Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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542 FXUS63 KLOT 120523 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1223 AM CDT Wed Jun 12 2024 .KEY MESSAGES... - Following very warm conditions away from the immediate IL lakeshore on Wednesday, some decaying showers/t-storms may push into parts of mainly far northern IL Wednesday night. - Chance for widely scattered thunderstorms on Thursday PM in parts of the area. Some storms may be strong to severe. - Hot and humid conditions are expected early next week, though thunderstorms may occur at times and modulate the heat. && .UPDATE... Issued at 918 PM CDT Tue Jun 11 2024 The only item of note for tonight remains a small chance for a few showers and perhaps even a few storms, although whether any precipitation develops in our area in the first place remains unclear. Most model guidance suggests a region of pretty steep 850-700 mb lapse rates will push across the area late tonight into early Wednesday morning. Modest warm advection will also be present in this layer as an attendant surface and low-level trough axis swings through. Most evening guidance remains just a bit too dry to "convect" within the zone of steepened lapse rates, but if cooling and moistening within the 800-700 mb layer is just a bit more significant than currently advertised, some elevated showers and storms--rooted above 800 mb--would be possible very late tonight and into early tomorrow morning. Carlaw && .DISCUSSION... Issued at 310 PM CDT Tue Jun 11 2024 Through Wednesday Night: A primarily quiet and much milder night is in store tonight, with the occasional minor exception of a few isolated showers or sprinkles at times. Lead ascent pushing across the far northwest CWA and southern Wisconsin as of this writing may be able to wring out spotty high-based sprinkles/brief light showers near and north of I-88 over the next few hours. The southern flank of a respectable short-wave trough will push southeast across the area this evening and overnight. Expectation is that we`ll stay capped to elevated convection with dry low to mid levels persisting, so maintained only slight chance showers/sprinkles mention. If any isolated/widely scattered thunderstorms develop near the upper MS Valley this evening, a few may survive into the northwest third or so of the CWA. Will let the evening shift assess whether a slight chance of thunderstorms will need to be re-inserted into the gridded forecast. Most areas will remain dry tonight, with PoPs only in the 10-20% range. Wednesday will be the warmest day for the CWA as a whole since back on May 21st, thanks to full sun, southwesterly warm air advection, and afternoon dew points mixing out into the 55-60F range. 850 mb temps rising to the upper teens Celsius and 925 mb temps into the mid 20s C supports highs in the upper 80s to lower 90s away from the immediate Illinois shore. The magnitude of low-level flow may be just weak enough to allow a lake breeze hugging the shore to slip just inland along the immediate Illinois shore Wednesday afternoon, though confidence is low. Indicating highs in the mid 80s there, which will drop back to the upper 70s if the lake breeze does push in. Convection erupting over the upper MS Valley near a cold front late Wednesday afternoon into the evening will push southeast in the late evening and overnight. It will encounter a much more hostile environment for convective maintenance locally with antecedent capping and unfavorable timing diurnally. For this reason, maintained general idea of chance (25-40%) PoPs over the northwest third of the CWA and slight chances (~20%) down to just northwest of the I-55 corridor. It`s certainly possible that any thunderstorms will fall apart before reaching the CWA, though for now official forecast shows broad-brushed chance/slight chance of showers and thunderstorms late Wednesday night. It will be the first warm summer night of this month with lows in the upper 60s to lower 70s due to cloud cover and a nocturnal low-level jet keeping moderate southwest winds going. Castro Thursday through Tuesday: Any showers that make it to the area in the pre-dawn hours of THursday may hang around early Thursday morning, but will dissipate and move out by midday. Partly cloudy skies will allow temperatures to climb into the low 90s. Storm chances return Thursday afternoon and evening as a shortwave moves north of the area. Southwesterly low-level flow will bring moisture into the area, building instability in the afternoon. Model guidance depicts CAPE values in the 1500-2500 J/kg range. On the aggressive end, the NAM and GFS seem to overdo the moisture return with dewpoints in the low-to-mid 70s (most models cap dewpoints in the mid-to-high 60s) allowing CAPE values to balloon over 3000 J/kg. Southwest winds ahead of the front will also limit the amount of shear that is present, with values capping out around 30-35 kts in the afternoon. A surface cold front will advance across the area in the late afternoon/early evening providing lift for storms to initiate. The frontal gradient looks fairly broad early in the afternoon, so convergence and lift will be on the weaker side. This may limit convective coverage until later in the day. This isn`t to say that storms won`t initiate earlier in the afternoon, however, the more favorable time frame appears to be closer to sunset as the frontal gradient tightens and low-level winds turn more southerly ahead of the front, increasing convergence and shear. This places the highest chance for storms across the southern portion of the CWA closer to sunset. It is even possible that the storms miss completely, initiating to the south of the CWA. Any storms that are able to form have the potential to become severe given the instability and shear values that are expected. Quieter weather returns Friday as a ridge builds into the central US. A return to seasonable temperatures looks likely Friday and Saturday with highs in the low-to-mid 80s. PoPs chances return on Sunday as models are in pretty good agreement that a shortwave will eject out of the Rockies and move across the central US. Most guidance shows that this shortwave will move just north of the area. The timing of this system is still uncertain, however. Left PoPs at slight chance across the northern half of the CWA. Moving into next week, the signal for hot and humid conditions continues to persist, however, there is disagreement on the magnitude of the heat. NBM guidance is aggressive, suggesting record highs on Monday. GFS ensemble guidance keeps the ridge much flatter than the ECMWF and Canadian ensembles resulting in cooler temperatures Sunday through Tuesday. The GFS is also much more convectively active, while the ECMWF remains quite dry. Opted to go closer to the NBM 10th percentile for high temperatures Sunday through Tuesday to tone things back a bit as confidence is not high enough right now to forecast record highs this far out with the potential for convection and a flatter ridge. However, if the high temperatures of the NBM and ECMWF do verify, heat indices will likely be over 100 degrees. Carothers && .AVIATION /06Z TAFS THROUGH 12Z THURSDAY/... Issued at 1223 AM CDT Wed Jun 12 2024 Only forecast concern is a chance of showers, possibly with isolated thunder, early Thursday morning, at the end of the 30 hour ORD/MDW tafs. A weak disturbance will move across the area early this morning with a mid deck expected to develop across the area. There is a small chance of sprinkles or a few showers toward daybreak this morning but confidence is too low to include any mention with this forecast and trends will need to be monitored over the next few hours. There is another chance for showers and possibly isolated thunderstorms early Thursday morning. This activity will be dependent on how widespread convection becomes northwest of the area Wednesday night and how far it can persist before it dissipates Thursday morning, with a better chance for RFD and northwest IL. Confidence is too low to include anything for the new ORD/MDW 30 hour tafs. Light southerly winds to start will turn more southwest by daybreak with speeds increasing to 10-12kt later this morning with a few gusts possible. Winds are expected to turn more westerly this afternoon then back southwesterly this evening. Low level winds will increase tonight, though they appear to remain below low level wind shear criteria. However, Its possible some gusts into the 15-20kt range may develop tonight. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago