Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
719 FXUS63 KLOT 242113 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 413 PM CDT Mon Jun 24 2024 .KEY MESSAGES... - It is increasingly likely that a line of thunderstorms will march through area late overnight into early tomorrow (Tuesday) morning. Damaging winds will be possible with these storms, particularly as you get closer to the Illinois- Wisconsin state line. - Another round of thunderstorms appears possible tomorrow (Tuesday) afternoon into Tuesday night, with some potential for severe weather and/or flash flooding, though confidence in this round of storms is low. - Another period of active weather with strong to severe thunderstorms and flash flooding is possible Friday into Saturday. && .DISCUSSION... Issued at 413 PM CDT Mon Jun 24 2024 Through Tuesday Night: The forecast through Tuesday night remains a challenging one, though there have been some signals that have emerged in the latest hi-res guidance that have increased confidence in some aspects of our forecast. First, convection is expected to erupt late this afternoon/early this evening near a surface triple point in the vicinity of the Twin Cities. Upscale growth of this convection into a bowing MCS appears quite likely late this evening, with Corfidi downshear propagation vectors and a northwest-southeast oriented thermal gradient across Wisconsin suggesting that the apex of this MCS will propagate southeastward into southeast Wisconsin overnight. West/southwest of the bow`s apex, convection should have a tendency to propagate in a more southerly direction, and the expectation is that this southward- propagating, quasi- zonally oriented portion of the MCS will track through our forecast area late overnight/early tomorrow morning. Almost ubiquitously westerly flow above 850 mb should concentrate the greatest damaging wind potential in the eastward-surging segments of the MCS, and these line segments would also likely carry some QLCS tornado threat as well. The current thinking is that these more intense line segments will probably remain to our north in Wisconsin, but it could at least be a close call for our northern counties. Regardless, the substantial degree of buoyancy aloft will still support the potential for damaging downburst winds within any deep convection associated with the MCS, though the near-surface stable layer will get increasingly stout with time and southward extent, putting into question how late into the night/morning the damaging wind threat will persist. The forecast for Tuesday remains shrouded in uncertainty and hinges heavily upon what the MCS does in the morning. With that being said, two general scenarios appear plausible. The first, and most likely, scenario is that the MCS plows through the entirety of our CWA and shunts its outflow to our south and west. This outflow would then serve as an effective frontal zone and would be the main focus for convective redevelopment Tuesday into Tuesday night, which would accordingly largely remain concentrated to our south and west. In such a scenario, most or all of our forecast area would find itself capped and sans precipitation through at least the early-mid afternoon. However, it`s still conceivable that isolated to scattered convection could redevelop thereafter -- either within a narrow ribbon of isentropic ascent or along any surface boundaries in the area, be it the lagging true cold front and/or a potential differential heating boundary along the northern periphery of the cloud shield associated with the MCS. The other, less likely, scenario is that tonight`s MCS largely misses us or weakens faster than expected. Either of these outcomes would likely result in either the MCS`s outflow boundary getting laid out across our forecast area during the daytime on Tuesday, or the outflow boundary becoming diffuse/getting washed out and allowing for the lagging true cold front to become the main focus for convective development in the afternoon and evening. Regardless of which boundary would become the dominant feature, it would likely get lit up with convection, much of which would become severe with damaging winds and large hail as the main threats given the strong to extreme degree of instability but somewhat lacking shear that would be expected to be present in this scenario. Perhaps even more concerning is that this setup would conceptually resemble a Maddox flash flood setup, with a low-level jet overriding and nearly paralleling the initiating boundary, allowing for convection to continuously regenerate and train over the same areas, which with precipitable water values near and in excess of 2" would almost certainly lead to flash flooding occurring somewhere. Though again, this scenario is the least likely of the two general scenarios for Tuesday, and if the first scenario were to verify, then the greatest potential for flash flooding should remain to the south/southwest of our forecast area. Last but not least, there is still unsurprisingly some uncertainty for our high temperature forecast tomorrow with the first scenario generally favoring cooler (but still fairly warm) temperatures than the second scenario. Fortunately, the heat and humidity shouldn`t be too oppressive in either scenario with 100F looking like a reasonable higher bound for peak heat indices tomorrow. Ogorek Wednesday through Monday: A cold front will press across the region on Wednesday. Depending on how things evolve Tuesday evening and overnight, some convection may be ongoing across the area Wednesday morning, mainly south of I-80, but the trend should be towards a drier one through the day. Some guidance does show some fairly deep boundary layer mixing developing through the afternoon, perhaps enough to pop a few high-based showers, but this doesn`t seem like a significant enough signal to justify PoPs at this point. A brief period of dry and quiet conditions will then prevail through Thursday as a transient region of high pressure drifts overhead. By Thursday night into Friday morning, the surface high will be departing and shuttling across the northeast, yielding a return of moist southerly trajectories in the region. A series of low- amplitude/subtle shortwaves emanating from a robust upper low pinwheeling across Montana and into Saskatchewan will likely get swept up within freshening mid and upper level west-southwesterly flow. This in turn will lead a commensurate increase in low-level warm advection across Iowa, which will eventually spill east of the Mississippi River during the day on Friday. Latest model guidance suggests that warm advection/isentropic upglide-induced convection will probably develop late Thursday night into Friday morning across Iowa within the plume of returning elevated instability. However, latest indications are that this activity may outpace the slowly-returning elevated mixed layer leading to a gradual weakening trend through the day. However, Friday night continues to look like a period we`ll have to watch closely, as another area of low pressure and associated cold front will be impinging on the region from the north. Ahead of this, intensifying warm advection acting on any remnant outflow from morning activity will also finally be able to impinge on the returning warm/moist sector as the kinematic parameter space also becomes more supportive of storm organization. Still pretty far out with plenty of spread and uncertainties, but this is a period we`ll continue to closely eye for a severe and also flash flood potential with PWATs forecast to surge into the 2 to 2.5 inch range. We`ll get into another brief quiet period over the weekend as high pressure returns. From a synoptic perspective, winds may surge a bit down the lake as the high arrives which could yield a building wave/rip current potential over the weekend. Carlaw && .AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 1240 PM CDT Mon Jun 24 2024 Aviation weather concerns are: - Increasing thunderstorm chances late tonight/Tuesday morning with some threat for strong to severe wind gusts. A lake breeze has brought a NE/E wind shift to ORD, MDW, and GYY. Whether it continues to surge at its current pace towards DPA or slows down remains a bit unclear, but elected to move up the timing to account for the faster movement. Regardless, winds will eventually return to a S and SW direction overnight. The main change was to convert the PROB30 groups to TEMPOs for TSRA late tonight and Tuesday morning. Expectation is for explosive thunderstorm development across Minnesota this evening within a highly unstable environment, with thunderstorms growing into a surging complex. While uncertainties remain regarding specific timing of this complex into the c90 (and refinements to TSRA timing will likely be needed), confidence was high enough to justify TEMPOs. While this thunderstorm complex should be in a gradual weakening phase, strong to severe wind gusts will remain possible as it moves through the region. A VCTS threat may continue a bit beyond the end time of the current TEMP groups, but this was not high enough confidence to justify a mention at this time. Conditions may remain largely quiet through Tuesday in the wake of this morning activity with gusty southwesterly winds developing through the morning and afternoon. Carlaw && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory from 1 AM to 1 PM CDT Tuesday for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago