Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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561 FXUS63 KLOT 011719 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1219 PM CDT Mon Jul 1 2024 .KEY MESSAGES... - Typical July warmth and humidity through the week with periodic chances for showers and storms Tuesday through Friday && .DISCUSSION... Issued at 308 AM CDT Mon Jul 1 2024 Through Tuesday Night: Dry and seasonably cool weather is on tap again for today. Easterly winds off the lake will once again hold temps in the 60s near the lake, but away from the cooling influences of the lake, the majority of the CWA should climb well into the 70s. Sunny skies this morning will give way to some increase in high cloudiness this afternoon. The dry weather and mild temps will continue most of tonight. Late tonight into Tuesday attention will turn to a pair of shortwave troughs. The first, a large trough over the Pacific Northwest and northern Rockies will move to the east today and tonight. A surface trough/dryline will develop and sharpen up today across the northern and central High Plains in response to the upper trough. Strengthening southerly flow in advance of this trough will transport higher theta-e air mass northward across the plains today and tonight. A second, small and more subtle shortwave trough over the southern Rockies is forecast to move into the central Plains tonight and likely be the impetus for a MCS developing across Nebraska spreading east toward the mid-Mississippi valley by 12z Tuesday. A few showers could reach our far northwestern CWA prior to 12z Tuesday, but most of tonight should be precip-free in our area. This MCS should be in a weakening phase late tonight and especially Tuesday morning as it tracks into a drier, more stable air mass to the east of the Mississippi River and as the low level jet diurnally weakens and veers tomorrow morning. How quickly this precip dissipates and how far south it spreads before it does varies considerably among the various models. The HRRR is generally farthest south and holds onto lingering showers and a few storms late into the morning hours Tuesday, while other guidance barely has any precip south of the IL/WI border. The ECMWF looks more HRRR-like and for that reason, opted to bump pops up Tuesday morning north of I-80. Debris cloudiness and perhaps rain, will likely keep temps somewhat cooler northern CWA. How much cooler will depend on the footprint of the potential rain/cloudiness. Opted to nudge temps down somewhat, but still warmer than the HRRR which holds temps in the low to mid 70s near the WI border Tuesday afternoon. Southerly winds Tuesday will advect warmer and more humid air north into the area, so even a couple/few hours of clearing in the afternoon could allow for a rapid recovery in temps (and dewpoints) across our northern CWA. Showers and thunderstorms are expected to redevelop along the cold front to our north and west Tuesday afternoon. Those showers and storms will track east toward our area Tuesday night, but it appears that they will be in a weakening phase as they outrun the better synoptic forcing which should remain well to our north and also encounters a more stable air mass. Still anticipate this activity to make it into the northwest half of our CWA where low end categorical pops from the NBM are within the realm of possibility. Precip chances will decrease with east and southeast extent across the CWA as this activity is expected to be weakening as it moves across the area Tuesday night. - Izzi Wednesday through Sunday: An upper-level trough will be in the process of pivoting into the eastern Great Lakes on Wednesday as its associated cold front continues to move southward across northern IL and northwest IN. Therefore, an area of showers and thunderstorms is expected to be ongoing along the front Wednesday morning, but should taper Wednesday afternoon as the front exits to our south. However, depending on the exact speed of the front there is the potential for some lingering showers and storms in the southern 1/3rd of our forecast area into the early hours of Wednesday evening. Regardless, the better kinematic field should be removed from the front by Wednesday afternoon which should keep the threat for any strong to severe thunderstorms low. Despite the front`s name, temperatures and dew points behind the front are not expected to be cool in any sense of the word. In fact, Wednesday afternoon is forecast to be one of the warmer periods of the week with highs forecast to be in the mid to upper 80s areawide. Couple these temperatures with dew points still in the upper 60s to around 70 and conditions look to be in place for many areas to see peak heat indices in the low to possibly mid-90s. While these values are nothing uncommon for this time of year, it will be a day where those sensitive to heat should use caution. The aforementioned front is expected to stall somewhere in south- central IL Wednesday night before lifting back north as a warm front on Thursday. At the same time, guidance continues to show a thunderstorm complex developing across the central Plains Wednesday night and tracking eastward along the frontal boundary into IL Thursday morning. Depending on how far north the warm front gets prior to Thursday morning will determine how much of our area will see thunderstorms for our July 4th holiday. The latest 00z guidance suite continues to favor a more southern track with the aforementioned storm complex which may keep the northern 1/3rd to half of our area (roughly I-80 north) dry through Thursday afternoon with better coverage of storms in our southern CWA. However, I will caution that the courser resolution of long range guidance can struggle with the mesoscale processes that drive storm complexes and their impacts on frontal boundaries especially 3+ days out. Thus, recommend keeping an eye on this period of the forecast for changes as you work to finalize your July 4th plans. Regardless of how the daytime hours on Thursday play out, another cold front is forecast to get pushed into northern IL Thursday night as an upper-level trough pivots into the Upper Midwest and Great Lakes. Given that the airmass ahead of the front will still be warm and humid have no doubts that shower and storm coverage will increase along the front and persist into the day on Friday. As the front exits the area Friday night, a surface high pressure center is expected move into the Ohio River Valley and southern Great Lakes. This high should bring the periods of showers and storms to a close as we head into next weekend, but guidance does continue to depict broad troughing residing over the Great Lakes through the weekend. Therefore, there is a chance (albeit low at this time) that some isolated showers and/or storms could materialize during this timeframe. Otherwise, expect temperatures and humidity to remain near typical July levels with highs in the low to mid-80s and dew points in the 60s into early next week. Yack && .AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 1219 PM CDT Mon Jul 1 2024 VFR conditions are expected through the period. E winds around 10 knots will veer SE late this evening, then settle S/SSE (160-180 degrees) with gusts over 20 knots by late Tuesday morning. While this TAF cycle depicts a dry forecast for the Chicago terminals, there are two low (<20%) chances of isolated TS late tonight into Tuesday. The first will be with a band of WAA lifting across the area roughly in the 09-12Z window, while the second window will fall in the 16-20Z window with a weak mid- level wave crossing southern Wisconsin. Kluber && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...None. && $$ Visit us at weather.gov/chicago