Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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769 FXUS63 KLOT 071147 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 647 AM CDT Sun Jul 7 2024 .KEY MESSAGES... - Chance (30 to 40 percent) for scattered showers and thunderstorms today, mainly north of I-80. - A cold front moves over the area on Monday providing another chance for scattered showers and thunderstorms as it moves west to east over the forecast area. - Increasing heavy rain potential for at least portions of the area Tuesday night and Wednesday. - Increasing heat and humidity next weekend. && .DISCUSSION... Issued at 330 AM CDT Sun Jul 7 2024 Through Monday: This morning will start out dry with higher clouds moving in from the west. A small complex of showers and thunderstorms is gradually moving northeastward from eastern Iowa into southwestern Wisconsin ahead of a an upper level short wave swinging over Minnesota. As this moves east, it will increase the probability for the development of showers and thunderstorms later this morning for areas along the state line, particularly around the Rockford Metro Area. Unfortunately, what happens after noon is fraught with uncertainty as models are still struggling with convection for that time period. There is another complex of thunderstorms that has a slight signature of an MCV in northern Missouri that is slowly making its way to southeastern Iowa. The HRRR is still suggesting that as it moves eastward, it will slowly decay and fall into just lightly scattered showers, keeping most of the area dry. Other high res models like the NAM Nest, the RRFS and RDPS are suggesting that a weak surface front extending from the Quad Cities to Milwaukee will develop and provide enough surface convergence to support the development of additional showers and thunderstorms, especially north of I-80. On top of that, models keep winds out of the south to southwest through much of the day, though there is the potential for a weak lake breeze to develop. Most model soundings, including the drier HRRR mentioned above, suggest that destabilization will occur, the question will be what exactly will be the forcing mechanism to set it off. The good news is that wind shear looks to be 20 knots or less and mid-level lapse rates should be less than 6 degrees C/km, so severe weather is not expected. While confidence remains low with the areal coverage of storms across northern IL this afternoon, we have opted to spread increased PoPs of 30 to 40 percent farther east into the Chicago metro area, especially north of I-80. Lastly, it does appear that these showers and storms (if they do develop) look to be more transient as they pass over. So perhaps the strongest cells could provide some momentary ponding on roads, the flooding risk remains low. The only area that is under the Level 1 out of 4 risk for excessive rainfall is a small portion of westernmost Lee, Ogle, and Winnebago counties due to the potential more frequent showers through the day where there are higher p-wats. The long wave upper level trough narrows, deepens, and becomes more positively tilted Sunday night and it slowly sags into northern Illinois during the overnight period into Monday morning. There could be a brief window of drier conditions after 6 PM tonight as it loses day time heating. However, as Monday approaches the probability of scattered showers and thunderstorms increases. Persistent southwesterly steering flow will continue to advect in p-wats around 1.5 inches with a surface cold front moving over the area to provide the forcing mechanism. Severe weather is not expected, but depending on what transpires with the rainfall today, potentially Monday could have some flooding issues to the west. This system will move in from the west and gradually pass to the east through the day Wednesday providing a nice soaking rain with seasonal temperatures in the 80s. DK Monday Night through Saturday: Over the past 24 to 48 hours model and ensemble guidance have continued to favor a faster and farther north track of the post tropical remnants of Beryl into the Ohio Valley late Wednesday into early Thursday. Accordingly, there is an increasing threat for moderate to heavy rainfall and breezy winds affecting at least parts of our area late Tuesday night and Wednesday. In collaboration with neighboring offices, we have thus opted to increase POPs from the NBM into the likely category (~60%). These likely POPs cover areas roughly along and southeast of the I-55 corridor, where the highest threat of heavy rain and potential flooding concerns currently reside. Farther refinements to the forecast for Tuesday night and Wednesday will undoubtedly be needed as we continue to hone in on the exact path Beryl takes into the mid-week period. Stay tuned! Following the passage of Beryl`s remnants later Wednesday, the remainder of the week is looking to be a bit more tranquil. However, with aggregate upper troughing lingering across the Great Lakes region Thursday into Friday, we cannot completely rule out the low (~20% chance) possibility some isolated to widely scattered afternoon showers and storms. However, a good amount of dry time is looking likely both days. Temperatures through Thursday are expected to remain seasonable, with daily highs generally in the upper 70s to low 80s, coolest near the lake. Thereafter, heat and humidity looks to build into next weekend as a large subtropical ridge builds across the central Rockies into the Mid-Mississippi Valley. If this upper ridge builds far enough northeastward into our area next weekend we could be looking at at least a short period of some significant summer heat later next weekend. Assuming we end up remaining far enough removed from any ridge riding thunderstorm complexes in this pattern through the weekend, we could end up with temperatures reaching well into the 90s (heat indices possibly in excess of 100) for next Sunday. KJB && .AVIATION /12Z TAFS THROUGH 18Z MONDAY/... Issued at 647 AM CDT Sun Jul 7 2024 The main forecast concern today continues to center around the threat and timing of any thunderstorms over or near the terminals this afternoon. All is quiet early this morning, with the exception of a small cluster of showers and storms across far northwestern IL. This activity will shift eastward and will be around KRFD through about 14z. Beyond this current activity, the trends with thunderstorms today remains of low confidence. The airmass is expected to destabilize across the area through the morning in advance of multiple weak impulses that will be streaming into the area from IA. These impulses may thus be able to foster some isolated to widely scattered thunderstorms across northern IL later this morning into the afternoon. However, because confidence remains low (~30%) with the coverage of storms, and the fact we cannot forecast a PROB30 within the first 9 hours of the TAF, we have opted to only mention a VCSH after 17z this afternoon at this time. If confidence increases, we will need to either add a VCTS or a tempo group for TS with later updates. The potential timing for any storms around the Chicago terminals could be as early as midday, and as late as 22z this afternoon. Brief heavy downpours and some locally gusty winds would be the primary threats from any storms. The threat of storms looks to wane for a period this evening, though more showers and storms may move into northwestern parts of the area sometime late tonight into Monday. With the better threat looking to be closer to RFD tonight, we did opt for a PROB30 mention for TSRA after 05z tonight. Prevailing wind directions today area expected to be from the south-southwest, though the winds in and around any thunderstorms will become more chaotic. It appears the lake breeze may struggle to move inland through the day, so we have opted to leave a wind shift out of the TAF until the larger scale flow shifts southeasterly for a period this evening. KJB && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago