Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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266 FXUS63 KLOT 171728 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1228 PM CDT Fri May 17 2024 .KEY MESSAGES... - Areas of dense fog this morning will quickly dissipate a couple hours after sunrise this morning - A few spotty showers & perhaps a t-storm this afternoon near & south of the Kankakee/Illinois River Valley - Very warm temperatures expected Saturday through Tuesday with highs well into the 80s, though some lake cooling possible - Active Monday and Tuesday with scattered thunderstorms with the threat for strong/severe storms and flooding. && .DISCUSSION... Issued at 303 AM CDT Fri May 17 2024 Through Saturday: Light winds and a moist boundary layer with dewpoints depressions nearing zero has allowed for ground fog to develop. As of this writing (0730 UTC), fog is still in the process of developing, but already have a few sites down to 1/4SM. See no reason that fog won`t grow more widespread and dense through sunrise, so plan to hoist a dense fog advisory for about the southern half of the CWA through early/mid morning. The high sun angle should help quickly burn off the fog this morning, despite fairly light winds. Pretty substantial moisture gradient expected across our CWA today with low levels in particularly still moist over southern 1/3 or so of the CWA. Heating of this moist boundary layer will result in modest afternoon instability with forecast soundings showing SBCAPE values 500-1000 J/kg and minimal CINH. Forcing will be lacking with weak height rises and no obvious surface/low level focusing/forcing mechanism. RAP/HRRR forecast soundings show evidence of mid level subsidence, which could help keep coverage sparse and might make it hard for updrafts to achieve charge separation and develop into thunderstorms. Measurable precip chances today of 15-20% south of the Illinois and Kankakee River Valleys seems reasonable. The light winds should allow for formidable lake breeze to charge well inland this afternoon. Most substantial cooling will be closer to the lake where shoreline readings will probably spend the afternoon in the lower 60s ranging to the lower 80s across most of the CWA. Saturday looks to be several degrees warmer than today with guidance progged 925mb temps in the lower 20s Celcius. These temps are warmer than the 90% percentile values for mid-late May and should easily support high temps at least into the mid 80s. More of a southerly gradient should result in far less pronounced of lake breeze Saturday afternoon, likely confined mainly to the IL shore, particularly the north shore communities. - Izzi Saturday Night through Thursday: After quieter conditions on Saturday night, warm temperatures are expected to creep back into the 80s on Sunday. A weak cold front will then move across northern Illinois on Sunday afternoon with a lake enhanced push of the front farther inland. The front itself can act as the forcing mechanism to create some shallow clouds and even the slight chance for showers. Even the NBM is now trying to resolve small areas where isolated showery activity may develop. Given the lower confidence in projecting the exact placement of shower development, most of the area was kept dry at this time. On Monday, an anomalously deep long wave trough will begin to eject out over the Rockies and move eastward toward the Plains with cyclogenesis occurring at the surface. A small upper level wave will move northeastward out of the Rockies sending a lobe of vorticity over Illinois. Stronger forcing will likely be better to the northwest under the right entrance region of the upper level jet. And while there is still expected to be enough instability for showers and a few storms to develop Monday afternoon, the projected lack of shear should keep the severe risk suppressed for the time being. Southerly winds will advect moisture northward resulting in precipitable water well over an inch. Localized ponding and flooding will be the primary risk keeping the northwestern portion of the forecast area in a level 1 out of 5 for heavier rainfall. By Tuesday, an upper level ridge builds over Texas reinforcing the southwesterly winds aloft, lifting the sub-tropical jet northward and sending the next surface cold front through the area. Model soundings are suggesting 40 to even 50 knots of deep layer shear, steep mid level lapse rates, and sufficient instability that would lead to Tuesday being the better chances for a set up that could lead to potentially strong to maybe even severe thunderstorms. Additionally, the persistent southwest flow will continue to push higher moisture content into the region allowing for a renewed chance for localized flooding. So it is no surprise that once again the forecast area is within a level 1 out of 5 for excessive rain. While it might be easy to get excited over Tuesday, the details still need to be ironed out as models are still disagreeing on how the low forms over the Southern Plains as well as its track (and speed) to the northeast over the area which influences timing and overall severity of storms. On Wednesday the surface low is expected to continue its journey northeastward away from the area, but models are on different sides of the coin on the "when". The GFS has it becoming slight more diffuse and taking a slight north- northeasterly track, keeping it closer to the area and thereby supporting storm redevelopment Wednesday morning and early afternoon. However, the Euro has it moving much quicker through the area, allowing much of Wednesday to be dry. Temperatures will finally cool behind the front with high temperatures on Wednesday and Thursday expected in the 70s. The next weaker upper level trough moves east of the Rockies on Thursday providing another chance for showers and storms to end the work week. DK && .AVIATION /18Z TAFS THROUGH 00Z SUNDAY/... Issued at 1228 PM CDT Fri May 17 2024 Key messages: - Easterly winds at the Chicago area terminals this afternoon following the midday lake breeze arrival. - Threat of fog tonight near the Lake Michigan shore and at KGYY. Main threat area looks to remain out of ORD and MDW at this time. Surface high pressure centered over the western Great Lakes is aiding in an inland lake breeze surge over northeastern IL early this afternoon. With no good westerly wind push to hold/slow its inland progress, it is expected to move across ORD prior to 18z, and shortly after 18z at MDW. Easterly winds will then prevail for the remainder of the afternoon at the Chicago area terminals, before becoming light and somewhat variable in direction tonight. Winds will settle into a south-southeasterly direction (190-200) around 8-10 kt Saturday morning following the eastward passage of the surface high. Dense fog is likely to expand across southern Lake Michigan into this evening, and it remains possible that some of this fog could work onshore across parts of northeastern IL. However, it remains a bit unclear if the fog would be able to reach far enough inland to directly impact ORD and MDW. For this reason, I have continued with a more optimistic forecast for the main Chicago terminals late tonight into early Saturday morning. KGYY stands the best chance to experience any significant reduced VSBYs due to fog tonight. KJB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago