Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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629 FXUS63 KLOT 271130 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 630 AM CDT Mon May 27 2024 .KEY MESSAGES... - Threat for a few strong to locally damaging wind gusts with scattered thunderstorms tomorrow afternoon, after about 3 PM. - Additional showers and thunderstorms Tuesday afternoon. && .DISCUSSION... Issued at 248 AM CDT Mon May 27 2024 Through Tuesday: A cold front is making its way out of the CWA early this morning shunting all of the precip off to our east. Cool air advecting in behind the front is spreading a low stratus deck across the area with even a few instances of light fog being reported, mostly up in southern WI so far through 2 AM. The low clouds will likely hang around well into the morning before burning off, but additional mid level clouds will fill in during the afternoon. The cloudiness and cold advection will keep conditions just a bit cooler today with highs forecast in the lower to middle 70s. A tight pressure gradient behind the departing storm system will also bring breezy conditions to the area today. During the day today, a low amplitude mid level vort max will dive from the northern Plains southeastward into the Midwest. Low level lapse rates are anticipated to steepen up with diurnal heating through the afternoon while mid level cooling will steepen up lapse rates upstairs a bit. The result will be a cluster of southeastward-moving showers and thunderstorms expected to develop today ahead of the vort max and move across the CWA during the latter part of the afternoon through about mid-evening. With CAPE and shear both on the awfully marginal side when considering severe potential, most thunderstorms are expected to be sub-severe. However, steep low level lapse rates with a modest boundary layer hydrolapse could be enough to produce strong, if not marginally severe, thunderstorm winds. Models are really favoring the southwestern half of the CWA to see the strongest storms, following a distinct corridor of maximized PVA. However, the strong wind threat should wane with southeastward extent as low level stability builds into the evening. Instability should fizzle away by the latter part of the evening, though a few showers may linger past midnight. Behind the departing trough, a second wave will follow closely behind and will impact the region on Tuesday with another vort max diving into northern Illinois during the day. As a result, another swath of scattered showers and storms is expected to move across the CWA during the afternoon. Guidance is unsure how much instability will build through the day with models ranging from no more than ~500 joules of MUCAPE to over 1,500. Models hone in on the Chicago metro as having the highest instability in any case being nearer to the cold core of the passing wave. Understandably, this is also where most are resolving the deeper convection. The signal appeared strong enough to warrant introducing a swath of likely PoPs through the metro during the afternoon. Weak cloud-layer shear should keep storms from organizing all too well, but the possibility of higher CAPE values keeps the potential for a stronger storm or two on the table. Didn`t think the potential was quite high enough to justify coordinating an outlook for severe weather in this morning`s Day 2, but this is something that we`ll continue to monitor closely. Instability looks to give out early in the evening which should do away with the thunder potential by around sundown, though a few showers may persist through the evening. Doom Tuesday Night through Sunday: The mid/upper-level ridge axis centered across the Rockies early this week will build eastward across the Midwest into the western Great Lakes for the second half of the week. As it does so, the active belt of enhanced northwesterly flow aloft across the Great Lakes region in the near term will get shunted east of our area. This will set the stage for a few day period of dry and quieter weather through the end of the week as surface high pressure sets up shop over the Great Lakes region. While the weather will be quieting down by mid-week, below average temperatures will persist into Thursday before conditions begin to moderate back above normal for the weekend. Therefore, expect high temperatures for Wednesday and Thursday to only be in the 60s to low 70s (coolest lakeside). Overnight low temperatures also look to get downright chilly for the end of May, with readings likely to drop into the mid 40s outside of urban areas. Temperatures then will warm back to around 80 degree highs for the upcoming weekend. Unfortunately, with the warming temperatures, comes increasing chances for showers and thunderstorms. It appears the area will gradually transition back into a more active weather pattern sometime this weekend into early next week. Accordingly, our current forecast is carrying some low end chances (20-30%) for showers and storms for the weekend. However, it is worth noting that there is a considerable amount of ensemble spread with how quickly this transition will occur. For this reason, the chances currently highlighted in the forecast are likely to broad-brushed and hence will need refinement as forecast certainty grows. KJB && .AVIATION /12Z TAFS THROUGH 18Z TUESDAY/... Issued at 630 AM CDT Mon May 27 2024 Main forecast messages/concerns: - Deck of low MVFR stratus persists through mid morning, then improves. - Another threat for late afternoon and evening thunderstorms. A deck of low MVFR stratus (bases of 1,000 to 1,500 ft AGL) will continue for a few hours this morning before gradual diurnal improvement back to a scattered to broken VFR deck occurs around midday. Thereafter, the main concern shifts towards pinning down the most favorable timing for scattered wind producing thunderstorms early this evening. A well defined weather disturbance across the Dakotas early this morning will track southeastward into our area by this evening. This feature is expected to foster the development of organized storm clusters capable of producing locally strong winds gusts to our northwest this afternoon. These storms are then likely to shift southeastward into northern IL by early this evening. Confidence with the occurrence of these into this evening continues to be on the high side. However, with the coverage potentially remaining more widely scattered in nature across our area, we have opted to continue with the going PROB30 mention for the 12z TAFs. However, a couple hour tempo group will need to be considered for later TAF amendments. Otherwise, west-northwesterly winds will persist through the period as surface low pressure tracks northeastward across the Upper Great Lakes. KJB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago