Expires:No;;137944 FXUS63 KLOT 131357 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 857 AM CDT Mon May 13 2024 .KEY MESSAGES... - Showers and thunderstorms will move in this afternoon and persist through the day on Tuesday. These storms may produce locally strong wind gusts in excess of 40 mph this afternoon and early evening. - A localized area of heavy rainfall likely this afternoon through tonight, but uncertainty remains in exact area. - Chances for showers and storms appear poised to return by the end of the week. && .UPDATE... Issued at 855 AM CDT Mon May 13 2024 Broader cloud cover has been generally non-existent across much of the forecast area this morning, allowing temps to increase to around 70F prior to 9am. Recent CAM guidance that is inline with current temp and cloud cover trends suggest max temps should reach or surpass 80F nearly areawide, especially into the Chicago metro. Dew points may also struggle to rise as quick as previously expected given robust low-level mixing this morning. Moisture advection from the southwest is expected to slowly increase dew points into this afternoon, albeit a bit more subdued given under-performing dew points upstream. As for forcing, a mid-level wave with ongoing convection near/over the AR/KS/MO/OK common borders is expected to be the main driver of increasing coverage of convection this afternoon. This feature is observationally trending slightly slower than with previous guidance, so arrival of convection into the forecast area may be delayed into mid-afternoon. With the additional mixing this morning, a currently growing DCAPE reservoir up to 1000 J/kg should begin to diminish with time this afternoon with increasing dew points. However, any delay with an increase in cloud cover will result in a slightly warmer and more deeply mixed PBL ahead of any upstream convection in central Illinois. Expectations are therefore that outflow driven single cells in central Illinois may eventually congeal into loosely organized multicell clusters capable of producing pockets of sub- severe wind gusts in excess of 40 mph across northern Illinois late this afternoon into early evening. Kluber && .DISCUSSION... Issued at 343 AM CDT Mon May 13 2024 Through Tuesday: The broad upper trough and associated upper low over the central Plains will continue to advance into the mid-Mississippi Valley today increasing low-level warm-moist advection as it does so. Temperatures aloft will also cool as the trough approaches with surface temperatures and dew points climbing into the upper 70s and lower 60s, respectively. These conditions will allow for ample destabilization and give way to widespread shower and thunderstorms this afternoon. At the same time, a cold front, currently analyzed across central Wisconsin, will advance southward into northern IL during the later afternoon and evening hours. As the front arrives the low-level moisture is expected to pool along the boundary, which in combination with relatively slow storm speeds (around 10 to 20 mph) and front parallel motions, is expected to establish a focus for training storms and locally heavy rainfall. While guidance remains in great agreement on this evolution, the convective nature of this event continues to provide uncertainties as to where exactly the axis of heaviest rainfall will occur. The latest 00z guidance suite has started to hone in a couple of potential areas, with global models have continued to favor areas along the I-88 corridor while the higher resolution CAMs favor areas along and just south of I-80. Unfortunately, the aforementioned convective nature will likely lead to notable changes in these favored areas until storms develop and we can see how their respective outflows are effecting the movement of the cold front. In the meantime, our official forecast has decided to take a more middle of the road approach and highlight the areas between I-88 and I-80 with locally higher POPs and rainfall totals to reflect the 00z guidance consensus. Additionally, our official forecast also aligns well with the slight risk (level 2 of 4) area for excessive rainfall from WPC. Regardless of where the axis of heavier rain set ups, those within this axis should expect rainfall totals between 1.0 and 2.0 inches with amounts generally in the 0.5 to 1.0 inch range elsewhere. Given the aforementioned uncertainty and the fact forecast rain values are under Flash Flood Guidance thresholds, confidence is still too low for the issuance of any flood outlook products at this time. However, if these amounts occur over urban and/or suburban areas, some instances of ponding or minor flooding may occur. Heading into Tuesday, the cold front is expected to push into central IL and IN which will move the axis of heavier rainfall out of our forecast area for Tuesday. However, the upper trough will still be pivoting overhead as it tracks into the Ohio River Valley which means showers and storms will persist through Tuesday afternoon. Though the greatest coverage of showers and storms is expected to be south of I-80 so expect gradually improving conditions for those to the north. Aside from the precipitation, breezy northeast winds are also expected on Tuesday behind the front with gusts of 20 to 25 mph likely. The northeast winds will also advect in notably cooler air which will keep highs in the low to mid-60s for most of the area with those near the lake expected to remain in the 50s due to onshore flow. Yack Tuesday Night through Sunday: On Wednesday, a surface ridge will move through the Great Lakes allowing for plenty of sunshine and a rebound in highs back into the upper 60s to lower 70s. With an onshore component to the wind, temperatures along the Lake Michigan shoreline will remain in the upper 50s to lower 60s. Outside a few lingering showers in the morning in northwestern Indiana, Wednesday looks dry. The forecast from Thursday onward carries much lower forecast confidence than is typical due to the apparent differences in how a myriad of upper-level shortwaves riding through the jet stream in the northern Pacific Ocean will interact with a soon- to-be eastward-drifting cut-off low emanating from southern California. Will the cut-off lows phase with the northern-jet stream leading to aggregate troughing across the Great Lakes, or will they have a missed connection leading to pronounced ridging across the northern US? Timing will be everything, and right now, the ensemble envelope is as vast as can be. (Not to belabor the point, the range in EPS 500mb height and 850mb temperature spans nearly 30 dam and 10C, respectively, by Saturday afternoon). Perhaps the only item of confidence is that there should be a wave (or waves?) of precipitation sometime in the Thursday to Sunday timeframe, warranting more or less continuous low- to mid-range chance PoPs (20-50%). Anything beyond that is pretty much a throw of a dart at this point. Borchardt && .AVIATION /12Z TAFS THROUGH 18Z TUESDAY/... Issued at 608 AM CDT Mon May 13 2024 Key Messages: - Scattered showers and thunderstorms are expected this afternoon and evening, primarily between 19-01Z. - While southwesterly winds are preferred during daylight hours, erratic wind directions and magnitudes are possible in and around thunderstorms. - Winds will turn northeasterly after sunset as ceilings build downward into MVFR and possibly IFR. Showers will prevail overnight and tomorrow morning. Discussion: An elongated low pressure system with embedded mesoscale convective vortices continues to lift northeastward toward the Mississippi River and will set the stage for a busy TAF period. Heating of the moistening boundary layer will lead to minimal convective inhibition as early as 17-18Z, supporting blossoming showers and thunderstorms on a scattered basis. Coverage of showers and storms is expected to maximize sometime in the 21 to 01Z timeframe, before the loss of sunlight leads to primarily showers prevailing after dark. When it will be all said and done, there may be an unusually long period of thunder at least in the vicinity if not directly overhead some or all of the terminals this afternoon and evening. For this reason, the outgoing TAFs will feature prolonged periods of VCTS with targeted TEMPOs. On-demand adjustments to the TAF appear unavoidable this afternoon as the first convective cells develop and overall trends become established. Confidence remains lower than average on the forecast for wind direction from this afternoon onward due to chaotic and merging thunderstorm outflows. While a general southwesterly direction is still preferred this afternoon, the wind direction and magnitude may vary considerably from time to time in and around thunderstorms. As the coverage of thunderstorms relaxes after sunset, winds should slowly adopt a northeast to easterly component before a cold front turns them decidedly northeasterly between 06-07Z. Primarily VFR ceilings and visbys are expected today, though pockets of MVFR ceilings IFR visibility may occur beneath thunderstorms. After sunset, ceilings should start to build downward especially behind the aforementioned cold front, with high confidence in MVFR developing and medium confidence in pockets of IFR developing. Cigs may only gradually improve toward the end of the TAF period as showers prevail. Borchardt && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory from 4 AM Tuesday to 7 AM CDT Wednesday for the IL nearshore waters. Small Craft Advisory from 7 AM Tuesday to 7 AM CDT Wednesday for the IN nearshore waters. && $$ Visit us at weather.gov/chicago