Area Forecast Discussion
Issued by NWS Twin Cities, MN
Issued by NWS Twin Cities, MN
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904 FXUS63 KMPX 172001 AFDMPX Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 301 PM CDT Tue Sep 17 2024 .KEY MESSAGES... - Active weather pattern returns, with the chance for showers and thunderstorms Wednesday and Thursday. - Slight risk (Level 2/5) for severe weather across eastern MN/western WI Thursday. Large hail and damaging winds are the main threats, however tornado potential does exist. - Additional chance for widespread precipitation during the second half of the weekend, followed by significantly cooler air next week. && .DISCUSSION... Issued at 256 PM CDT Tue Sep 17 2024 Well defined upper-low over the Rockies is progged to lift northeast and occlude over northern CONUS/southern Canada over the next few days. The Upper Midwest is positioned in a region of broad southerly flow to the northeast of this feature. 850 mb analysis reveals a 30- 40 kt LLJ stretching from northeast Nebraska to west central Minnesota. Scattered morning showers that developed on the nose of the LLJ in southwestern MN have all but dissipated, with just a few lingering showers near the Iowa/Minnesota border at this hour. We expect the remainder of the afternoon to remain dry for most of the region, however we cannot rule out a few isolated showers or a storm within the ongoing southerly flow regime. Coverage of any convection should be very limited given subsidence in the mid-levels. Highs are generally on track to reach the mid to upper 80s, although peak heating has been slower across southwestern Minnesota where cloud debris from the morning showers has been slow to clear. We`ve included slight chance PoPs in western Minnesota overnight, as the intensification of the nocturnal LLJ will likely produce isolated to scattered convection. Our discussions on the ops floor have centered around the eastern extent of PoP coverage tonight, given the poor handling of showers along the eastward shifting LLJ the past few evenings. Essentially, this means we wouldn`t be entirely surprised to see some showers work east towards the Twin Cities tonight/early Wednesday. The aforementioned upper-low is forecast to become negatively tilted as it occludes over the northern CONUS tomorrow. The approach of the upper-low will swing an initial zone of ascent across the Upper Midwest Wednesday, which will lead to the reintroduction of PoPs across the area. Forecast soundings capture a thermal nose around 5k feet, owing to the ongoing warm air advective regime. We`ll lean against earlier solutions for convective initiation due to this complication, however we do expect PoP coverage to increase later in the day as the low-level jet intensifies. Our latest forecast features 20-30% PoPs across western MN through 00z, followed by an increase to 40-60% PoPs across south central MN through 06z. Showers and thunderstorms will continue eastward through very early Thursday. The SPC has included a Day 2 Marginal Risk across western Minnesota, where initial storm development may pose a large hail/damaging wind threat. By Thursday morning the closed upper-level low is forecast cross the international border north of the Dakotas and move over southern Manitoba. The trailing surface cold front will be the focus point for renewed convection and potential severe weather later Thursday morning and into the afternoon. SPC has expanded the Day 3 Slight Risk with the newest outlook, such that all of eastern Minnesota and much of western Wisconsin are now highlighted under the area of greatest severe weather potential. The environment is forecast to be supportive of severe weather given steep mid-level lapse rates, strong instability, and sufficient shear tied to the strong vertically stacked system. There are questions in how far east the cold front will travel prior to convective initiation, as well as the state of the environment following Wednesday night`s round of convection. Right now we are leaning on a solution that supports a large hail threat around/shortly after initiation, followed by more of a damaging wind concern as the convection blossoms into an organized line of storms. Low-level curvature in the forecast hodograph will support the potential for brief tornadoes. Storms will end from west to east as the front moves through the area. Dry weather returns for Friday and most of Saturday, though scattered showers will be possible as a clipper-type wave advances across the Dakotas Saturday morning. Better chances for widespread rain will be tied to the advance of a surface low in the central Great Plains on Sunday. The evolution of the northern wave (mentioned above) and the southern surface low has been handled differently across recent ensemble runs, such that confidence in rainfall amounts remains in flux. Should wetter members come to fruition, it`s plausible that some locations across the area may observe 7-day rainfall totals upwards of 1.5-2 inches. On the flip side, the drier nature of several ensemble members makes this a low confidence forecast at this time. What is more certain is the arrival of cooler September air for the second half of the weekend and into early next week. The latest blended guidance advertises highs in the upper 60s/low 70s and lows in lower 50s for much of next week. Fall lovers do not get too excited yet -- while it`s a little beyond the current forecast period, all signs point upward for a warmer than normal end to the month. && .AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/... Issued at 1215 PM CDT Tue Sep 17 2024 It will be another 24-30 hours of southerly winds and VFR skies. Like yesterday, not anticipating storms near the western IA/MN border to survive the afternoon, with even RWF expected to remain dry. The big question for tonight is do we see showers develop again tonight like we have seen the last 2 nights. Sunday night they were across eastern SoDak & Neb, last night it was western IA into southwest MN. If we were to see it happen again tonight, it would likely shift east to be more aligned with the I-35 corridor. Confidence in them happening again is too low to include in any of the TAFs at this point, but it is something to keep an eye on. KMSP...No weather is expected today. We don`t have any showers in the TAF for Wednesday morning, but if we repeat what happened with shower development the last 2 nights to the west, they would likely form a bit farther east than they did the last 2 nights, meaning MSP would be in line to start Wednesday with some shra/tsra activity. Nothing shows this happening, so we don`t have anything in the TAF, but it is something we`ll have to continuously reconsider through the night. /OUTLOOK FOR KMSP/ THU...VFR. SHRA/TSRA likely. Wind SSW 15G25 kts. FRI...VFR. Wind WNW 5-10kts. SAT...VFR. Chc shra/tsra. Wind SE 5-10 kts. && .MPX WATCHES/WARNINGS/ADVISORIES... MN...None. WI...None. && $$ DISCUSSION...Strus AVIATION...MPG