Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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707 FXUS63 KDMX 220821 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 321 AM CDT Sat Jun 22 2024 .KEY MESSAGES... - Heavy rain to continue for northern Iowa this morning. Storm total rainfall generally between 3 and 5 inches expected with locally higher amounts possible. - Severe weather possible for primarily southern and eastern portions of the state this afternoon. Damaging winds will be the primary threat with a tornado or two possible. If storms can initiate early, heavy rain will again be a factor for northern counties. - Storm chances return late Monday as well as Tuesday. && .DISCUSSION... Issued at 317 AM CDT Sat Jun 22 2024 Synopsis: Early morning shortwave was located over the Siouxland area with an associated warm front across northern Iowa which connected back southwest to a surface low in Southwest Nebraska. The H850 jet with speeds over 40kts continued to advect in dew points around +15 degrees C from the southwest. Its magnitude and direction is similar to the mean wind and parallel to parts of the aforementioned boundary in western Iowa, leading to training storms on the more SW to NE oriented portions of the front. An MCS tracked slowly southeast in northern Iowa with advection fueled updrafts sustaining it. Saturday Morning: With the warm front stalled over northern Iowa, expect heavy rain to continue in at least the pre-dawn hours of Saturday. Previous rainfall rates of 2-3 inches per hour have relaxed into the 1-2 inch per hour range. Severe weather chances have waned as storms have decreased in intensity overnight, but gusty winds are possible along the leading edge of the MCS. Storms further down along the line show signs of weakening with warming clouds tops on IR in Nebraska. This area of rain will move northeast into northern Iowa over the coming hours, focusing with the forcing from the shortwave and developing surface cyclone over southern Minnesota. Have kept rain mentions in the north through the morning as the cold pool from the MCS will behave similar to yesterday in that it will sink further south into Iowa and fester convection except now with more forcing in play. Would expect rain to be lighter compared to overnight, but adding to an already impressive rainfall total. H850 jet sustains itself through the morning and will lead to gusty winds in the clear skies of the warm sector in the far south. Saturday Afternoon: MLCAPE values in the warm sector exceed 2000 J/kg especially where skies are allowed to clear this afternoon. Deep layer shear values have trended towards 40 kts, especially nearest the triple point at the state line. Low level SRH values remain high due to the jet and hodographs are straight at higher levels, making rotating updrafts favorable and also sustainable. CAMs illustrate updraft helicity swaths nearest the triple point in the north early this afternoon, but some factors of uncertainty are in play with this outcome. The main one namely being what the near storm environment will be this afternoon. If the cold pool from overnight`s storms does not retreat towards the state line, the aforementioned outcome is less favorable and would at least offset the initiation time of the convection, delaying it until the northeast part of the boundary almost exits the CWA. Further southwest along the front, convection will be possible, but will be farther away from the better forcing. As for threats: hail appears to be the least favorable threat as columns remain very saturated. This however will favorable strong cold pools for damaging winds. The tornado threat looks more favorable closer to the triple point. Storms will be more progressive in nature due to the cold front, but there remains lingering concerns for heavy rain in the already saturated northern counties. If storms initiate earlier on in the afternoon, another 1-2 inches of rain could fall, leading to more flooding issues. Confidence is lower in this outcome at this time so the Flood Watch has been preserved at its original 1pm expiration, but trends will be monitored through the morning. Long Term: The upper level ridge retreats into the Desert Southwest region after today and will keep Iowa dry Sunday into much of Monday. A passing shortwave to the northeast might at least yield clouds Sunday afternoon. Southerly flow builds Monday with warmer temperatures returning. Somewhere nearby in the state will be a surface boundary with enough instability and forcing to cause concern for more severe weather, hence the Marginal Risk. Details on this day will be better defined as mesoscale features are better captured. Storms will again be a possibility on Tuesday. Highs for much of the area are likely overdone both of these days due to the factors mentioned. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1053 PM CDT Fri Jun 21 2024 Main concern continues to be storms building over northern Iowa affecting mainly FOD/MCW/ALO with potential for IFR cigs and low vsby during periods of very heavy rainfall. Have not modeled in wind gusts, but there continues to be a slight potential for brief strong wind gusts in periods of heavier storms between 06-12z at those locations. Farther south, much lesser impacts other than an isolated storm or two for DSM/OTM during the overnight/early morning hours. There is increasing confidence that we will see another round of storms 18z Sat to 02z Sunday as the main cold front sweeps southeastward across the state, but confidence in the timing of storm impacts is too low at this point to include anything more then SHRA/VCTS in the TAFs. && .HYDROLOGY... Issued at 317 AM CDT Sat Jun 22 2024 We are looking at a high-end river flood event especially across our far northwestern CWA. The river basins of most concern include the West Fork Des Moines River, East Fork Des Moines River, mainstem Des Moines River above Saylorville Lake and the Cedar/Winnebago Rivers. This higher end river flooding is due to the heavy QPF axis shifting south and now affecting many of our river basins. The West Fork Des Moines River basin is of most concern. At some locations the expected river flooding will approach the flood of record and will exceed the levels seen in 1993 (which is the benchmark major flood for many locations in Iowa). The river is rising rapidly and is above flood stage already at some locations. As for the East Fork Des Moines River, significant flooding is expected there too. The river flooding will be in the top three crests of all time at some locations. River forecasts presently include QPF through Sunday morning due to the widespread, heavy nature of this rainfall event. The river forecasts are based on our best forecast rainfall amounts and locations. That being said, location is everything in terms of rainfall and river response. If the QPF axis shifts even farther south than expected then the river responses may be even more significant and include more rivers/locations. If the QPF ends up being higher then the same thing may happen. Saylorville Lake on the mainstem Des Moines River above Des Moines is expected to rise significantly in response to this runoff. Presently the lake elevation is just under 840 ft. The latest U.S. Army Corps of Engineers forecast has it rising to between 865 and 870 ft in early July, or a rise of between 25 and 30 ft. As a point of comparison the lake reached an elevation of 860.1 ft in late May in response to the heavy rainfall from April into May. Although the forecast rise with this event will be significant it will still be well below the spillway elevation of 884 ft. Of course like the river forecasts the Saylorville Lake forecast depends heavily on the forecast rainfall. If the rainfall ends up being heavier or further south then expected then the response may be more significant than presently expected. && .DMX WATCHES/WARNINGS/ADVISORIES... Flood Watch until 1 PM CDT this afternoon for IAZ004>007- 015>017-023>028-033>039. && $$ DISCUSSION...Jimenez AVIATION...Percha HYDROLOGY...Zogg