Area Forecast Discussion
Issued by NWS Quad Cities, IA IL
Issued by NWS Quad Cities, IA IL
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376 FXUS63 KDVN 200138 AAA AFDDVN Area Forecast Discussion...UPDATED National Weather Service Quad Cities IA IL 838 PM CDT Thu Sep 19 2024 .KEY MESSAGES... - Scattered storms (30-60% coverage) mainly along and north of U.S. Highway 30 in Iowa through late evening that may make it to the Quad Cities and northwest Illinois before fading; isolated severe threat that will wane if not end in the overnight hours. - Active weather pattern this weekend to bring several chances of much needed rain to the area. - Seasonable temperatures and mostly dry conditions next week. && .UPDATE... Issued at 838 PM CDT Thu Sep 19 2024 The forecast area will continue under glancing effects from the deep upper low over southern Manitoba through late evening. A pronounced cap due to a well-mixed boundary layer was observed on our 00Z sounding this evening. While that capping is presumably less north of the U.S. Highway 30 corridor (where isolated initiation has occurred), there likely is still some presence of that CIN. With increasing upper and mid-level flow as seen on water vapor imagery, as well as the system low-level jet of 25-35 kt developing over the region, storms are likely to maintain some organization and not fade in overall coverage until early overnight. In fact, they may even uptick prior to midnight within general low-level confluence. Also of note, there was a cell with explosive growth just norht of the CWA in Fayette and Clayton Counties in northeast Iowa near 8 PM. As for severe potential, the message continues that the later any storms are the more minimal potential for severe and more so small hail and 40 mph wind type hazards. While strong synoptic systems in a transition season (early meteorological autumn) can maintain kinematics sufficient for severe potential at night, that is more ideal if storms are already of an organized mode by this time of evening moving into that air mass. If there are any storms to reach severe limits, they look more likely to be semi-discrete and hail and/or wind and mainly along/north of U.S. Highway 30. && .SHORT TERM /THROUGH FRIDAY/... Issued at 228 PM CDT Thu Sep 19 2024 Radar mosaics have depicted scattered showers and thunderstorms just to our west across north central IA for much of the day. These storms developed along an instability gradient and have remained below severe limits, with just a 29 mph wind gust reported at the Waterloo airport. Latest water vapor and satellite imagery shows CI occurring near the IA/MN border, where there is stronger upper level support. 12z DVN sounding showed a cap around 770 mb that has kept any clouds that developed locally fairly shallow today. Forecast focus in the near term is on storm trends and severe weather risk. Assessing surface analysis and model trends today, I noticed models were initially overdoing Tds this afternoon. This caused some of the 06z model runs particularly the NAMNest, to convect storms too early. In any case, recent CAM runs have matched reality and show storms developing across southern MN and northern IA this afternoon in the warm sector, with the greatest upper level support. Discrete supercells are expected initially, with some indication of 0-3 km shear values becoming perpendicular to the storm motion and allowing for some bowing segments. A narrow 1-3 hr corridor of a tornado threat remains between 23-01z over northeast IA tonight. Otherwise, a fat CAPE sounding and DCAPE values over 1200 J/Kg may allow for a few severe storms tonight with marginally severe wind/hail before the sun goes down and instability quickly wanes. Regarding timing, the HREF local probability matched mean (LPMM) composite reflectivity shows storms to move east southeast through the CWA between 7 pm and Midnight tonight. With high PWs increasing between 1.7 and 2", any storms will be proficient rain producers with rain rates 0.5"/hr expected. Rainfall totals between a quarter and three quarters of an inch are forecast. This would be much welcomed rain with most of the area not seeing much rain since Aug 30th. The 12z HREF LPMM Accumulated QPF even shows some potential for locally higher amounts approaching 2" in Benton and Iowa counties overnight! There is some potential for fog along and north of Hwy 30 overnight, but left out of the forecast for now due to expected cloud cover. Friday...high pressure will build overhead with clouds diminishing in the morning leading to dry conditions. Afternoon highs will top out in the mid to upper 80s. && .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... Issued at 228 PM CDT Thu Sep 19 2024 Friday Night-Sunday Night...broad ridging aloft will begin the period, with the latest deterministic and ensemble models in agreement ejecting the closed low over the southwest CONUS east of the Colorado Rockies by Sunday afternoon. A lead shortwave is progged to move along a frontal boundary Saturday night bringing increasing clouds and precipitation chances. High moisture levels indicated by PWs in the 1.6 to 1.8" range (200% of normal) and rich 850mb theta-e air in place to support heavy rain amounts. ENS/GEFS/GEPS 24-hr probabilities of at least 0.5" of rain are now in the 50-60% range across the CWA ending 00z Monday. This is indeed good news for rain prospects! Regarding the severe potential, there remains too low of probabilities and uncertainties in instability/shear overlap at this time. That said, Day 3 CSU machine learning probs suggest there could be some risk added as we get closer to the weekend. Early Next Week...the upper low mentioned above to move over the Midwest, with continued clouds and precip chances through at least Tuesday. Cooler, more seasonable temps, dry conditions, and comfortable humidity levels are forecast behind this system, with highs in the 70s and lows in the 50s Tuesday-Thursday. Perfect for the 1st full week of astronomical Fall! && .AVIATION /00Z TAFS THROUGH 00Z FRIDAY/... Issued at 617 PM CDT Thu Sep 19 2024 VFR conditions will prevail through much of the TAF period, with brief periods of MVFR vis/cigs possible this evening and into the night. Cold front passing through the region tonight will bring through a line of showers/storms between 02-08z, impacting the area from west to east. Some storms may be strong to severe, bringing frequent lightning, hail, and gusty winds at times. Winds ahead of the front will be southerly, becoming light and variable during and after frontal passage. After 15z tomorrow, winds will become light and northwesterly, with decreasing cloud cover. Thus, the main timeframe for aviation impacts tonight will be between that 02-08z hour, give or take a 1-2 hours. Currently, a TEMPO is being used to cover that timeframe, but once confidence in timing/coverage increases, we will amend accordingly. && .DVN WATCHES/WARNINGS/ADVISORIES... IA...None. IL...None. MO...None. && $$ UPDATE...Friedlein SHORT TERM...Gross LONG TERM...Gross AVIATION...Gunkel