Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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549 FXUS63 KDMX 190447 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1147 PM CDT Wed Sep 18 2024 ...Updated for the 06z Aviation Discussion... .KEY MESSAGES... - An increase in precip chances later tonight into Thursday morning, but warm and humid with the potential of strong- severe storms late Thursday afternoon and evening. - Marginal (1/5) to Slight (2/5) Risk of severe weather late Thursday afternoon and evening - Mainly dry Friday, but chances for showers and storms return this weekend, becoming likely (60+%) in many locations Sunday && .DISCUSSION... Issued at 248 PM CDT Wed Sep 18 2024 Our pattern will become increasingly more active Thursday, and again over the weekend into early next week as influences from the current western CONUS mean trough continue to progress eastward. The weather is relatively inactive right now however, with the first lobe of mid-high level kinematic forcing associated with the current MT/ND upper low producing mid clouds and light precip, which is mainly aloft due to high cloud bases. The next lobe of deeper forcing ahead of the SD PV anomaly will increasing forcing through the Mid MO Valley later tonight however and attempt to activate the fairly deep moisture fetch from the central Plains through the Siouxland area. Effective shear will remain fairly weak this far east into Thursday morning however, so while scattered nocturnally influenced convection is expected overnight into early Thursday, it should remain on the weaker side. More appreciable convection is anticipated late Thursday afternoon and evening however. The aforementioned forcing will continue to slowly advance into Iowa with peak heating development anticipated along a weak frontal boundary. Mixed layer CAPEs are expected to reach 2000-3000 J/kg with negligible CINH, and with a modest but strengthening jet segment ejecting into the Plains ahead of what will be our weekend system, effective shear will increase to 30-40kts resulting in an increased potential for strong-severe storms. With that effective shear oriented near normal to the boundary, that should support the potential for discrete/supercell development, at least initially, but drier low levels and higher LCLs would seem to diminish tornado potential somewhat with wind and/or hail the more likely severe weather threats. Convection should exit overnight leading into a break from precip Friday with subsidence as the initial upper low lifts into Canada, and a bit more southern CONUS ridging influence. Precip chances will increase once again overnight into Saturday morning however with a nocturnally influenced theta-e advection surge into IA associated with fairly deep forcing also increasing ahead of a trough along the US/Canadian border. Chances will linger later in the day and night, although there may be a break later in the day ahead of the Four Corners system which will sharply increase rain chances Saturday night into Sunday as the upper low passes through the region. While the location of appreciable rainfall is uncertain north to south depending on model solution, both EC and GEFS 24hr median QPF suggest 1"+ amounts somewhere in the state, with locally higher amounts possible in convective elements. Neither GFS deterministic shear and instability parameter space, or experimental CSU ML/AI algorithms suggest much in the way of severe weather this weekend. There isn`t a whole lot to latch onto in terms of forcing with short wave strength and timing ill-defined in various guidance sources, but the central CONUS mean trough will linger into the middle of next week keeping at at least low precip chances in the forecast, and temperatures fairly close to seasonal values. && .AVIATION /06Z TAFS THROUGH 06Z FRIDAY/... Issued at 1145 PM CDT Wed Sep 18 2024 VFR to begin the period. Chances for showers and a few storms will increase from west to east early in the morning. There should be a break before more storms develop in the afternoon. Gusty winds are possible with any storms in addition to MVFR cigs and lower vsbys in rain. Breezy southerly winds much of Thursday before switching with fropa by late in the period. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Small AVIATION...Donavon