Area Forecast Discussion
Issued by NWS Chicago, IL
Issued by NWS Chicago, IL
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767 FXUS63 KLOT 101107 AFDLOT Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 607 AM CDT Fri May 10 2024 .KEY MESSAGES... - Showers with isolated embedded thunderstorms are likely tonight. - Breezy northwest winds are expected on Saturday, but otherwise dry and seasonable conditions will prevail. - Swinging temperatures will continue next week with additional opportunities for showers and a few thunderstorms. && .DISCUSSION... Issued at 220 AM CDT Fri May 10 2024 Through Saturday: A positively tilted shortwave trough continues to pivot overhead this morning and has generated some isolated showers along the Mississippi River. These showers have been dwindling in coverage and should dissipate over the next couple hours. Therefore, dry conditions are expected for our Friday with partly cloudy skies and seasonable temperatures in the mid to upper 60s. While some guidance does show a weak lake breeze developing this afternoon, the elevated westerly synoptic winds should keep the lake breeze confined to the immediate lakeshore in IL and within the nearest mile or so in northwest IN. If the lake breeze is able to penetrate inland, those nearest the lakeshore may see temperatures remain a few degrees cooler especially in northwest IN where further inland progress is forecast. Our next chance for rain will arrive tonight as another upper- level shortwave dives into the western Great Lakes and forces a cold front across northern IL and northwest IN. While the threat for thunderstorms continues to look meager at best, forecast soundings do show some steepening mid-level lapse rates as the trough passes overhead which could support an isolated rumble or two of thunder. Thus, I have decided to keep a slight chance (around 20%) mention for thunder in the forecast primarily prior to midnight. Regardless, no severe weather is expected but there is a non-zero chance that more robust showers and/or storms could generate localized wind gusts in excess of 25 to 30 mph. Showers and any thunderstorms are forecast to exit into northern IN prior to daybreak on Saturday which will once again leave us with another dry and seasonable May afternoon. However, a lingering pressure gradient in combination with deeper atmospheric mixing Saturday afternoon does look to promote breezy northwest winds gusting in the 20 to 30 mph range. Yack Saturday Night through Thursday: Saturday night into Sunday, the aggregate upper-level troughing responsible for the recent stretch of cooler temperatures and waves of showers will finally drift eastward, opening the door for a return of warm (and above-average) temperatures. Sunday accordingly looks like a splendid spring day with highs in the upper 70s, a southwesterly breeze, and plenty of sunshine. Monday into Tuesday, the upper-level wave currently in the process of becoming "cut-off" in the southwestern United States is expected to become "re-absorbed" into the upper-level flow while drifting eastward toward the mid Mississippi River Valley. Owing to differences in phasing opportunities with upper-level shortwaves propagating around the perimeter of an upper-level low settling over southeastern Ontario, ensemble model guidance offers varying evolutions of the early week system. With that said, there remains a reasonable signal (>50% chance) for a wave or two of showers and perhaps a few thunderstorms sometime in the Monday to Tuesday timeframe. Prospects for cloud cover suggest highs on Monday should be a few degrees cooler than on Sunday and in the low to mid 70s. By Tuesday, the Great Lakes should be on the backside of the system, leading to pronounced northeasterly onshore flow (10 to 15 mph) and cooler temperatures in the mid 60s (upper 50s near the Lake Michigan shoreline). Wednesday and Thursday, a surface high pressure system and upper- level ridge will move over the Great Lakes allowing for a rebound in temperatures into the mid to upper 70s. With the core of the surface high pressure system expected to pass north of our area, the surface wind will maintain an onshore component. As a result, temperatures will continue to be cooler near the Lake Michigan shoreline (it`s that time of year, after all). Ensemble model guidance favors the next opportunity for showers and thunderstorms to arrive sometime toward the end of next week. Borchardt && .AVIATION /06Z TAFS THROUGH 12Z SATURDAY/... Issued at 1238 AM CDT Fri May 10 2024 Aviation Key Messages: * Light northwesterly winds (around 5kt) this morning will back westerly and increase in magnitude (to 10-12kt) this afternoon. * VFR clouds based above 5kft will prevail through the TAF period. * A band of showers with northwesterly wind gusts as high as 30kt will swing through the terminals this evening (generally between 03-06Z). Discussion: Surface winds will gradually back westerly throughout the morning and increase in magnitude to 10-12kt by early afternoon. A few gusts of 15-20kt appear likely this afternoon, though opted to withhold mention of gusts for brevity and the expectation for limited impacts to aviators. Broken mid-level clouds based between 5-10kft will transition into a cumulus field based near 5kft this afternoon. With forecast soundings showing appreciable depth and instability within the cumulus layer, a stray shower or two cannot be ruled out particularly between 20 and 00Z. For now, will withhold any formal mention of showers this afternoon in the outgoing TAF package given the expectation for a relatively limited coverage (not to mention the probability of occurrence being less than 20%). A vigorous upper-level shortwave and associated cold front will race southeastward across the Great Lakes this evening accompanied by a strongly forced band of showers. Chances for thunder appear to be less than 10% at the terminals owing to a loss of diurnal instability, though thunder may be observed earlier in the day further to the north in Wisconsin. With a pocket of dry air expected to be in place beneath cloud bases this evening, the band of showers may produce gusty northwesterly winds perhaps as high as 30 kt in spite of the development of a shallow near-surface stable layer. A few showers may continue behind the front as mid-level lapse rates steepen within the core of the passing upper-level wave. After daybreak Saturday, winds will turn northwesterly and increase in magnitude with gusts of 25 to 30 kt a few hours after sunrise. Borchardt && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. LM...Small Craft Advisory until 3 AM CDT early this morning for the IL and IN nearshore waters. && $$ Visit us at weather.gov/chicago