Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
503 FXUS63 KDMX 260520 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1220 AM CDT Wed Jun 26 2024 ...Updated for the 06z Aviation Discussion... .KEY MESSAGES... - Strong to a few severe storms late afternoon into the evening with main concern from large hail, damaging winds and locally heavy rainfall central and south. - Larger complex of storms overnight across southwest half of Iowa with damaging winds main concern. - Hydrologic discussion below main discussion details ongoing river flooding and future locally heavy rainfall potential - Drying out this weekend with more seasonal temperature and humidity this weekend && .DISCUSSION... Issued at 248 PM CDT Tue Jun 25 2024 Boundary has settled down into the state this afternoon with lingering isolated convection into southern Iowa. Temperatures have once again climbed into the low 90s over the southern two-thirds of Iowa outside the areas with clouds with heat index values hovering around 100 in most locations in central and southern Iowa. The convection in the south has persisted but coverage has slowly been decreasing into the early afternoon with a bit of a col region in the low levels moving into southern Iowa. However, another weak wave is currently over the Nebraska Sandhills and is forecast to move southeast into tonight helping to produce the upcoming weather across the state. Instability is once again very strong with the high dewpoints and temperatures but is still be suppressed by relatively warm mid level temperatures. CAMS have continued to produce some isolated to scattered convection starting late this afternoon near the boundary in an east west corridor across the central portion of the state. Low level shear is relatively weak and the main concern with these storms will be large hail with the initial updrafts and then possible strong to severe wind gusts as the storms mature. There is the possibility of a brief tornado during the development stage, but given the weak surface flow and low level shear, this threat remains very limited. However, with the high amounts of moisture, the storms may also produce locally heavy rainfall and perhaps an isolated flash flood event, but widespread heavy rainfall is not anticipated with these storms. As the evening progresses, the weak wave in Nebraska moves east with additional convection developing in eastern Nebraska into western Iowa. The HRRR has been steadfast since late yesterday in organizing this convection into a line and pushing it southeast through central and southern Iowa tonight along the instability gradient. More recently, the larger scale models also indicate an MCS developing as the low level jet intensifies this evening and passing through the state in similar fashion into tonight. Therefore, have increased the PoPs through this evening into the early morning hours central and south in anticipation of this convective system. Damaging winds will be the primary threat with the storms with hail as a secondary. The 0-3KM shear vector is marginal at 25-30kts, therefore any QLCS tornado threat may be somewhat limited. Otherwise, the other concern will be from heavy rainfall although the progressive nature of the system is likely to limit the overall flash flooding threat. This activity departs the south towards daybreak on Wednesday with drier and cooler air arriving from the north. Quiet conditions are then forecast through Wednesday into much of Thursday morning as subsidence and a drier airmass remain in place across much of the state. However, warm advection and increasing theta-e advection by later in the day and particularly Thursday night into early Friday is likely to lead to widespread convection into Iowa, particularly across the southern half of Iowa. Some redevelopment of storms may occur in the far south on Friday afternoon prior to a front passing through the entire forecast area, but it appears that a good portion of the area should remain dry. Cooler and drier conditions return for the weekend with relatively pleasant weather for outdoor activity. A return to more active weather is then anticipated into next week as upper flow becomes more west to southwest across the western half of the CONUS. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1220 AM CDT Wed Jun 26 2024 Main concern is lingering convection at DSM/OTM with potential for some modest wind gusts and brief reductions of vsby. Aft 12z, all of area will be dry with light northwest winds. Likely the remainder of the period will remain VFR aft 15z where some lingering MVFR will be present near OTM. /rev && .HYDROLOGY... Issued at 248 AM CDT Tue Jun 25 2024 Moderate to record river flooding is occurring or is forecast on the West Fork Des Moines River, mainstem Des Moines River and parts of the Cedar River basin. The greatest immediate concern is river forecasts from the Humboldt area down to Fort Dodge. The crest may come in lower than expected but still significant. We continue to assess the data including real-time, in-situ river flow measurements made by our partners at the U.S. Geological Survey (USGS) which help us in the river forecast process. Some of the USGS staff endure dangerous conditions to make these measurements because they sometimes require wading or boating into the floodwaters and swift currents. NASA SPoRT relative soil moisture (RSM) continues to show more limited capacity over northern Iowa mainly north of U.S. Highway 20. Fortunately, much of the rainfall for later today into tonight is forecast to fall south of Highway 20 where RSM is lower/more capacity is available. This is also where USGS daily and 7 day streamflows shows river basins in the normal percentile versus northern Iowa at much above of high streamflows. Thus, again suggesting some capacity for rainfall that falls later today into tonight. The next chance for rainfall will be later Thursday through Friday night. It remains to be seen what kind of flash flood risk may result with the latest WPC excessive rainfall outlook at slight (2 out of 4) for a good portion of Iowa. While the experimental Hydrologic Ensemble Forecasting Service (HEFS) based on GEFS QPF points to mainly within bank rises at the 10% chance exceedance level, it will depend on where and how much rain falls in the basins. This will ultimately determine whether a given river response is a quicker rise, higher or prolonged crest, or a slower fall. 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...Cogil AVIATION...REV HYDROLOGY...Zogg