Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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887 FXUS63 KDMX 011746 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1246 PM CDT Sat Jun 1 2024 ...Updated for the 18z Aviation Discussion... .KEY MESSAGES... - Trending drier and brighter today - Fog potential tonight, especially eastern half of the state - Uncertainty on daytime Sunday rain/storm chances, but storms look to enter the state Sunday night with a few stronger storms and locally heavy rainfall possible - Storms later Tuesday into Tuesday evening bring a higher chance of strong to severe storms and possible locally heavy rainfall && .DISCUSSION... Issued at 318 AM CDT Sat Jun 1 2024 GOES-East upper level water vapor imagery shows a closed 500mb low meandering through Missouri early this morning. This is the impetus for the low level moisture and lift that is causing the scattered showers over portions of central Iowa at this hour. This low will slowly move into Illinois and the upper Ohio Valley later today and thus we should see conditions turning drier with some clearing in at least western Iowa by later today. Temperatures will be well into the 70s to around 80 degrees over the western half of the state while the slower departing showers and clouds will keep temperatures lower in the 70s in eastern Iowa. Tonight looks like a good setup for radiational fog development with forecast soundings showing an inversion anywhere from 500 feet (RAP) to around 1000 feet (NAM, GFS) deep. This combined with recent rainfall, which keeps pooling of moisture beneath the nocturnal inversion high, and light to calm winds as a transient high pressure moves over the state should allow fog to form after midnight. High resolution models as well as the 0z HREF mean visibility are favoring the eastern half of the state (roughly east of I-35) for fog development with the MAV/MET MOS guidance showing around a half a mile visibility at Waterloo/ALO and under a half a mile at Ottumwa/OTM. The latter is supported by 50-60% probability of a half a mile or less visibility in the 0z HREF. As is typical with fog, there are detractors and the NBM short is not as aggressive with its visibility reduction. After any fog dissipated Sunday morning, a subtle shortwave trough will be approaching the state in the zonal flow with timing differences remaining in the global suite of models. Low level thermal lift ahead of this wave will allow for an increase in cloud cover through the day. Confidence in details such as timing for showers and thunderstorms is rather low looking at the spread in the models. Some models such as the RAP, NAM, and ECMWF indicate shower and thunderstorm chances increase over western Iowa persisting or waning as they slowly move into central Iowa through the day. Other models such as the NAMNest, HRRR, FV3, and NSSL WRF show the morning activity dying leaving the bulk of the day dry across central Iowa. Looking into Sunday night, there is better agreement in the convective allowing models, though there is still inherent uncertainty despite the agreement as the outcome of whatever happens during the daytime Sunday may influence Sunday night`s storms. Overall, it looks like severe convection kicks off over western and central Nebraska that grows upscale/congeals into a line of storms that advances towards Iowa reaching the state sometimes Sunday evening or after midnight. This is shown in the latest FV3, NAMNest, and the RRFS and as they reach our forecast area, their intensity should be on a downward trend. The SPC`s day 2 outlook highlights much of our forecast area with a marginal with a slight risk over northwest Iowa and this seems to capture the risk fairly well. On the hydrology front Sunday night, WPC highlights the western half of Iowa with a slight risk of excessive rainfall. Precipitable water values increase above 1.5 inches with favorable warm cloud depths for efficient rainfall and forecast soundings Sunday night also are juiced showing skinny elevated instability and near saturation over much of the column. If this does end up being a progressive line of storms as currently shown, this may lessen the flash flooding concerns with those CAMs showing rainfall of 1 to 2 inches with perhaps isolated pockets approaching 2.5 inches. Approaching those higher end forecast rainfall values would near the 3 or 6 hour flash flood guidance values. Of course, this doesn`t take into account the uncertainty of rainfall during the day Sunday, which if it occurs would prime conditions a bit more. Rivers show a little response, but nothing beyond a return to action stage at some points based on latest contingency forecasts. The storms from Sunday night will be pushing out of the state Monday morning, though NBM has broadbrushed PoPs through the day into Monday night. Have lowered PoPs a bit in coordination with neighboring office for part of Monday, which seems supported by the idea that the shortwave energy will have moved east of the area and some level of subsidence will be passing over the state. Highs on Monday and into Tuesday will be a few degrees above normal in the low and middle 80s. Storm chances do return later Tuesday as shortwave vorticity lifts from Kansas into Illinois through the day while a longer wave trough moving over the northern states deepens and approaches the state. Moisture will pool ahead of this shortwave and more so the cold front with precipitable water values nearing 1.75 inches and warm cloud depths at or above 3500m. 850-300mb flow is weaker earlier in the day, but increases as the trough deepens and the forcing approaches. While the mean flow is not as favorable for flash flooding at this time horizon, convective elements may still produce locally heavy rainfall and for now WPC has placed a marginal risk of excessive rainfall over the state so will need to monitor the flash flooding threat. As for severe weather risk, instability really blossoms with MUCAPE nearing 3000 J/kg by late Tuesday. However, the deep layer shear is displaced from this area and closer to the front with hodographs generally showing linear structure to messy/disorganized. So, at present would expect strong to perhaps severe storms later in the afternoon into the evening as the cold front pushes through the state. As the storms depart Tuesday night and the cold front moves east of the state, drier air with dewpoints back into the 50s will arrive with high temperatures only stepping down a few degrees and more in line with early June climatological values. The longer wave trough mentioned above wraps up into a sprawling upper level closed low over the Ontario and Manitoba provinces. Depending on its proximity to Iowa, this could bring additional shortwaves and rain chances into the region on northwesterly flow. Finally, while it will depend on how much and where the rain falls this weekend into the first part of next week, an examination of experimental HEFS (Hydrologic Ensemble Forecasting Service) forced with GEFS rainfall does shows several points in the Cedar and Des Moines basins returning to flows above action stage with a 30% chance that ESVI4/West Fork Des Moines River at Estherville and CEDI4/Cedar River at Cedar Falls exceed minor flood stage middle to later part of next week. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 1246 PM CDT Sat Jun 1 2024 Patchy MVFR ceilings linger east early this afternoon, but should slowly rise and improve to low end VFR by 19-21z. SCT- BKN lower VFR conditions can be expected elsewhere into the evening as well. There is some potential for isolated weak convection through 02z, but the confidence isn`t great enough to mention as of yet. IFR/MVFR fog may develop overnight as well behind the lingering low clouds, with the better potential east where those conditions have already been included near KMCW/KALO/KOTM starting at 07-08z. && .DMX WATCHES/WARNINGS/ADVISORIES...None. && $$ DISCUSSION...Ansorge AVIATION...Small