Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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714 FXUS63 KDMX 292331 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 631 PM CDT Sat Jun 29 2024 ...Updated for the 00z Aviation Discussion... .KEY MESSAGES... - Quiet and dry Sunday. - Storms return Monday into Monday night. Some severe storms possible west with primarily damaging winds. Heavy rain possible north and west. - Additional severe storms and heavy rain possible Tuesday afternoon and overnight. Severe weather primarily in the southeast with heavy rain primarily east. - Rain chances to return Thursday. && .DISCUSSION... Issued at 243 PM CDT Sat Jun 29 2024 The main upper level trough moved off to the east through the day with remnant showers from a decaying MCS getting eroded by dry air associated with high pressure and northwest flow. As the center of high pressure sinks down this afternoon from the Dakotas, the pressure gradient across the north will increase and lead to some continued breezy conditions through the evening. Decoupling will keep winds light overnight and high pressure overhead will keep winds light Sunday. Skies will be mostly clear and highs will only be in the 70s. We`ll get a reprieve from mugginess as dew points drop into the 50s. For early this week, the H850 high center will be slow to move across the Great Lakes region, allowing for an available channel of Gulf moisture to make its way into the Upper Midwest as a series of upper level shortwaves move across Canada. PWATs will be in the 2- 2.3" range with warm cloud depths pushing 14kft. Precipitation is anticipated to arrive late Monday beginning in the northwest as isentropic ascent commences. This precipitation will increase in intensity and coverage as the H850 jet forms along the nose of the LLJ. Right now, this looks to be near the unfavored region of southern MN/northern IA. Further to the west somewhere in Nebraska, an MCS will form along the warm front and move towards Iowa. It`s not certain this far out where exactly this MCS would move, but it is certain that locations impacted by it will be subject to heavy rain. There`s a nonzero chance of severe winds with is as well, but the main instability axis will lag behind as it arrives into the state, making that chance wane over time. The Marginal Risk for severe weather in the west highlights this well. The QPF forecast currently has up to 2 inches near the IA-MN state line, but convective processes can easily push this amount higher in localized spots. What can inhibit this is the aforementioned MCS that will pass south of the main H850 front and track into western and central Iowa. Depending on its pace, it will eat away at the instability axis and moisture transport that would have otherwise continued to be pushed farther north. Trends will be monitored. It`s still important to note that the flash flood guidance for 3 hours near the border is just below 2 inches. The mature corn crop osmosis will add some buffer room to this value, but regardless, it will not take much water to cause problems. The main cold front will make its way into the state Tuesday and will have plenty of deep layer shear and instability to work with in at least the southeast, hence the Day 4 outline for this region. There remains some uncertainty in warm sector placement as the cold pool from the overnight MCS could be nearby. Aside from severe weather, heavy rain will focus in the southeast which is an area where basins can take on extra rain. The mean wind Tuesday evening will be parallel to the cold front with low dew point depression at low and mid levels. PWATs will still be well over 2 inches. The highest precipitation looks to be in northeast Iowa with this round, but much of eastern Iowa could see 1-2 inches. Beyond Tuesday night, the boundary will stall somewhere in Missouri and block moisture transport further north on Wednesday. The front tries to retreat into the state Thursday ahead of the next trough. Accompanying the trough may be another MCS, according to deterministic guidance. Both subsystems translate to more chances for heavy rain nearby near the end of the work week. Due to the lingering rain chances and clouds hanging around this week, highs will largely be held out of the 90s. && .AVIATION /00Z TAFS THROUGH 00Z MONDAY/... Issued at 623 PM CDT Sat Jun 29 2024 VFR conditions expected to continue through the TAF period with any lingering breezy winds early this evening continuing to decrease becoming light overnight, generally out of the north. Winds settle out of the northeast Sunday morning and then east to southeast by Sunday evening, but generally staying under 10-12 knots. && .HYDROLOGY... Issued at 301 AM CDT Sat Jun 28 2024 Despite the rainfall over some of the region into early Friday, little additional rain fell in the Winnebago and Shell Rock River watersheds late this afternoon or evening. So far, the rivers have not risen as quickly as forecast. We may see the Winnebago River remain just below FS at Mason City while the Shell Rock River at Shell Rock may yet just get to or slightly above flood stage this afternoon. In the longer term, a much bigger concern is QPF next week and its potential impact on rivers. Again, with our current river forecasts taking into account 24 hrs of QPF they do not include the QPF for next week. The last few meteorological model runs have been trending higher in QPF especially across the southeast two thirds of the state with mean ensemble values of 2 to 2.5 inches over much of the region. Our latest QPF ensemble hydrographs and Hydrologic Ensemble Forecast System (HEFS) runs are latching on to this QPF. Unlike the rainfall a week ago, the QPF next week will affect more of the state so additional river basins may come in to play. The Cedar/Iowa River basins look to be the most affected however our guidance is showing possible rises on tributaries to those rivers as well as portions of the Des Moines and Skunk River basins. At this point the most likely scenario will be slowing the rate of fall of our area rivers, however some ensemble members are suggesting the possibility of additional or new crests with additional minor flooding. Some of the lower probabilities are suggesting moderate to major flooding at some locations. We cannot totally rule out these possibilities. Ultimately the location, amount and timing of the rain next week will be the biggest factor. In addition to the river flooding, we will also need to monitor for localized flash flood potential next week given the favorable heavy rainfall parameters Monday night and Tuesday night through Thursday. Though this is a rather broad overview for now, refinement of details will likely be a 6 to 12 hour process at most and will be much better defined prior to each potential event. 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...Jimenez AVIATION...KCM HYDROLOGY...Zogg