure that situation out.
Here's to a good weekend and hopefully a good rain for those of you who need it! Roll weather...TS
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