Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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509 FXUS63 KDMX 010829 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 329 AM CDT Mon Jul 1 2024 .KEY MESSAGES... - Seasonal temperatures through Friday with more active boundary near Iowa/Missouri through Tuesday and back into the area by Thursday. - Two periods of locally heavy rainfall possible late tonight and again Tuesday afternoon; another possible period southeast Wednesday night into Thursday evening. - More spotty showers/storms next weekend && .DISCUSSION... Issued at 329 AM CDT Mon Jul 1 2024 .Short Term /Today through Tuesday/... Confidence Short Term: Medium to High Little overall change in the forecast in the near term. Typical uncertainties with convective rainfall location will persist after 24 hours, though today is coming into better view for location. Tonight most of any storms continue over the panhandle of Nebraska and eastern Colorado. High pressure remains anchored this evening from Minnesota south to the Gulf of Mexico. This will help the cause briefly yet this morning. The high is beginning a slow slide east but the ridge axis will be able to keep enough dry air in the region this morning to weaken any approaching storms after 12z. The main moisture channel at H850, still very robust with 15C dewpoints, stretches from the Gulf back northwest into the eastern Rockies. Within the next 18 hours, this will be shifted back into Iowa as expected the past few days. Both synoptic and CAMS have developed an MCS over Kansas this morning and will lift that north northeast into southwest/western Iowa after 12z. This should weaken rapidly by 9 to 10 am as it encounters drier air north and east of the system. As the system over the Rockies lifts east northeast this evening, the moisture channel and better forcing will arrive this evening with a line of showers/storms developing over northwest Iowa. This will generally ride east northeast along the better thetae gradient, bringing most of the rain from the first round into the north half of the state. There has been no real change from the previous forecasts for the amount of available moisture and warm cloud depths tomorrow night and again Tuesday/Tuesday night. We continue to see PWATs of 2 to 2.25 inches feeding the area of convection tomorrow night over northern Iowa along with warm cloud depths over 13kft. The good news, if we can call it that, is that the trough appears progressive enough to alleviate a higher risk of significant rainfall in any one location. Despite that, we may still be looking at isolated 1 to +1.5 inch totals in bands of storms that fire generally in an east west fashion over western to northeast Iowa. The greater instability later today will be over eastern Nebraska into far western Iowa. This should quickly wane into the evening with only a rather low chance for any severe storms in our area. Tuesday will see the boundary sag farther east and south with a more typical and higher levels of instability in the afternoon/evening over the forecast area. The greatest shear will be along and behind the front, but 0 to 6km bwd of 40 to 50kts ahead of the front will be more than adequate to promote storms. The main upper level wave will pass across the region tomorrow between roughly 23z and 06z with storm coverage beginning to ramp up aft 18z from southwest areas northeast to around Waterloo. The line should quickly fill in south of I80 between 20 and 22z tracking east southeast with time. The severe threats tomorrow afternoon into the evening will include some potential for a brief spin up as well as large hail and wind with the higher risk south of I80 aft 20z. We are now in an enhanced risk in that area. Even with storms being progressive, the same efficient rainfall processes will result in some rainfall totals exceeding 1 to +2 inches over the south as the storms ramp up in the afternoon. The highest totals should be with the more organized storms south of I80 at that time. As clouds return to the region this morning with a small chance of lingering showers in the afternoon, highs will be held back in the upper 60s to lower 70s. Tonight will be much milder; especially over the south/southwest where mins will hold in the mid to upper 60s. Tuesday will likely see periods of showers and storms over the north with partly sunny skies at best in the south where passing showers and storms will increase in coverage after noon. Generally, highs will reach to near 80 north while the south will see the mid 80s. .Long Term /Tuesday Night through Sunday/... Confidence: Medium The storms will be moving south of the region by 02z Tuesday evening with a few areas of lingering showers/iso storms through 06z. We are still on track for a break Wednesday with a drier west northwest flow arriving. Temperatures will actually be warmer given decent H805 temps and mixing during the afternoon. We are still expecting highs to recover to the mid to upper 80s region wide. The break will likely be short lived, however. There is better consistency today for rain chances ramping up again on the 4th, though the deterministic GFS is likely too far north and convecting too much on Thursday. Another wave will approach the region Wednesday night with a renewed push of moisture and isentropic lift across the same retreating boundary from Tuesday night. An MCS is expected to develop southwest of Iowa and track into southern/central Iowa by 12z Thursday. The EC is suggesting the low level jet is veering and remaining farther south/east by Thursday afternoon/evening while the GFS has a slightly stronger/more northwest wave solution and keeps the low level jet backed into Iowa Thursday night, creating a large MCS over the southeast/east. This solution looks to be contaminated by convective feedback and creates some high qpf; likely not going to occur to that extent and somewhat farther east into Illinois by that time. Unfortunately, nearer the upper level low, there is some consistency of moderate rainfall upstream of the Western Fork of the Des Moines River over southwest Minnesota. This will need to be monitored moving forward. Rainfall from late Wednesday night to Thursday night may add another 1 to 2 inches of rain most focused over the southeast quarter of the state. This is supported by the GEFs and it appears that the 00z GFS is quite the outlier in precipitation amounts. The synoptic low that wraps up late Thursday night into Friday is likely going to create some lighter showers over the region for Friday, much cooler temperatures and a fair amount of cloud cover to round out the week. We should see some improvement into the weekend, but as bad luck would have it, a northern stream front is anticipated to arrive into northwest Iowa Saturday afternoon with a round of showers and possible thunderstorms both days. Highs Thursday may need to be trimmed a few degrees with 70s to around 80 or so more likely in our area. Friday will struggle to reach the mid 70s. By Saturday a break and milder H850 temps should bring everyone back to the 80 to 85 range. Sunday will be similar to slightly warmer than Saturday. && .AVIATION /06Z TAFS THROUGH 06Z TUESDAY/... Issued at 1155 PM CDT Sun Jun 30 2024 VFR conditions expected to continue overnight through much of the daytime hours Monday, though clouds will be on the increase overnight into Monday morning. Although a few showers remain possible around midday west to central, confidence in showers impacting KFOD or KDSM remains lower than 50% needed for mentions in the TAFs due to dissipating nature of showers and encountering dry air. Higher confidence in rain or storms moving in towards the end of the TAF period northwest to southeast with mentions included in future issuances. Light winds overnight will become out of the southeast and increase on Monday morning with gusty conditions expected by Monday afternoon into Monday night. && .HYDROLOGY... Issued at 329 AM CDT Mon Jul 1 2024 There will be two periods between now and Tuesday night with another late Wednesday night through Thursday night when moderate to locally heavy rainfall may occur. Details of all three are in the main discussion and it appears that tonight will be fairly progressive and what falls will be across the northern basins. Tuesday will see more storms from north central to south; and especially along and south of I80 in the mid to late afternoon. The somewhat more progressive nature of the expected rainfall over the next 2 days will likely slow the fall of some of the area rivers in the north. Most of the HEFS guidance suggests most basins will see some slowing in the fall of already flooded rivers or rises that show no significant impacts for the others. Depending on the amount/coverage of rainfall tonight/early Tuesday northeast near the Winnebago and in the Cedar River basin, the Cedar River may have additional rises and minor flooding again at Cedar Falls this week. Across the central and south, the main concern will be localized heavy rainfall and some urban flooding concerns for any hourly rainfall rates over 1.5 to 2 inches. We will continue to monitor Saylorville as well with any added rainfall across the north later today and tomorrow. The latter week rainfall is generally forecast south of Saylorville and at this time, is not expected to cause additional rises there. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...REV AVIATION...KCM HYDROLOGY...REV