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Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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779 FXUS63 KLOT 151725 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1225 PM CDT Thu Aug 15 2024 .KEY MESSAGES... - Showers and isolated t-storms expected this morning, likely decreasing in coverage/ending this afternoon. - Second round of scattered showers/t-storms probable tonight, but details in timing, coverage, and location not particularly high. - Scattered showers and t-storms expected later Friday afternoon into early evening, especially northern CWA. && .DISCUSSION... Issued at 320 AM CDT Thu Aug 15 2024 Through Friday Night: Early morning radar mosaic across the Midwest shows just a largely unorganized mess of showers and thunderstorms across the region. This activity is being driven by low level theta-e advection within broad 30-40kt southerly low level jet as well as modest height falls and synoptic ascent associated with approaching shortwave trough. Low level jet should weaken as it begins to veer and flop over into northern IL this morning. While low level jet is expected to weaken, continued (albeit weakening with time) low level theta-e advection and eventually ascent from the approaching trough should keep showers and isolated to scattered t-storms going through the morning as the activity spreads gradually eastward. Highest thunder chances will likely be early this morning and focused more over the western CWA closer to the better instability, though isentropic ascent could still be enough for some "slantwise" convection persisting east of the axis of upright instability. Lead shortwave trough axis should be pushing into our eastern CWA by early afternoon, with threat of showers/storms largely ending from the west coincident with the passage of this trough. Seems probable that eastern portions of the CWA could be socked in with cloud cover much of the day, limiting heating, though some breaks are more plausible western CWA this afternoon, allowing a better chance for temps to rebound. NW-SE oriented warm front will move across the area this afternoon, with a much more humid air mass spreading across the area. Tricky convective forecast for tonight. It is unclear if and where boundary layer rooted convection will develop late this afternoon or early this evening, but if it does, it seem northeast MO, eastern IA, west central IL would be area most likely to experience sufficient heating today to erode convective inhibition and have the forcing necessary to initiate convection. Farther east into our CWA, forecast confidence wanes considerably. It does seem probable that some destabilization is likely as a result of low level moisture advection beneath steepening of mid-level lapse rates this evening. Cold front west of the Mississippi River that could initiate BL based convection late this afternoon or early this evening is forecast to weaken/wash out as it tries to move east tonight. However, shortwave trough axis is still progged to rotate around the slowly deepening mid and upper level low as it wobbles southeast into the upper Mississippi Valley. A couple of the more probable of the possible scenarios for us tonight include: 1) BL based convection west of us maintains itself as it moves east into all or parts of our destabilizing CWA. 2) BL based convection either doesn`t develop to our west or stays south of the area and new, likely elevated convection develops over our CWA tonight as synoptic ascent increases in association with the approaching shortwave trough. There remains a large spread in guidance with respect to magnitude of the deep layer shear (and instability too), so severe threat tonight is highly conditional and uncertain. There are scenarios that could evolve that would result in a severe weather threat into our area, but many more that would result in minimal severe threat. Going to need to watch observational trends today to get a better handle on severe threat, if any, tonight. With the initial cold front expected to largely dissipate tonight, it seems unlikely that there`ll be much of a scouring out of low level moisture. Skies are expected to clear out Friday, allowing the boundary layer to heat up and destabilize, the sunshine and deeper mixing could also allow the dewpoints to mix out some in the afternoon as well which could mute instability somewhat. A secondary cold front and associated trough axis pinwheeling around the upper low appear likely to ignite at least scattered t-storms over southern WI or far northern IL late in the afternoon. This convection should move southeastward through the early evening before dissipating after sunset. Greatest coverage/chances of convection look to be north of I-80, with weaker/later arriving forcing farther south likely limiting chances. Certainly can`t rule out a severe weather threat late Friday afternoon into Friday evening as 0-6km bulk shear is progged to increase to 30 to perhaps as high as 40kt. Big uncertainties regarding coverage of storms and magnitude of the instability, so the marginal risk in SWODY2 is quite reasonable. If confidence increases in convective coverage and threat of dewpoints mixing out/limiting instability decreasing, then being a window of severe wx potential will increase. - Izzi Saturday through Wednesday: The upper-level low currently over North Dakota will drift ESE to the eastern Great Lakes by Sunday night. Guidance continues to waffle on whether sufficient low-level moisture will linger far enough west to support diurnally-enhanced showers Saturday afternoon or if gradual drying limits precip coverage. However, guidance has trended more in favor with an embedded mid-level trough rotating across the area on the southwest flank of the upper low. Backward trajectories indicate that this is not a new trough but actually today`s wave rotating completely around the upper low over the next couple days. The latest forecast shows scattered showers with perhaps a few thunderstorms Saturday afternoon, though as is the case with upper lows, the details and evolution of embedded impulses around the low likely will not be fully realized beyond 24-36 hours, so some forecast changes are quite possible. Following suit with Saturday`s forecast guidance, there has been a slower/westward shift with the upper low on Sunday. Previously, Hurricane Ernesto was progged to shift far enough west to pull the upper low farther east late weekend into early next week. With the forecast track of the hurricane trending farther east, little to no interaction between the two systems is expected. Ultimately, this may result in continued isolated shallow showers across far eastern Illinois and northwest Indiana on Sunday. Mid-level ridging and an approaching surface high will provide quiet conditions across the area Monday into Tuesday. A loose omega block is then expected to develop across much of North America by the middle of next week. By this time, guidance shows developing spread with the strength of this blocking pattern as a strong wave over the northern Canadian Prairie provinces potentially dampens the amplitude of the ridge. This may result in a slightly more favorable set-up for a round or two of convection to the north to come precariously close to our area late next week as low-level moisture finally begins to return northward. Otherwise, generally seasonable to seasonably cool temperatures are expected through the period. Kluber && .AVIATION /18Z TAFS THROUGH 00Z SATURDAY/... Issued at 1225 PM CDT Thu Aug 15 2024 The main aviation weather concerns are: - IFR/MVFR cigs and showers/drizzle this afternoon - Period of brief S/SSW winds early this afternoon at ORD - Potential for stratus to build down to IFR/LIFR this evening and overnight, but low confidence in this - TS chances (around 30-40 percent chance) overnight MVFR cigs will continue to gradually expand eastward this afternoon, with a period of IFR cigs and IFR/MVFR vsbys more probable at RFD. Showers/drizzle will also continue this afternoon, possibly lingering into the evening hours. At ORD, winds may briefly trend near a 180-190 direction, but are expected to back SSE again after 21-22z. Large uncertainty exists in the forecast tonight. Guidance trends have been towards more expansive MVFR and even IFR cig development. Confidence is not high enough to warrant this in the outgoing TAFs at this time, but if trends continue, a trend towards more pessimistic TAFs may be needed. Did not make significant alterations to the PROB30 TSRA groups overnight with lingering uncertainties around convective initiation. If low cigs do materialize tonight, they will scatter fairly quickly through Friday morning with breezy southwesterly winds developing. The next shower and TS chances will arrive towards the tail end of the extended ORD/MDW TAFs, and more likely during Friday evening. Carlaw && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Beach Hazards Statement until 10 PM CDT this evening for ILZ006. IN...None. LM...Small Craft Advisory until 10 PM CDT this evening for Winthrop Harbor to Wilmette Harbor IL. Small Craft Advisory until 4 PM CDT this afternoon for Wilmette Harbor to Northerly Is. IL. && $$ Visit us at weather.gov/chicago