Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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477 FXUS63 KDMX 270833 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 333 AM CDT Thu Jun 27 2024 .KEY MESSAGES... - Increasing clouds with storm chances returning late today into Friday morning with another round of storms later Friday afternoon and evening. Flash flooding is looking less likely given current rainfall, but severe storms are possible Friday afternoon and evening. - B-E-A-utiful late June weekend with more seasonal temperatures, low humidity, and dry conditions. - Storm chances could return as early as later Sunday night, but more likely later Monday into Tuesday. Severe and locally heavy rainfall may result, but will be more defined in the coming days. - Hydrologic discussion below main discussion details more on ongoing river flooding and future locally heavy rainfall potential. && .DISCUSSION... Issued at 333 AM CDT Thu Jun 27 2024 This morning, Iowa resides on the northeastern periphery of the southwestern US mid-level ridge with a longwave trough over the Eastern US and another one on the other side of the ridge over the Pacific Northwest. A shortwave trough topping the ridge over western Nebraska this morning is sending mid and and high level clouds eastward over portions of western Iowa this morning as viewed on GOES-East Nighttime Microphysics RGB imagery. The shortwave along with strong theta-e advection will bring a continued increase in cloud cover and eventually a return of showers and thunderstorms. While some convective allowing models (CAMs) are aggressive with their arrival time of these chances, have tempered them with forecast soundings showing residual low level southeasterly flow and some remaining low and mid-level dry air that will need to saturate. So while there may be a few showers over the north later today, the main round of showers and storms will arrive later this evening into the overnight into Friday. MUCAPE values will be generally under 500 J/kg and while shear is more appreciable given the 35 to 50 knot 850mb low level jet, the severe risk will be quite limited during this period. The 0z HRRR surface wind gusts did show near severe gusts arriving into western Iowa, but the simulated reflectivity is unimpressive. HRRR forecast sounding at CRL show no eye popping downdraft CAPE values around this time, but there is some mid-level dry air that grows with time, which may be why there is this signal in the HRRR. This may be what SPC is keying off of for bringing their marginal risk into our western areas, but their discussion notes uncertainty in eastward extent and bolsters our confidence in a very limited severe risk. Precipitable water values will be on the increase as well nearing or topping 2 inches by Friday morning with deep warm cloud depths for favorable warm rain processes. However, the latest National Blend of Models (NBM) and the 100 member ensemble have both shown a decrease in probability of one inch lessening concerns for flash flooding. Rainfall amounts from deterministic models through midday Friday are at most 2 inches (e.g. ECMWF, WRF-ARW) with the 0z HREF localized probability matched mean showing just a bullseye or two of up to 1.25 inches. While models show that the showers and storms will be moving into eastern Iowa later Friday morning, it remains to be seen what kind of break in cloud cover will occur on Friday. Moisture recovery will be going well with dewpoints back towards 70 degrees into the afternoon hours as strong southerly breezes prevail. Forecast soundings show plenty of deep layer shear for storm organization at 35 knots plus, but there is a warm capping layer. Soundings also show that the NAM and GFS struggle to warm at the surface with lingering cloud cover debris and thus do not realize the surface instability and have more meager instability overall. In contrast, the HRRR shows more potential of realizing the surface instability from I-80 southward with the CSQ sounding having nearly 2000 J/kg of SBCAPE by late Friday afternoon. So, it`s possible storms may struggle more over northern and central Iowa with weaker low level lapse rates, though the cold front and shortwave may provide the necessary forcing for strong to severe storms capable of large hail and gusty winds later in the afternoon/evening. Over southern Iowa, looking at the same HRRR CSQ sounding, it shows high values of 0-1km storm relative helicity so a tornado or two may be possible, especially in the vicinity of a surface low and warm front that may move into this portion of the state. This severe potential will hinge on more clearing to allow for more instability by mid to late afternoon. Rainfall amounts, despite an otherwise favorable heavy rainfall setup, look to largely stay below 1.5 inches, which will be spotty. Total rainfall from Thursday night through Friday night will be half an inch to an inch in most areas with a few locations between 2 to 3 inches. Thus, the concern for heavy rainfall and flash flooding looks quite limited. As the cold front pushes through the state Friday night, rain chances will end by Saturday daybreak with cooler and gradually drier air filtering into the state as high pressure moves across the region this weekend. Conditions will be quite nice for late June with low humidity and highs in the middle 70s to middle 80s on Saturday and 70s on Sunday with lows in the 50s Sunday morning. With the high departing later Sunday, moisture return from the Gulf of Mexico will ensue with the southwestern US mid-level ridge moving eastward into the southern Plains and lower Mississippi River Valley with the Pacific Northwest trough advancing eastward as well. While some showers and storms may return Sunday night into Monday morning with the start of the low level thermal lift, the higher chances will come as the trough approaches and provides additional forcing later Monday into Monday night. This could be a window for strong to severe storms with high amounts of deep layer shear and appreciable instability. WPC`s day 5 excessive rainfall outlook also has a marginal risk over the state with a slight over northern Iowa due to their antecedent conditions. The heavy rainfall parameter space does look favorable, but 850-300mb flow also looks fast so will need to assess in the coming days further. Additional strong to severe storms are possible later Tuesday into Tuesday evening as a cold front moves through the state. Temperatures on Tuesday have the potential to reach or exceed 90 degrees with the NBM having a 30 to 50% chance of this south of Highway 20, but this will ultimately depend on no lingering cloud cover. While the heat should be short-lived as the cold front settles south of the state at midweek, it may still be close enough that additional storm chances will persist as shortwave troughs move through the northern flow. && .AVIATION /06Z TAFS THROUGH 06Z FRIDAY/... Issued at 1118 PM CDT Wed Jun 26 2024 VFR conditions with light and variable winds continue through the night. Patchy fog chances look low, but still can`t rule out some brief ground fog, especially in areas over northern Iowa (KMCW, KFOD, KALO) where more surface moisture is present. Cloud cover will increase through Thursday, with the chance for a few sprinkles or an isolated shower in the morning, followed by increasing chances for scattered showers Thursday afternoon into the evening. Some lightning is possible later Thursday into Thursday night, but have kept thunder mention out of the TAFs for now. && .HYDROLOGY... Issued at 333 AM CDT Thu Jun 27 2024 Major to record flooding continues on the West Fork of the Des Moines River early this morning. It looks like both Estherville and Emmetsburg have reached their final crest and while they may remain there for a bit, should start to see a slow fall in the next day or two. Humboldt and Fort Dodge are cresting now or will be later today. The remainder of the rivers in northern Iowa, including the East Fork Des Moines, Cedar, Winnebago, Shell Rock, and Iowa Rivers are all in recession. Both Dakota City and Shell Rock are forecast to fall below flood stage within the next 12 hours or so based on current forecasts. The only rise is on the mainstem Des Moines at Stratford as the water works its way down towards Saylorville Reservoir, which should reach its peak around 875 feet around the Fourth of July and remain there for a period of time. The next chance for rainfall will be later today through Friday night. The flash flood risk in this time period is less likely given lower rainfall amounts as well as lower areal coverage in higher amounts, which may top out around 2 to 3 inches in a few locations for the event. As for possible infiltration, northern Iowa north of Highway 20 has shown a slight improvement with the roughly top 4 inches of relative soil moisture (RSM) showing a 2 to 4% drying compared to this time yesterday. The 4 to 16 inch layer RSM continues to show 60% or higher values over this same area so while the top layer has shown a bit of drying, these areas will continue to be prone to saturation and more rapid runoff response. Elsewhere in Iowa, the rainfall from Tuesday has lowered capacity in southern Iowa with streaks of 60% RSM. Thus, there may be isolated areas that may see more runoff response. However, in this part of the state USGS streamflows are generally at the normal percentile at both the daily and 7 day average. While there is more rainfall forecasted early next week, there should be some time for soil moisture and infiltration capacity recovery, but will need to monitor QPF amounts as the event approaches given the favorable heavy rainfall parameters. On the river flooding, the plan is to use just 24 hours of QPF today. Thus, the first part of this late week rain event through 7am Friday will make it into today`s river forecasts. For areas that are not currently experiencing or forecast to go above flood stage, namely central and southern Iowa, the experimental Hydrologic Ensemble Forecasting Service (HEFS) based on GEFS 10 day QPF continues to paint within bank rises at the 30% chance exceedance level. The 10th exceedance level would bring a few locations on the Iowa, Raccoon, and Des Moines into at least flood stage. However, this would require the higher end rainfall. For northern Iowa and rivers with above action stage conditions (Des Moines above Saylorville, Cedar, Shell Rock, and Winnebago), the rainfall Thursday night into Friday as well as early next week may slow the recession and prolong above action stage flows. 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...Dodson HYDROLOGY...Ansorge