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Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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878 FXUS63 KDMX 141745 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1245 PM CDT Wed Aug 14 2024 ...Updated for the 18z Aviation Discussion... .KEY MESSAGES... - Back-to-back rounds of thunderstorms this morning and again this evening into Thursday morning. Heavy rain is possible both rounds with a few strong to severe storms possible tonight. - Additional storm chances Thursday afternoon into evening. - Warmer and drier forecast this weekend. && .DISCUSSION... Issued at 340 AM CDT Wed Aug 14 2024 Multiple rounds of showers and storms are expected today through Thursday with heavy rain and strong to severe storms all possible. Attention early this morning is on the first round as a lead shortwave ahead of the surface low noted over the northern High Plains at 06Z moves eastward through Nebraska. Storms continue to traverse southern into central Nebraska early this morning and are expected to continue into Iowa fueled by favorable moisture transport into eastern Nebraska, increasing theta-e advection, and a 35-45 knot low level jet. As this activity arrives in western to southwestern Iowa, SPC mesoanalysis and hi-res guidance continues to depict marginal instability in Iowa which will limit storm strength and as such, generally expect a continuing weakening trend with this convection, as has already started to occur in eastern Nebraska. The warm air advection wing will be the first to arrive which was just arriving at the IA/NE border in far southwest Iowa around 715Z with this activity gradually lifting northeastward. Then, the main line arrives shortly after generally moving eastward across southern Iowa, though precipitation extends well north and west behind the main line. Lots of lightning has been noted with both the warm air advection wing and the main line which will remain the primary hazard along with locally heavy rainfall given the more minimal instability values previously mentioned. With pwats around 2", precipitation amounts may near or exceed 1" over especially southwest Iowa, with localized higher values possible, into midday. Coverage should diminish as storms push eastward through the morning hours as the overnight low level jet fades with time with most guidance in pretty good agreement of only lingering precipitation in northern Iowa by early to mid afternoon. With this, clearing should begin to occur over at least far southwest into southern Iowa setting the stage for round two later this evening. The extent of clearing and as such, atmosphere recovery and resultant instability values heading into round 2 will have a significant impact on storm mode and strength. Forcing continues to look stronger with this second round as the aforementioned low nears the area dragging a front with it, in addition to the potential for any remaining outflow boundaries from the morning activity. Latest guidance has varying amounts of MUCAPE remaining heading into the evening hours with amounts varying from 1000 J/kg to robust values of a few thousand J/kg. While these highest values may be offset from the increasing shear, there remains the ingredients for some strong to severe storms with damaging wind gusts and possibly large hail. However, soundings do continue to suggest the potential for long, looping hodographs, low LCLs and enough helicity to keep a conditional tornado threat if any storms can become surface based despite otherwise elevated storms being the most likely. As noted in the previous discussion, HREF helicity tracks fade with time through the late evening hours with overall hi-res guidance continuing to depict messy storm modes which would all lead to a low tornado threat overall, but it is conditionally higher than zero. With pwats once again near to exceeding 2", favorable warm cloud depths, and a stronger low level jet in most guidance, heavy rain will also remain a threat this evening into early Thursday, especially given the rain this morning. HREF Localized Probability Matched Mean 24-hour QPF values ending 12Z Thursday reach towards or even exceed 2" in much of southern Iowa with pockets of higher values, and additional little areas of similar amounts in isolated locations further north and east. Thus, we will continue to have to monitor for any localized flooding issues especially with the repeated rounds of rain, and given the overall amounts into Thursday, some ponding in especially low-lying areas seems likely. Round 2 storms exit the area by mid-morning Thursday (note, round 2 begins near/after 00z tonight) but additional showers and storms are possible on Thursday afternoon into evening. Guidance is not in good agreement at this time with the amount of instability present on Thursday afternoon with the better forcing also generally east of the area. Timing of the frontal passage, any lingering outflow boundaries, and amount of recovery after early day storms will all influence the strength of any storms that occur, which could also be east of the area. Will continue to monitor the mesoscale details that will impact our storm threat Thursday, but for now, see the SPC Day 2 Outlook for some further details. Although some wrap around showers could occur especially east on Friday as the low to the northeast consolidates further, conditions generally look to dry out for much of the weekend though some isolated precipitation chances remain as some waves move through the otherwise continuing to build northwest flow as the upper ridge continues to nudge further east into the region. Temperatures will be closer to normal levels with higher humidity as we continue through the week. && .AVIATION /18Z TAFS THROUGH 18Z THURSDAY/... Issued at 1245 PM CDT Wed Aug 14 2024 MVFR to IFR conditions are expected for much of the forecast period as a system passes through the Midwest. Showers persist across the north this afternoon with widespread convection overnight helping the poor aviation conditions persist. Surface wind gusts in and around the storms may approach 35-40kts this evening in western Iowa. Low ceilings persist overnight into Thursday morning with some minor improvement by late in the period. Surface winds are expected from the southeast this afternoon becoming more southwest toward the end of the forecast. && .DMX WATCHES/WARNINGS/ADVISORIES...None. && $$ DISCUSSION...KCM AVIATION...Cogil