Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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593 FXUS63 KDMX 140856 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 356 AM CDT Sun Jul 14 2024 .KEY MESSAGES... - Few areas of non-severe storms and gusty winds that may impact the area early this morning. - Hot and humid conditions continue area-wide today and then south again tomorrow, with heat indices of 100 to 105F. - Shower and thunderstorms chances continue tonight into Monday, a few of which could be strong to severe. Gusty winds and heavy rainfall will be the main threats with storms. && .DISCUSSION... Issued at 351 AM CDT Sun Jul 14 2024 We continue to watch multiple areas of convection early this morning as weak waves kick off thunderstorms along the periphery of the upper level high in the southwest CONUS. Fortunately, much of the convection to the north of the state looks as if it will mostly stay to our east and the MCS that produced 80 mph winds previously tonight over South Dakota has since weakened as its drawn nearer to the state, likely due to weaker kinematics ahead of it to keep the cold pool in balance, as well as the 12C 700 mb temperatures it resides in. Weve also seen a few storms develop along the leading edge of the LLJ in southern IA/northern MO tonight, but these have stayed sub-severe. Therefore, barring any additional development with the MCS to our west or along the outflow from storms to our north, not expecting anything other than a few storms this morning. That being said, some gusty winds may accompany these features as they move into warm, dry environments. In addition to storms lingering into the morning hours, the heat will continue to build in today, leading to another hot and humid day for much of the state. Observed highs yesterday underperformed the forecast by 1 to 3 degrees across the area, likely due to a combination of 850 hPa temperatures being a bit cooler than expected, as well as some patchy cumulus/shower activity through the afternoon hours. Likewise, having the warm layer aloft could be limiting how deep of mixing can occur, which would then translate to surface temperatures. Fortunately, it seems guidance has picked up on the slightly cooler 850 hPa temperatures, which has therefore brought NBM highs down a tad from yesterday. On the flip side, dewpoints overperformed yesterday, which kept apparent temperatures over 100F, albeit still slightly lower than forecast. Keeping these trends in mind, have tweaked high temperatures and dewpoints slightly, but still anticipating hotter temperatures today with slightly better mixing. That being said, the aforementioned MCS and additional convection through the morning hours will likely have some effect on highs today. Even if the precipitation dissipates prior to arriving here, residual cloud cover will still likely drift over the state in the morning. Therefore, because of the uncertainty with convection today, decided to maintain the same area of counties for the heat advisory. As we get into tonight, more chances for storms will be possible as the upper high begins to break down and the upper trough to the north drops down into the region. Guidance is again fairly messy with the progression of storms tonight, with convection first developing with the better forcing to our north tonight and then potentially dropping into the state Monday morning. Confidence is low in these storms impacting the forecast area, but the mostly likely area would be northern Iowa with strong winds being the main concern. Confidence is then higher in the occurrence of convection associated with the frontal passage Monday afternoon. These storms could be more area wide, but the warm layer aloft will likely inhibit widespread storm development. Strong winds will be the primary concern with the afternoon storms as well, thanks to the high bases and dry low levels. Locally heavy rainfall could also become a concern with 2"+ PWATs, but storms should be fairly progressive as the front pushes south. A supercell would be needed to produce any larger hail, as any small hail will melt as it falls. Finally, the tornado threat should be low, as LCLs will be around 1500m or higher. Not to be overshadowed by the storm chances, temperatures on Monday will again be quite warm, primarily over southern Iowa where the boundary will be arriving a bit later in the day. Theres potential for Monday to also be the warmest of the three, with 850 hPa temperatures nearing 30C ahead of the front. That being said, Mondays highs will suffer from the same uncertainty of previous days and be dependent on both location of the front, as well as any convection/outflow boundaries that result from it. Regardless, those that avoid these features on Monday could see heat indices exceeding 105F and potentially even approaching 110F. After Monday, temperatures return to more normal values and high pressure overhead mellows out the pattern through the week. && .AVIATION /06Z TAFS THROUGH 06Z MONDAY/... Issued at 1215 AM CDT Sun Jul 14 2024 Maintained VFR conditions through the period at all terminals, though there remains a high degree of uncertainty in what, if any, storms may affect the state. One cluster over central Minnesota is dropping southeast at this hour and current trends would keep it likely east of MCW and ALO. Another area of storms over western South Dakota may reach western Iowa before sunrise if it holds together, but any terminal impact would be after 12z. Even if storms do not directly move over the state, there could still be outflow related wind shifts that may cause a variation in wind direction that is not reflected in the official forecast later in the day Sunday. Staying with the assumption of no storms moving into the state, it looks like another round of diurnally driven cumulus that could be around FL030 starting mid-morning. For now, have capped at SCT, though Saturday ceilings were in BKN in MVFR and will need to && .DMX WATCHES/WARNINGS/ADVISORIES... Heat Advisory from 1 PM this afternoon to 8 PM CDT this evening for IAZ004-015-023-024-033>035-044>048-057>062-070>075- 081>086-092>097. && $$ DISCUSSION...Dodson AVIATION...Ansorge