Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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136 FXUS63 KDVN 260450 AFDDVN Area Forecast Discussion National Weather Service Quad Cities IA IL 1050 PM CST Wed Nov 25 2020 ...06Z AVIATION UPDATE... .SYNOPSIS... Issued at 310 PM CST Wed Nov 25 2020 The latest sfc analysis was indicating the main sfc low was rolling acrs central IL, while aloft the upper level low was tilted slightly to the west with the center of circulation northeast of STL looking at the water vapor imagery. The brunt of the warm conveyor was streaming well to the east of the region up the eastern OH RVR Valley, while a pinched off TROWAL fed weakening DEF zone was moving east acrs the north half of the CWA/north of I80. The thunder chances have shifted off to the southeast under the main low/ convective looking clusters acrs central IL this afternoon. 24 hour QPE suggests a swath of 1 inch to near 1.75" has fallen from Sigourney IA, to Tipton IA and to just west of MT Carroll. The low pressure system will pull away and eventually take the precip and clouds with it for a decent late week and into the weekend period. && .SHORT TERM...(This Evening through Thursday) ISSUED AT 310 PM CST Wed Nov 25 2020 Tonight...The main challenges tonight will be timing the exiting cloud deck and if there will be renewed fog development. the slightly off-stacked low complex is on track to reach north central IN by 6-7 PM this evening. The weakening def zone will largely be moving out of the area or decayed into marginal light rain patches acrs NW IL at the start of the period. Still some satellite vort lobe influences on the western flank of the departing cyclone to possible induce enough lift to ring out some patchy drizzle in spots(especially the east half through mid evening). Then upstream ridging currently acrs the central plains will look to press eastward acrs the area by midnight and into Thu morning. Thus the lift should be shut off by then, but the incoming subsidence inversion may trap the extensive stratocu deck that`s in place acrs the region and prevent it from clearing from the west like most model solutions want to do. Night time during late fall also conducive to keeping clouds trapped in longer than the models want them to. Like the previous shift stated, with sfc winds backing to the west and possibly southwest while decreasing late tonight, and any clearing skies would foster dense fog development into Thu morning especially over the heavier rain swaths. But with the uncertainty of clearing, will keep patchy to areas of fog possible especially o the west after midnight and before dawn in areas more likely to clear from the west, if they can. The HiRes models now target central to northeast IA in converging, backing southwest sfc wind fields toward dawn with dense fog, and just clip the northwestern CWA with visibility reduction. But with uncertainty, will mention fog in much more of the area than that. Back to this evening, there may be transient localized swaths of fog development in areas where north wind back and converge, but again hopefully the cloud cover will prevent a more widespread fog problem before midnight. Will bank on post wave boundary layer cool advection from northwest IA to get the low temps down into the low to mid 30s west of the MS RVR, and upper 30s to near 40 in the east. If clouds hold tight these values will be too cool in the west. Thursday...Will walk out or mix out any lingering stratus or fog as the morning progresses for mostly sunny skies. That until a passing frontal system down acrs MN and WI possibly brings a cloud increase acrs the north by late afternoon. Increasing southwesterly boundary layer mixing flow to drive highs in the mid 40s in the far northeast, to the lower 50s southwest. More cloud lingering will make these optimistically mild, while deeper mixing more than H925 MB indicate by the fcst soundings would add a few degrees to the fcst highs. All in all a decent Turkey Day probably in store for tomorrow. ..12.. .LONG TERM...(Thursday Night through next Wednesday) ISSUED AT 310 PM CST Wed Nov 25 2020 Key Messages: 1. Sensible quiet weather through the period. 2. Active pattern will lead to mainly temperature changes through the period. Discussion: From a sensible weather perspective the period will be quiet with a a couple of cold fronts and shifts in temperatures along with some windy conditions. Aloft, a much different story with strong dynamics through the period and a highly energetic flow. As is usual in these cases, the overall forecast skill/confidence is low as small changes to locations and timing of waves can greatly affect the sensible weather. While the current forecast has the area mainly dry, this could change and later forecasts should help improve on the skill and forecast confidence. At the beginning of the period, largescale flow will be SW aloft leading to mild temperatures on Friday. Upstream, a strong shortwave is expected to move into the area and bring cooler temps to the area for Friday night. This cold front looks to be a dry passage across the area. Saturday, zonal flow will lead to mild temperatures again. Sunday another wave is expected to move SE into the area and interact with a closed low across Arklatex. Models keep this low east area and then set up what appears to be an omega block for next week with cooler air again. Next week could see more active weather but this dependent on where those closed lows are and I just don`t have high confidence in where they will be. && .AVIATION...(For the 06Z TAFS through 06Z Thursday Night) ISSUED AT 1049 PM CST Wed Nov 25 2020 LIFR and IFR will dominate the start of the period. However, expect a slow gradual improvement to predominantly MVFR with pockets IFR by morning and then eventually VFR by Thursday afternoon in clearing, as low pressure continues to pull away from the region and developing west/southwest winds usher in drier air. && .DVN WATCHES/WARNINGS/ADVISORIES... IA...NONE. IL...NONE. MO...NONE. && $$ SYNOPSIS...12 SHORT TERM...12 LONG TERM...Gibbs AVIATION...McClure

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