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SEVERE WEATHER THREAT...



A spring storm with strong dynamics will produce showers and thunderstorms over the region Tuesday and Tuesday night. All modes of severe weather are possible, including tornadoes, large hail, and damaging winds. While storms could roam parts of the area much of the day, the most active periods should be in the morning and then again late afternoon and evening.


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WHAT WE KNOW...

At the time of this post, the Storm Prediction Center has much of the central Midwest highlighted as an area of concern for severe weather. In fact, 26,000 people are in a slight to enhanced risk of strong storms.



In my immediate area, much of eastern Iowa and WC Illinois are in a level 3 enhanced risk. The rest of the region is in a level 2 slight risk. You can see the geographic distinctions below.


The tornado risk is a bit of a mystery yet, but as I write this, SPC has a 10 percent hatched potential indicated over much of eastern Iowa. That means a 10-14% risk of a tornado within 25 miles of a point. The hatched region implies a 10% chance of a significant tornado of EF2 status or greater.



As for hail, the pink represents a 30-44% chance of severe hail within 25 miles of a point. The yellow is a 15-29 percent risk. Hatched areas point to a 10 percent chance of hail greater than 2" in diameter within 25 miles of a given point.



Damaging wind does not look like as big a concern as tornadoes and hail. However, a widespread 15-29% risk within 25 miles of a point exists.


POTENTIAL PROS:


We do know low level helicity is going to be significant and shear for rotating thunderstorms looks very high in the lower levels of the atmosphere, (0-1 KM). This should be enhanced even more by a warm front that sets up near I-80 in the afternoon. Potent effective bulk shear pegged up to 70 kts., along with very high storm relative helicity, points to significant hail and tornadoes (that is If discrete supercells can form). Cloud bases from any surface based storms (LCL's) are going to be low, which increases the chances of rotation reaching the ground. Moisture, another key ingredient, surges into the region during the day, with most areas seeing dew points of 60 north to 65 south.


POTENTIAL CONS:

Capping currently looks minimal, which should allow shower and thunderstorm development in the morning. How widespread this activity is and how long it (or its clouds) persist could eat into temperatures and instability, especially over the north. The degree of cloud cover or lingering precipitation in the afternoon, will determine the amount of CAPE (instability). If storm coverage and cloud cover remains high much of the day, it will drastically cut destabilization and severe weather potential.


The bottom line going into Tuesday, is that despite impressive parameters, the amount and severity of any storms will be highly contingent on heating which is reliant on breaks in the clouds and diminished rain Tuesday morning or afternoon.


One thing that did stand out to me was a Monday run of the 3K NAM (a convective allowing model called a CAM). It shows a long track helicity signature indicating a rotating supercell that tracks from NE of Kansas City all the way to just SW of Muscatine before dissipating. Several more significant tracks are noted surrounding that. That's the type of thing you would expect to see with a long tracked tornado.



These are just model simulations, and hopefully future runs do not show anything like that. That was a very ominous run. As expected, that run indicated a PDS sounding, which stands for particularly dangerous tornado sounding. The hodograph top right is classic, as are the veering wind barbs from surface to 5,000 ft.



Temperatures are going to be cooler Tuesday, but the air will become far more humid by evening, with dew points increasing more than 25 degrees. Temperatures will be highly dependent on cloud cover and rain. Readings could range substantially from 64 north to 76 south. Models are all over the board.


With the major increase in moisture, rainfall could be heavy (1-2 inches), especially north of the warm front and in stronger updrafts. The potential exists for a very active day if things come together right. I personally will be out in the field chasing, expect updates if needed and if I get any money shots, I will pass them along. Until then, roll weather...TS

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