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Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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814 FXUS63 KDMX 262000 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 300 PM CDT Wed Jun 26 2024 .KEY MESSAGES... - Quiet weather continues rest of today into Thursday afternoon - Shower and storm chances return west into Thursday evening, with severe weather possible by Friday - Dry and generally quiet conditions return Saturday and especially Sunday - Active weather potential into early next week, with gradual warming && .DISCUSSION... Issued at 258 PM CDT Wed Jun 26 2024 Fairly mild conditions today across Iowa, with surface analysis showing a large area of high pressure centered over the Dakotas and extending across the Upper Midwest. Satellite imagery shows clear skies across Iowa, though scattered cumulus can be seen across much of Minnesota and Missouri, which is expected to diminish into the evening. Despite a weak boundary now over southern Iowa, temperatures this afternoon have increased into the upper 70s to 80s, though dewpoints are more varied with most areas in the upper 50s to low 60s, while southern Iowa sits in the mid to upper 60s. Dry conditions are expected to persist through the rest of the day and into Thursday, with overnight low temperatures falling into the mid 50s to low 60s. Given expected quiet conditions with very light winds and residual moisture, especially east, a few areas of patchy fog cannot be ruled out into early Friday, though uncertainty is on the higher side given gradually increasing clouds, but monitoring will continue. Much of Thursday is looking to be dry as high pressure gradually tracks east/southeast out of the region, with slightly cooler temperatures as values in the upper 70s to low 80s are expected. Low level flow per guidance is expected to increase out of the southwest, especially through the evening and into Friday as a 40+ knot LLJ sets up over the region. This is ahead of a developing low pressure system centered over Montana, which looks to drop a cold front into the state later Thursday afternoon to evening, gradually track eastward into Friday. An increased pressure gradient will lead to breezy winds late Friday morning to afternoon, with gusts upwards of 25-30 mph expected. Further analysis in the environment depicts low-end CAPE and mid-level lapse rate values, along with bulk shear values around 30-40 knots Thursday evening into Friday. Deterministic models struggle with the timing of arrival, which is expected to track west to east, as RAP, NAM and GFS have little if any activity over western Iowa until late Thursday, while the CAMS have showers and storms arriving in the afternoon to evening. Have taken the middle ground solution at this time, given some potential for dry air to be a limiting factor in the earlier arrival. Regardless, not expecting severe weather with this initial activity into Friday. As the front gradually tracks east through Friday afternoon, showers and non-severe storms are expected. However, a boundary is expected to set up over southern Iowa Friday evening into Saturday, with additional showers and storms expected. Despite the remaining timing differences, the overall setup for strong to severe weather is looking higher, with higher CAPE values generally in the 1000-1500 J/kg range, bulk shear values around 40 knots and mid-level lapse rates around 7 C/km, supporting a higher potential for strong to severe storms. This is highlighted with the Day 3 SPC outlook which outlines a Slight Risk across southern and eastern Iowa into Missouri. The main concerns at this time include damaging winds and some possible larger hail, along with a few tornadoes. On the hydrological side, PWATs around 1.75`` with warm cloud depths just over 15kft are suggested per model guidance, which would allow for an increased potential for heavy rain with this activity. Ensemble guidance has the highest rainfall in southern Iowa into Missouri, with total rainfall amounts 1-2 inches expected and locally higher amounts possible, with areas further north seeing lesser values. Though the differences in timing remains into Saturday regarding how quickly any remaining showers and storms depart, dry conditions are gradually expected to return and remain through the rest of the weekend as high pressure settles overhead. Highs in the upper 70s to 80s are expected Saturday, before cooling Sunday in the 70. Into early next week into the beginning of July, an active weather pattern is signaled to return, with the Colorado State Machine Learning site pinging portions if not all of Iowa with at least a lower end potential for severe weather. Further analysis shows moderate CAPE and shear over the region, though uncertainty on this potential remains on the higher end given timing ahead of this activity, as changes are surely expected. Warmer temperatures also look to return Tuesday/Wednesday with values in the upper 80s into the 90s. Stay tuned for updates as more information becomes available. && .AVIATION /18Z TAFS THROUGH 18Z THURSDAY/... Issued at 1243 PM CDT Wed Jun 26 2024 VFR conditions are expected throughout the period across the terminals. Mainly clear skies are expected for the rest of the day, before high clouds increase from the west and north before gradually spreading eastward through the end of the period. Slightly breezy winds from the north will diminish into this evening, before shifting SE Thursday as winds increase, gusting to 15-20 knots. A small signal is in place for patchy fog over the eastern terminals, though confidence on occurrence is low at this time, and will continue to be monitored. && .HYDROLOGY... Issued at 259 PM CDT Wed Jun 26 2024 The river forecasts have transitioned mainly to maintenance mode for our CWA. Having said that, major to record flood is still ongoing or forecast. Major to record flooding continues to be forecast on the West Fork Des Moines River. The upper portions of the West Fork and the East Fork are either leveling off or in recession from their crest. Farther downstream on the West Fork, rises are still forecast. Significant flooding is also forecast on the Des Moines River at Fort Dodge. On the Cedar River, Janesville has crested and Cedar Falls and Waterloo are nearing their crests. The Shell Rock and Winnebago are also in recession. Rainfall on Tuesday fell largely south of US 20 and away from the basins mentioned above. There were pockets of rainfall of 2 to 3 inches along and south of I-80. With about 36 to 48 hours before new rain falls over these areas, there should be some recovery of infiltration capacity. Not surprising, NASA SPoRT relative soil moisture (RSM) has changed little since this time yesterday showing more limited capacity/saturation over northern Iowa with some limited capacity of 50 to 60% in the 0-40cm layer. Rivers and streams did respond with the rain that fell yesterday with up to a couple foot rises generally at worst (e.g. NRWI4, DESI4, DMOI4, DEMI4, PRMI4 and CHTI4) so capacity remains in many central and southern Iowa streams after this rainfall. The one exception was BDFI4 where the river rose significantly but still crested below action stage. The next chance for rainfall will be later Thursday through Friday night. The flash flood risk remains uncertain in this period with WPC excessive rainfall outlooks painting broad slight risks (2 out of 4) Thursday night into Friday. Depending on where the rain falls with the expected more widespread storms Thursday night into Friday morning and if the storms later Friday fall over the same area, then there could be an isolated flash flood risk if things line up just right, or wrong depending on your perspective. On the river flooding, QPF only goes out 24 hours so this late week rainfall is not reflected in forecasts and won`t be until Thursday morning`s updates. 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 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 flood stage, but that would need the higher end rainfall. For northern Iowa and rivers with above flood stage conditions or forecasts (Upper Des Moines, Cedar, Shell Rock, and Winnebago), the rainfall Thursday night into Friday may prolonged the high flows or slow the recession. 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...Bury HYDROLOGY...Zogg/Ansorge