Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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780 FXUS63 KLOT 301805 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 105 PM CDT Tue Apr 30 2024 .KEY MESSAGES... - Decaying showers and thunderstorms may move into the area late tonight (30-50% chance). Severe weather is not expected. - Unseasonably warm temperatures continue through Thursday. - Additional periods of showers and storms expected Wednesday night through Friday. && .DISCUSSION... Issued at 342 AM CDT Tue Apr 30 2024 Through Wednesday: All is quiet early this morning with clear skies across the area and surface high pressure settles across the region in the wake of yesterday`s weather system. Fog has developed across portions of east central Illinois and west central Indiana. So far this has not expanded further north into Benton and Iroquois Counties but felt prudent to include a patchy fog mention through daybreak just in case. Can`t rule out shallow fog in a few other spots (typical low- lying fog-prone areas) given residual low-level moisture from rains the past couple of days. Temperatures this morning start out notably cooler than what we have seen the past few days, in the 40s to lower 50s. Ample sunshine and height rises lifting into the area ahead of yet another system developing across the Central Plains should allow temperatures to readily warm into the 70s today, with the exception being along the immediate lakeshore. The lake breeze looks to hold off on pushing further inland until late afternoon/early evening. The low-level jet begins to ramp up heading into this evening as showers and storms from our west approach the area. This will allow winds to remain breezy overnight out of the southwest and help limit overnight cooling to the upper 50s to near 60. The narrow axis of instability associated with these storms is expected to weaken with eastward extent. Accordingly, these storms are expected to decrease in coverage and intensity as they approach. A few rumbles of thunder can`t be fully ruled out, mainly west of the Fox River valley and I- 55 as scattered showers move through. Severe weather is not expected. Northwest Indiana could very well end up dry with showers dissipating prior to reaching the stateline. With such a warm start to the day, temperatures on Wednesday look to easily warm into the mid 70s with generally light westerly flow. Strong subsidence in the low-levels and mixing will allow for dew points to dip into the 40s resulting in RH values in the lower 30% range. Given how much rain has fallen the past couple of days, not expecting any fire spread concerns even if wind gusts overperform. A lake breeze/backdoor front will cool temperatures late afternoon/early evening along the lakeshore and further inland across northwest Indiana. Any precipitation with our next system looks to hold off until after sunset. Petr Wednesday Night through Monday: During the latter half of the week, a deepening upper-level trough will swing into the Upper Midwest from the northern Rockies and help induce a somewhat disjointed episode of cyclogenesis to our west. As a surface low spawns in the central Plains, broad warm air advection/isentropic ascent on its eastern flank will likely encourage scattered showers and thunderstorms to develop and track northward into our area late Wednesday night into Thursday morning as an associated warm front lifts northward. Shower coverage should be greater closer to the center of the incipient surface low, where forcing will be maximized, so currently have chance PoPs going across the western 2/3rds of our forecast area with slight chance PoPs farther to the east during this time frame. This initial wave of precipitation is likely to lift north of the area by Thursday afternoon, and should afford at least part of our forecast area some dry time on Thursday before the surface low and its cold front approach from the west and bring another band of rain that should feature at least isolated storms into the area sometime Thursday afternoon or Thursday night. This secondary wave of rain is then expected to linger into Friday before departing eastward along with the cold front. Given the recent rainfall, the potential for training convective elements ahead of the cold front, and that precipitable water values near or in excess of 1.5" (near-record values for this time of year here, per sounding climatology for DVN and ILX) are progged to be drawn northward out ahead of the cold front, we will need to monitor the potential for hydrologic concerns during this late Thursday through early Friday time frame. However, a good deal of spread still remains in forecast guidance regarding the overall evolution of the low pressure system, and some solutions (such as the operational 00Z ECMWF) advertise a low track/occlusion that would favor the heavy rainfall threat remaining west of our forecast area, and could even result in little to no rainfall being observed here Thursday night into Friday. Thus, we`ll continue to hold off on more formally messaging the potential hydro concerns until there is some more agreement amongst forecast guidance that such a threat should actually materialize. In other news, breezy southerly flow should help temperatures rise into the upper 70s to mid 80s south of the warm front on Thursday, though onshore flow should keep lake-adjacent locales in Illinois several degrees cooler. High temperatures in our western and northern counties may also end up being lower than currently advertised in our gridded forecast database if the warm front doesn`t lift as far north as presently expected, or if the second wave of rain arrives earlier in the afternoon. In the wake of Friday`s cold frontal passage, temperatures are favored to return closer to normal going into the weekend, when another incoming disturbance looks to bring another chance for precipitation to the region. Slightly warmer temperatures are then favored into early next week with the overall weather pattern continuing to look rather active across the central CONUS. Ogorek && .AVIATION /18Z TAFS THROUGH 00Z THURSDAY/... Issued at 105 PM CDT Tue Apr 30 2024 Main Concerns: - Wind shift to southeast early this evening - Decaying SHRA moving across the area late tonight - A period of strong (>40 kt) south-southwest wind gusts possibly accompanying the decaying SHRA, with at least 25-30 kt range gusts appearing probable - Potential for a period of MVFR CIGs early Wednesday, primarily at RFD West-southwest winds around 10 kt with occasional gusts will shift to southeasterly early this evening. A true lake breeze shift to easterly appears unlikely. Tonight, expected severe TS west of the MS River will weaken quickly as they move east-southeast across the region, with TEMPO mention in the TAFs. The HRRR model has shown a fairly consistent run to run signal for very strong south-southwest winds gusting to 40-50+ knots associated with the rapid decay of the SHRA. Change with this TAF issuance was to bump up prevailing south-southwest winds to ~15-20 kt with ~25-30 kt gusts, which appears probable, even if the HRRR scenario doesn`t pan out. Forecast soundings suggest MVFR CIGs are briefly possible behind an early morning cold front passage. Included TEMPO mention in the RFD TAF where confidence was a bit higher. Winds will shift to westerly with 20-30 kt gusts behind the front, strongest Wednesday afternoon. Castro && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago