Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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164 FXUS63 KDMX 281737 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1237 PM CDT Fri Jun 28 2024 ...Updated for the 18z Aviation Discussion... .KEY MESSAGES... - Non-severe thunderstorms moving out this morning. - Storm redevelopment later this afternoon into this evening as a cold front moves through the state. There is uncertainty on the amount of storm coverage and severe hazards, but most likely hazards would be large hail and damaging wind gusts. Flash flooding continues to look less likely. - Wonderful weekend weather for late June - low humidity, no rain, cooler - Storm chances return later Monday into Tuesday. Severe and locally heavy rainfall may result with details in the coming days. - Hydrologic discussion below main discussion details more on ongoing river flooding and future locally heavy rainfall potential && .DISCUSSION... Issued at 408 AM CDT Fri Jun 28 2024 Showers and thunderstorms were prevalent in the evening into the early hours of this morning as the warm air advection kicked in in earnest with an 850mb 55 knot low level jet along the Missouri River. This has resulted in light rainfall amounts with rates generally up to around 0.1 to 0.15 inches per hour. Farther to the northwest, a mesoscale convective system was trucking across South Dakota with the HRRR, RAP, WRF-ARW, and RRFS having a generally good handle on timing and placement. GOES-East clean IR imagery has shown cloud tops warming with a southern flank trying, but failing to sustain itself. FSD WSR-88D imagery shows that storms are struggling to develop south of I-90, likely due to the 14C 700mb temperature cap in this area. At the same time over northeastern Kansas and southeastern Nebraska, clouds tops have been cooling as new storms develop. Both of these systems may scrape our border areas this morning, but all expected to remain below severe limits. This along with our current activity over central Iowa should be moving off into eastern Iowa by mid-morning. The question for the next round of storms later this afternoon into this evening will be the amount of recovery for possible destabilization ahead of an incoming cold front as well as spatial extent of storm coverage. It is less of a concern of dewpoints as those will be back into the upper 60s to low 70s by this afternoon as breezy southerly winds prevail. However, the amount of solar insolation is in question with the 0z SPC HREF mean cloud cover showing quite a bit of low clouds. NAM forecast soundings show more low level saturation suggesting clouds and this is reflected in a lack of surface warming and surface based instability availability over much of Iowa. In contrast, the GFS and HRRR forecast soundings at DSM and CSQ shows more recovery potential and surface heating to allow it to overcome any warm capping aloft by late afternoon. By that time, SBCAPE and entrainment CAPE will have built to around 1500 (GFS) to 2000 to 2500 J/kg (HRRR) with favorable 0-500m and 0-1km storm relative helicity for rotating storms. However, streamwiseness is not all that great for vorticity ingest unless the storm deviates to the Bunkers right motion vector or the surface winds back, which is possible given potential outflow boundary from early day storms. If this were to occur, a tornado or two would be possible, primarily over southern Iowa where it is believed that destabilization is most likely. Otherwise across central Iowa, low level lapse rates improve to around 6C/km with similar mid-level lapse rates based on forecast soundings. The cold front should help provide needed convergence and lift for storm development, which may contain large hail up to around golf ball sized and damaging wind gusts up to around 60 mph. Bottom line, the amount of recovery will be key in what kind of hazards may evolve and storm coverage in this period. As for heavy rainfall, additional rainfall up to 1 to 1.5 inches is shown from deterministic models over parts of central and southern Iowa with the 0z HREF localized probability matched mean showing a few areas with 1.5 to 2.5 inches. With the cold front moving through the state tonight, storms will end by Saturday morning and we`re looking at a wonderful late June weekend weatherwise. Winds from the northwest will deliver drier, lower humidity air and lower highs back into the middle 70s to middle 80s Saturday, lows in the 50s Sunday morning, and highs in the middle 70s on Sunday. Much needed dry conditions for those water logged Iowans will prevail Saturday into Sunday as high pressure moves across the state. Monday looks to be a breezy day as the high pressure moves away from the region and a cold front approaches from the west. Storm chances will be increasing most likely later on Monday as theta-e advection ramps back up over the region ahead of a longwave trough advancing from the Pacific Northwest over the north central states. Deep layer shear looks favorable for storm organization and a severe threat, but any appreciable instability doesn`t arrive until Monday night and more so on Tuesday ahead of the cold front. Tuesday also is looking like a potential hot day with NBM and the 100 member ensemble showing a 40 to 70% chance of reaching or exceeding 90 degrees roughly south of Highway 30. Of course, this will depend on the extent on any cloud cover with the current forecast right near 90 degrees in these areas. Storms ahead of the cold front later Tuesday afternoon into Tuesday evening could be strong to perhaps severe. Locally heavy rainfall will also be possible as precipitable water values of 1.75 to 2 inches Monday night increase to above 2 inches with favorable, deep warm cloud depths for efficient warm rain processes on the day Tuesday. 850-300mb winds are a bit on the fast side, though could potentially parallel the cold front. Therefore, will need to monitor the flash flood threat in the coming days, but the WPC excessive rainfall outlooks on Monday and Tuesday seem fair at this time given antecedent conditions and uncertainty. It seems largely agreed upon that the cold front will push south of the state by Wednesday, but the GFS pushes it farther south while the ECMWF keeps it closer and thus lingers storm chances over at least southern Iowa. Both of these models suggest that a shortwave trough moves down late in the week the possibility of more widespread chances. Details will become more clear in timing and location as we approach the latter half of next week and Independence Day. && .AVIATION /18Z TAFS THROUGH 18Z SATURDAY/... Issued at 1225 PM CDT Fri Jun 28 2024 Messy aviation forecast over the next 12 to 18 hours as the area deals with variable cig restrictions, rounds of shra/tsra, then some patchy fog tonight into early Saturday morning. As of 1730z, cigs over much of the area are MVFR with pockets of IFR. Radar imagery shows a complex of showers and storms moving into southeast Iowa. This will continue to push east, with additional development already noted over northwest Iowa. Expecting further development by later this afternoon and evening, impacting terminals from west to east into the evening hours. Timing of tsra and extent of categorical impacts to individual terminals remains uncertain at this time, although sporadic strong wind gusts and heavy rain reducing visby below 2sm is possible at times. Conditions should improve as storms push east by late evening into the early overnight hours. Model guidance suggests patchy fog may develop east of I-35, and have included mention of MVFR visby at KALO and KOTM. VFR conditions prevail area-wide by mid-morning Saturday and through the rest of the TAF period. && .HYDROLOGY... Issued at 408 AM CDT Fri Jun 28 2024 With the exception of the Des Moines River at Stratford, rivers have now crested and are in recession. There is still moderate to major river flooding on the West Fork Des Moines with record flooding still ongoing in Humboldt. This morning, we also ended the flood warnings at Shell Rock and Waterloo as they have dropped below flood stage. Cedar Falls is tracking below the evening forecast and will likely fall below flood stage later today. Rainfall early this morning is focused more over the northern half of the state, which has less infiltration capacity based on NASA SPoRT relative soil moisture. Fortunately, the rates are on the lower side and not consistent with some breaks between rain episodes. Looking at worst case flash flooding guidance, it still shows that flash flooding is about 1 to 1.5" away from a concern at the 1 hour and 3 hour time windows. Additional rainfall of 1 to 1.5 inches with a few spots up to around 2.5 inches is expected today, though this is expected to be more focused over central and southern Iowa. On river flooding, we will continue to use the traditional 24 hours of rainfall. This rainfall may cause some re-rises over northern Iowa with an example that Shell Rock River at Shell Rock may near flood stage on Sunday. Any re-rises from this rainfall will be captured by this morning forecasts and can be addressed in a hydro update later today. Looking at a longer time horizon using the experimental Hydrologic Ensemble Forecasting Service (HEFS) and the GEFS 10 day QPF, it continues to look like below flood stage flows will be the most likely outcome over this period at forecast points that are not currently experiencing above flood stage flows. For rivers with currently above action stage conditions or higher (Des Moines above Saylorville, Cedar, Shell Rock, and Winnebago), the ongoing rainfall now through today as well as next week will prolong the above action stage flows or slow the recession and in some instance may cause a secondary, lower crest. Of course, will need to monitor the rainfall for next week given the favorable heavy rainfall parameters Monday night into Tuesday for any flash flooding potential and impact on rivers. Make sure to stay up to date on the latest forecasts and river information, avoid flooded areas and rivers, and never drive around barricades or through flooded areas. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Ansorge AVIATION...Martin HYDROLOGY...Ansorge