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Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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241 FXUS63 KLOT 131719 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1219 PM CDT Sat Jul 13 2024 .KEY MESSAGES... - Today will be more humid than yesterday, with peak afternoon heat indices in the mid to upper 90s. - Several waves of thunderstorms may occur from this afternoon into Tuesday with threats for severe weather and flash flooding. - Sunday and Monday may feature dangerous heat, though the timing and coverage of thunderstorms may limit the intensity and coverage of the heat. && .UPDATE... Issued at 1052 AM CDT Sat Jul 13 2024 Current radar imagery shows an ongoing MCS moving from eastern MN into WI. A mid-level shortwave is associated with the MCS and is expected to dig to the southeast and into the CWA this afternoon. A plume of steeper mid-level lapse rates (7+ C/km) has been slowly working its way northeastward out of MO and into southeastern IA and western IL. Satellite imagery shows some ACCAS associated with this area of steeper lapse rates. As the steeper mid-level lapse rates continue to advect northeast and are intercepted by the shortwave, current thinking is that there may be enough lift to initiate some convection across northwestern IL and the western half of the CWA this afternoon. Timing of this looks to be late afternoon (21-22 Z) through the early evening. Many of the 12Z CAMs (particularly the 3kNAM, WRF-ARW, and WRF-NSSL) seem to have caught onto this scenario playing out as well. Although confidence is a bit higher in this scenario than anything else at this point, confidence remains low in this scenario actually playing out. Thus have opted for chance (20-30%) PoPs for the western portion of the CWA through 00Z. Any storms that develop this afternoon could become strong to severe given the large reservoir of instability (2000+ J/kg MLCAPE) that is expected this afternoon. Relatively weak shear values (20-25 kts) should facilitate more of a pulse/multi- cellular storm mode today (as opposed to more organized/supercellular convection), with locally damaging downburst winds looking like the main severe threats. Moving into the evening, thinking remains the same in that some sort of ongoing convection will move into the area, though guidance continues to be in disagreement on the timing and progression of these storms. Carothers && .DISCUSSION... Issued at 305 AM CDT Sat Jul 13 2024 Through Sunday: Regional water vapor imagery depicts several clusters of thunderstorms across the northern Plains, each apparently tied to subtle vort maxes or upper-level shortwaves along the northern periphery of an expansive summertime ridge centered over the central US. In addition, several ephemeral areas of mid-level ACCAS are evident on nighttime microphysics imagery along the implied eastern edge of an expansive EML plume, with one such area in southern Minnesota and another in west-central Iowa. Ahead of the thunderstorm clusters and beneath the EML plume is an expansive reservoir of deep low-level moisture arcing from the Gulf Coast to southern Manitoba along the southwestern side of a surface high centered over the eastern Great Lakes. Taken altogether, the surface to upper-level pattern is quintessential of summertime. Today, the surface high pressure system in the northeastern United States will drift further away, allowing for the plume of low-level moisture to slosh northeastward into northern Illinois and northwestern Indiana. Today will hence feel noticeably more humid than yesterday, with peak heat indices poised to climb into the mid to upper 90s. An initially weak surface pressure gradient should support the development of a lake breeze, though inland penetration may be stunted by mid- afternoon as synoptic southwesterly winds increase in magnitude. With several clusters of thunderstorms already ongoing well upstream of our area, chances for some sort of convectively- augmented vort max, MCV, or shortwave, not to mention remnant or reinvigorating convection itself or some combination thereof, to propagate southeastward into our destabilizing airmass and/or activate the eastward-spreading EML plume seems inevitable. Indeed, within the myriad of forecast scenarios advertised by successive runs of CAMs is a commonality of at least one but potentially several episodes of thunderstorms in our general region in quick succession, perhaps as early as later this morning but more likely from this evening and through the overnight hours. There is always a degree of inherent low forecast confidence in these types of patterns, but felt there was enough of a signal to cautiously increase PoPs from around 20% from late morning to the 40 to 50% range from sunset onward. (If I had to pick which CAMs that appears most realistic, the 00Z NAM3/HRDPS looked pretty reasonable). In terms of the threat for severe weather today and tonight, pattern recognition of a deep plume of moisture-laden instability displaced south of the upper-level jet supports outflow-dominant clusters with locally severe winds (e.g. not a widespread severe weather threat). Perhaps more concerning would be the threat for flash flooding, given repeated/reinforcing outflows would orient perpendicular to the increasing nocturnal low-level jet and associated replenishing feed of 1.75"+ PWATs. Such a regime can support repeated rounds of convection over the same area, if not outright backbuilding, leading to a focused zone of copious rainfall amounts greater than 4". At this point, the level 1 to 2 threat for damaging winds (consistent with isolated to scattered reports of wind damage) advertised by SPC looks appropriate. The Level 1 threat level for flash flooding (consistent with isolated reports) is appropriate for now, but may need to be increased in tandem with the issuance of a Flood Watch if confidence in repeated rounds of thunderstorms tonight increases. On Sunday, the upper-level ridge and associated pool of warm 925- 850mb temperatures is expected to build eastward setting the stage for potentially dangerous heat. With prospects for at least remnant convective debris and remnant outflow lingering through the morning, not to mention regenerating convection as the low-level jet veers, confidence in the high temperature forecast is lower than average. In forecast scenarios where the convective footprint is minimized, high temperatures in the low to mid 90s and rising dew points into the mid 70s would support heat indices in the low to mid 100s across part if not all of the area. However, in forecast scenarios where the convective footprint is higher, the threat for dangerous heat would be lowered. We`ll take it one day at a time in this pattern, and consider the need for a heat headline once we have a better picture of the coverage of (or are outright dealing with) thunderstorms tonight. Borchardt Sunday Night through Friday: Sunday evening and night, a low-mid level shortwave disturbance is progged to track right over northern IL providing another potential for some nighttime showers and storms. While a few isolated storms may go up earlier in the day, the better chances arrive in the evening and overnight as the system moves across the area, although coverage remains largely uncertain. An EML overspreading the region will park an impressive pool of MUCAPE ahead of the system. However, forecast soundings develop a stout, diurnally-driven capping inversion amid poor low level lapse rates during the evening just prior to the wave moving in. Plus, the strength of the shortwave and resultant quality of the forcing is also disputed which makes it difficult to decipher what roll the cap will play. Overall, the trend among guidance appears to be toward a cluster of storms moving across northern Illinois late Sunday, although there is still plenty of support for less. If forcing proves ample enough for parcels to take full advantage of the EML, some stronger storms will certainly be possible with the threat for gusty winds and heavy downpours. Oppressive heat looks to carry over from Sunday into Monday as a pronounced upper high will lock a hot, humid airmass over the central US early next week. High temperatures are expected to tag the lower and middle 90s with afternoon heat indices near or over 100F. Most models are thinking that an influx of low-mid level dry air behind the overnight convection will provide plenty of sunshine throughout the day. However, if cloud cover ends up struggling to clear out post-convection earlier in the day, temperatures could prove slightly cooler than expected. The heat on Monday in conjunction with the EML still in place will build a great deal of instability across the region during the day. Dry low-mid level air and a general lack of forcing look to keep our CWA mostly quiet during the day. A cold front will drop south through northern Illinois on Tuesday. This looks to drive a complex of showers and storms through the CWA beginning Monday evening into the night. The time of day should work in our favor with instability decreasing some during the evening ahead of the potential. However, still-noteworthy instability and increasing shear beneath the encroaching trough base could result in strong to severe convection ahead of the front during the evening and night. Additional thunderstorms are expected to form ahead of the front on Tuesday as the boundary works south through the CWA. The pattern should settle down after Tuesday with the upper trough axis progged to move through early Wednesday and height rises and weak flow aloft to follow into late next week. Doom && .AVIATION /18Z TAFS THROUGH 00Z MONDAY/... Issued at 1219 PM CDT Sat Jul 13 2024 Aviation weather concerns are: - Low-confidence potentials for TS late this afternoon/early evening and again overnight. - Breezy/gusty southwesterly winds on Sunday. The main focus today is on the potential for isolated to scattered TS late this afternoon and evening, and then again overnight. Overall, the TS/precipitation forecast through midday is low confidence. Latest indications are that an area of isolated to scattered showers/storms may develop this afternoon across northwest Illinois and scoot eastward through the early evening. Chances, however, remain under 50 percent, and too low for VCTS or TEMPO TS mentions at any of the terminals. Chances are the "highest" at RFD, but still under thresholds justifying a formal TS mention. If activity were to develop, the main time for on-station impacts at the Chicago-area sites is about 23-03z, and a few hours earlier than that INVOF RFD. The next chances appear to be overnight, but are contingent on activity currently across central Minnesota developing further into a mature convective complex. If this were to occur, another TS potential exists from late this evening into the early morning hours on Sunday. While additional chances for storms exist beyond early Sunday AM, chances realistically are too low to carry additional PROB30 groups. Sunday may remain largely precipitation-free, with the next round of potential storms arriving in the evening. Carlaw && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago