Sunday Night Update: Severe Weather Still Possible Monday

SUNDAY NIGHT UPDATE: Not much has changed in my thoughts on Monday's severe weather risk. A few questions still exist on timing and how morning convection will impact afternoon instability, but severe weather still appears very possible Monday. 

Sunday evening, a low-level jet is already showing up over Southern Illinois drawing warm, humid air north. The LLJ has advected dewpoints into the low 60's already Sunday evening. Showers and storms overnight are possible with fairly widespread convection by daybreak Monday near a surface warm front. Rain and clouds will likely limit instability, but a wind field that will continue to strengthen should make up for the lacking instability as the main line approaches from the west along and ahead of a cold front. Mid to late afternoon is still the time frame in my opinion for Southern Illinois with some models even suggesting early evening. I'm still thinking that the biggest threat is straight-line wind damage, however, a few tornadoes will be possible. In addition to the threat for severe weather, heavy rain will be possible with these storms and localized flooding is possible.

Severe weather is nothing to be scared of, as long as you are prepared. Our job as meteorologists is not to scare you or hold your hand through a severe weather event. Our job is to inform you of the possibilities and potential of approaching weather that may pose a threat. It is your job to make sure you have a plan in place and know how to act in the case of severe weather. Until storms are on the doorstep, it's simply impossible to know exactly who is in the path of the worse weather. We will continue to monitor as new model data rolls in this evening and throughout the day Monday and update on social media and break into programing if the scenario calls for it.

SATURDAY: ANALYSIS: Over the last few days, persistent cool, damp weather has plagued Southern Illinois on the northern side of a stationary front. While surface winds have generally been from the northeast, upper-level winds have been predominately from the southwest creating overrunning rainfall and resulting in a considerable amount of clouds. A strengthening storm system will move through the Rocky Mountains into the Plains Sunday, pushing the stationary front and associated energy to the east. Ahead of the system in the Plains, dominate south flow will return as we await the arrival of the system Monday.

As the storm system arrives Monday morning drawing a warm front north, storms are likely to develop, but will be below severe limitations. As the warm front lifts north, surface dewpoints, temperatures, and winds will begin to go up as Southern Illinois becomes planted in the "warm sector". By early afternoon, we'll be turning our attention to showers and storms that have developed along and ahead of the cold front as they begin to push into western parts of Southeast Missouri. Models still differ in timing, but I'm currently leaning towards a mid-late afternoon arrival in Southern Illinois. As storms move in, models are indicating a very impressive wind field, but only limited amounts of instability. These situations are common in the lower Ohio Valley throughout the Fall season and have a history of producing the potential for violent weather.

HiRes NAM 925mb 00Z Tuesday (Monday evening) -- Strong low-level winds can be seen over Southern Illinois as warm air gets wrapped counter-clockwise around the parent low.

HiRes NAM Skew-T Monday afternoon showing the wind field (along the right side) strengthening from the south/southwest and a stiff southeast surface wind. CAPE 1000J/kg+ still a few hours ahead of the front.

HiRes NAM Radar Simulation 21Z Monday (4PM). A line of strong storms can be seen entering Southeast Missouri.  Disclaimer: Radar Simulations are far from exact and simply one model's solution.

SUMMARY: My current thinking at the time of this writing: A "squall line" is likely to push through during the mid to late afternoon hours in Southern Illinois. I feel the biggest threat is for embedded line segments called bow echoes creating damaging straight-line winds. With the amount of wind, and the directional shear involved a few tornadoes are possible, especially along any bookends of bowing segments. In addition to severe weather, very heavy rain is likely and may cause some localized flooding.