Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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485 FXUS63 KDMX 272354 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 654 PM CDT Thu Jun 27 2024 ...Updated for the 00z Aviation Discussion... .KEY MESSAGES... -On and off showers, with a few non-severe storms possible this evening into Friday -Severe weather possible Friday afternoon to evening, but remaining conditional due to potential lingering clouds - Quiet and pleasant weekend with dry conditions, especially Sunday with cooler and less humid weather - Signal remains for more active pattern with multiple chances for on and off showers and storms next week && .DISCUSSION... Issued at 259 PM CDT Thu Jun 27 2024 Satellite imagery this afternoon shows mid-level clouds streaming eastward over Iowa, with mostly cloudy skies across much of the area. Radar returns have overall increased across much of western and northern Iowa throughout the day, though surface observations indicate very little if any rainfall associated with this activity into Iowa, thanks to low level dry air that has been stubborn to depart. Model guidance depicts increasing southwesterly flow and moisture return to the region into the evening with a strengthening low level jet ahead of the circulating low pressure system slowly entering into the Dakotas, which is expected to increase rain chances as the front passes through into Friday. Further analysis of model soundings over the region this evening into Friday shows CAPE values generally under 1000 J/kg, along with limited shear and mid- level lapse rates. A few stronger storms cannot be ruled out, though largely expecting storms to remain below severe limits this evening into Friday. Uncertainty remains on the overall coverage of this activity, as the CAMS show mainly scattered activity along a line pushing eastward across the state, though models such as the Euro and NAM have more widespread coverage of showers and storms. Regardless, some heavy showers are possible with rainfall amounts generally under an inch expected. Lingering shower activity is expected throughout the morning Friday, with non-severe storms possible as the front continues to track eastward. With the pressure gradient tightening, winds out of the south/southwest are expected to increase, turning breezy by late morning through the afternoon with gusts up to 30-35 mph, especially over the northern portions of the state. The big question remains in how much cloud cover will remain through the day. Models such as the NAM and GFS are trending with more clouds throughout the day, though the HRRR is more optimistic on cloud cover diminishing later into the afternoon after the initial wave of showers and storms push through the state. If clearing does occur later in the day, then the chance for showers and storms will be higher. Overall, guidance per model soundings show more appreciable CAPE values in the 2000-3000 J/kg range, along with 35-40 knots of shear and mid-level lapse rates around 7-7.5 C/km, which would be favorable for strong to severe storms later on Friday afternoon to evening. Damaging winds and large hail would be possible if conditions do become favorable, along with the potential for a tornado or two. Will be closely monitoring for any changes, which will be most important tonight into Friday in terms of how cloud cover plays out. On the side of heavy rain potential, conditions are still on the more favorable end with PWATS around 2 inches and deep warm cloud depths overhead, as surface dewpoints increase into the low 70s. With the better potential further south into Missouri however, not anticipating any major impacts at this time, with values generally around an inch or less expected, especially south. Will continue to keep a close eye, especially given the remaining river impacts over northern Iowa. As the cold front departs the state into Saturday, drying conditions with high pressure sliding into the Midwest region will lead to dry and quiet weather through the weekend. Highs are generally expected in the upper 70s to 80s Saturday, with cooler conditions Sunday in the 70s. The signal per long term guidance remains as more active with a series of waves within the larger scale flow passing through the region, and low level flow opens up more consistently from the south, bringing in warm and more humid conditions back into the region. This looks to result in on and off chances for showers and storms throughout much of the next week, though specific details on exact impacts are limited and will be better defined as more information becomes available. Temperatures gradually warm back into the upper 80s to low 90s Tuesday and through the next few days as well. && .AVIATION /00Z TAFS THROUGH 00Z SATURDAY/... Issued at 640 PM CDT Thu Jun 27 2024 VFR conditions prevail across much of the state early this evening, but showers are ongoing in parts of the north and west/southwest. Overall trends continue to suggest rain shower activity will increase this evening into Friday morning moving west to east with time. However, confidence in how widespread this activity is and exact timing is lower, making specific details in the TAFs more challenging. Have attempted to provide some overarching timing with the first round here tonight into Friday morning, with a secondary round also possible Friday afternoon into evening. With confidence in timing and exact impacts to TAF sites low, have opted to keep out any thunder mentions at this time, but storms could occur at times, especially on Friday. Heavier showers may also occur at times on Friday leading to reduced visibilities, but overall flight conditions look to become MVFR in the north as CIGs decrease through the morning hours, with low VFR remaining across the south at this time. Winds out of the south will also be on the increase Friday with gusts (outside of storms) of 20-30 knots. && .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...Bury AVIATION...KCM HYDROLOGY...Ansorge