Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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911 FXUS63 KLOT 121123 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 623 AM CDT Mon Aug 12 2024 .KEY MESSAGES... - Mainly dry conditions and warming temperatures through the first half of the week. - Periods of showers and thunderstorms starting Wed night and continuing into Thur night. && .DISCUSSION... Issued at 308 AM CDT Mon Aug 12 2024 Through Tuesday: A mid-level wave tracking ESE across Iowa early this morning has continued to generate scattered high-based showers and a few thunderstorms as it shifts away from modest mid-level lapse rates and into a drier antecedent airmass east of the Mississippi River. Overall expectations are for the mid-level showers to track toward central Illinois late this morning through this afternoon. It is possible that the entire forecast area will remain dry today, but have opted to maintain a slight chance of showers and sprinkles roughly southwest of a line from Dixon to Morris to Rensselaer. Otherwise, today will feature mostly cloudy skies, light winds, and high temps in the mid 70s to around 80F. The continued dry airmass will persist over the area through Tuesday. A seasonably strong upper-level jet will remain generally inactive in terms of precip generation, especially given rising mid-level heights and little flow (<10 knots) below 700 hPa. There is less than a 10% chance that some high- based cumulus along an afternoon lake breeze in northeast Illinois grow sufficiently deep to squeeze out a few sprinkles. Otherwise, expect mostly sunny skies with high in the low 80s (normal for mid-August). Kluber Tuesday Night through Sunday: Seasonably warm temperatures and comfortable humidity values (by mid August standards) expected Wednesday as transient shortwave ridging traverses the Midwest. After several days of return flow from the Gulf of Mexico, a very warm and moist air mass is expected to spread north into the central Plains by Wednesday. A formidable trough is progged to move east into the Cornbelt and upper Midwest Wednesday night with height falls in advance of this feature likely to lead to strengthening nocturnal low level jet Wed night. Strong theta-e advection is likely as this low level jet drags the very moist air mass over the Plains east toward the MS Valley, likely resulting in an MCS or two Wednesday night. Guidance would tend to favor areas to our west Wed night, but as low level jet veers and trough continues eastward, this activity will probably spill into our western CWA after midnight. Low confidence in how Thursday will play out with morning showers and thunderstorms expected across the CWA. Convection should generally be in a weakening phase, so greater threat for for heavy rainfall and possibly even some severe wx looks to stay west of our CWA. However, cold pool/outflow from the Thurs morning convection could stifle the synoptic warm front`s northward progress Thursday. This will play a big role in temps Thursday afternoon as well as potential recovery needed for a second round of storms Thursday evening as cold front and associated upper trough move across the region. In the wake of Thursday night`s cold front, look for somewhat less humid air mass to eventually filter into the region Friday and through the weekend. Confidence in the evolution of synoptic features heading into the weekend is lower than average. For several days now, some ensemble members of the GFS and ECMWF and even occasionally an operational run or two have been showing a mid and upper level low closing off somewhere in the region. The 00z ECMWF has come in with a closed mid-upper level low sitting and meandering across the Great Lakes region this weekend, but this time it has much stronger ensemble support. Interestingly, potential tropical cyclone #5 (PTC5) being monitored over the Tropical Atlantic may end up playing a role, as it is progged to develop into a powerful tropical cyclone and re-curse north across the western Atlantic this weekend into early next week. Often times when strong, large hurricanes (like what the ECMWF is showing) move north into the mid-latitudes, they frequently result in a slowing down of the progression of systems and some blocking developing. Obviously confidence in how this plays out is low with so many moving pieces, but should an upper low cut off over the region, there could be bouts of scattered showers and storms over the weekend, favoring the afternoon and evening hours with peak heating. NBM does carry chance pops Friday and into the first part of the weekend, which seems more than reasonable at this distance as there is still a lot to work out in how the pattern will evolve. It`s still possible that PTC5 isn`t as prominent as shown by the ECMWF and a more progressive trough moves east of the area and leaves us with dry and seasonable weekend. Stay tuned... - Izzi && .AVIATION /12Z TAFS THROUGH 18Z TUESDAY/... Issued at 623 AM CDT Mon Aug 12 2024 BKN-OVC VFR deck expected overspread the terminals this morning and continue through the afternoon. Can`t completely rule out a brief light shower or sprinkle, but more likely to be just virga, if anything, around the terminals. A lake breeze will result in a 5-10kt east to northeast wind later this afternoon, though confidence on timing is low as cloud cover should limit differential heating between land and marine boundary layer. Skies are expected to clear out this evening which could allow or patchy ground to develop late tonight, mainly in outlying areas. Have introduced some MVFR VSBY with patchy fog late tonight at RFD/DPA/GYY for now, but certainly possible for much lower conditions if fog becomes more widespread/dense. - Izzi && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago