Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
774 FXUS63 KLOT 012336 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 636 PM CDT Mon Jul 1 2024 .KEY MESSAGES... - Dangerous swimming conditions Tuesday morning for the Lake County (IL) shoreline. - Typical July warmth and humidity through the week with periodic chances for showers and storms Tuesday through Friday - Showers and storms are expected to move across the area Tuesday night through Wednesday morning. Some storms could be strong to severe. && .DISCUSSION... Issued at 304 PM CDT Mon Jul 1 2024 Through Tuesday: Convective debris clouds continue to filter over the region as a surface high drifts eastward across the Great Lakes region this afternoon. Expect partly to mostly cloudy skies tonight combined with steady S/SSE flow to support temps in the upper 50s to low 60s. With a slight onshore flow increasing over southern Lake Michigan tonight, dangerous swimming conditions will develop from building waves and long-shore currents along Lake (IL) County beaches from around sunrise through late Tuesday morning. While a NW to SE oriented arc of low-level isentropic ascent will lift across northern Illinois late tonight through sunrise Tuesday, it appears moisture will be insufficient for saturation and realization of elevated instability from marginal mid-level lapse rates. Have put slight chance of thunderstorms in the forecast roughly north of a Mendota to Chicago line from around 3-7am. Effective shear around 30 knots with low MUCAPE suggests the primary threats will be limited to perhaps very small hail and some sporadic gusts under 40mph. An area of convection is expected to develop across northwest KS and southwest NE late this afternoon, with the remnant convective wave progged to lift ENE across southern Wisconsin late Tuesday morning into the mid-afternoon. Marginal lapse rates around 6-6.5 C/km, poor deep layer moisture profiles, and stagnant if not slightly rising mid-level heights all point toward a notable weakening trend with any residual convection as it shifts toward eastern Iowa Tuesday morning. However, continued low-level isentropic ascent extending southward into far northern Illinois supports maintaining a chance of showers and some embedded thunderstorms north of a Dixon to Evanston line through the afternoon hours on Tuesday. Kluber Tuesday Night through Monday: Convection will be ongoing across Iowa and into western Illinois Tuesday evening as a cold front pushes east through the Mississippi River Valley. This convection will advance towards the CWA through the evening and into the overnight hours. Dry air in the low levels and poor mid-level lapse rates (5-6 C/km) will limit instability from building in our area. This will result in the storms to be weakening as they reach the western portion of the CWA, however some storms producing severe weather is not out of the question given deep layer shear values of 40-45 kts. SPC has placed the far western portion of our CWA in a level 1 out of 5 risk of severe weather for this threat. The cold front should pass through the area sometime early Wednesday morning with shower and storm chances dropping off through the morning for most of the area. Wednesday afternoon is expected to be dry for most areas though a few lingering showers are possible. Despite the frontal passage temperatures will still climb into the mid-to-upper 80s with dew points in the low 70s making conditions hot and humid. Model guidance depicts the front slowing down and eventually stalling out just south of the area through central Illinois. A couple of the models show the possibility of the front actually stalling out within the southern third of our CWA. If this scenario were to play out then showers and storms would likely continue through the day on Wednesday, though they would primarily be confined to areas south of I-80. On Thursday cyclogenesis in the lee of the Rockies will latch onto the stalled front and allow it to slowly meander back to the north as a warm frontal boundary. Modest instability (1000-2000 J/kg of MLCAPE) and deep layer shear (25-35 kts) will develop behind the front, in the open warm sector of the developing low pressure system. Thus, depending how far north the warm frontal boundary is able to traverse, this could put the area under the gun for another round of showers and storms on Thursday. For now have gone with likely (50-70%) PoPs across the southern CWA with chance (30-50%) PoPs north of I-80, however, as previously mentioned, this is highly dependent on the ability of the boundary to move northward. Chance (30-40%) PoPs will once again be over the entire area on Friday as the cold front of the low pressure system moves across the area. A plume of modest (6-7 C/km) mid-level lapse rates and instability (1000-1500 J/kg of MLCAPE) are expected to precede this frontal passage, bringing the potential for thunderstorms. However, uncertainty still remains in the timing and progression of this front and system. Moving into the weekend, dry conditions are expected behind the cold front with temperatures in the low-to-mid 80s. Chances for showers and storms return late Sunday into Monday as there is a signal for a shortwave to move across the area, although uncertainty is quite high in how this will play out. Carothers && .AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 636 PM CDT Mon Jul 1 2024 Forecast concerns include... Chance for showers Tuesday morning. Gusty south/southeast winds Tuesday. Chance for showers/isolated thunderstorms Tuesday night. Current scattered showers across northeast IA are expected to move east/northeast tonight into far northern IL and southern WI. This activity will be moving into a drier airmass and some weakening is expected though some showers may affect northwest IL and RFD overnight through mid morning Tuesday. Thunder chances appear low and no mention for now. Much of this activity will dissipate as it approaches the Chicago terminals later Tuesday morning and confidence is too low to include mention in this forecast. There will be another chance of showers and possibly thunderstorms later Tuesday evening into early Wednesday morning. The bulk of any activity that forms is expected to remain southwest of the terminals. East/southeast winds this evening will become more southeasterly tonight and south/southeasterly Tuesday morning when gusts will increase into the lower 20kt range. Winds may turn more southerly by Tuesday evening, when gusts are expected to diminish. cms && .LOT WATCHES/WARNINGS/ADVISORIES... IL...Beach Hazards Statement from 5 AM CDT Tuesday through Tuesday morning for ILZ006. IN...None. LM...Small Craft Advisory from 5 AM to noon CDT Tuesday for Winthrop Harbor to Wilmette Harbor IL. && $$ Visit us at weather.gov/chicago