Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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718 FXUS63 KDMX 180925 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 425 AM CDT Tue Jun 18 2024 .KEY MESSAGES... - A few severe storms possible late this afternoon and early evening, mainly west of Interstate 35. - Heavy rain possible, especially into central Iowa tonight and Wednesday. This could lead to flooding or flash flooding in some areas and headlines may still be needed. - Unsettled weather continues into the weekend. && .DISCUSSION... Issued at 422 AM CDT Tue Jun 18 2024 Surface low pressure is over northwest Nebraska early this morning with a trough extending north/northeast into southeast North Dakota and far northwest Minnesota. Much of the thunderstorm activity is in that region currently and in advance of the approaching short wave trough moving through Montana. This leaves Iowa firmly in the warm sector where 3 am temperatures were well into the 70s. Strong southwest flow aloft is over Iowa this morning as well due to that short wave trough moving into the Northern Plains and the large upper high over the eastern CONUS. That upper level flow will remain southwesterly into Wednesday but it will relax a bit as the eastern upper high tilts and ridges into the Southern CONUS. This all will help allow the cold front to move into northwest Iowa later today then will settle slowly south into the day Wednesday. Strong moisture advection is ongoing this morning from Texas northward into Kansas and northward into the western Iowa and eastern South Dakota. Areas of stratus are already beginning to develop from eastern Kansas and southward within this band of the moisture/theta-e advection and a few storms are beginning to bubble further south into Oklahoma and Texas. This is all of note as it is a possibility into Iowa yet this morning. Model proximity soundings show a warm layer aloft near 3.5-4 kft and with the influx of moisture arriving below this layer, stratus may develop into Iowa. In addition, some elevated weak convection may develop above this layer with enough saturation and forcing near 700 mb for parcels to begin to lift from that level. Do not have much for precipitation in the forecast for this morning but trends suggest that PoPs may need to be increased for the period soon. Finally, mixed layer winds below the warm layer are 35 kts or higher and this will likely result in windy conditions at times today with gusts of 30-40 mph possible. High temperatures today will continue to follow the trend of not reaching NBM forecast values as the bias correction continues to push temperatures too high. In addition, limited mixing depth, potential increasing clouds and cooler 925 mb temperatures to begin the day all suggest cooler than Monday temperatures for most areas. The main forecast challenge will be thunderstorms that will have an early attendant severe weather threat with damaging winds large hail and eventually transitioning to a heavy rain threat as profiles saturate. The cold front should reach the far northwest forecast area by mid to late afternoon and then will move slowly southeast overnight. By mid evening, the threat with any storms should have shifted to heavy rain potential. PWAT values along the instability axis will be in the 2 inch range and these values and the instability axis do extend north/cool side of the surface boundary so post boundary convection is likely and expected. The mean storm motions should be near parallel to the boundary therefore training of storms is likely as well. Warm cloud depths of 13 kft or higher will promote warm rain processes leading to efficient rain producing storms. There are a couple things to monitor for the heavy rain potenitial. One, the surface low will lift rapidly northeast and will be over Ontario tonight. The cold front is linked to another area of low pressure currently over southeast Colorado which will settle south into New Mexico as high pressure moves into from the west. This will stretch the boundary and forcing thin and the main low level jet will become more focused into Kansas later tonight. Second, the Gulf moisture flow will not have a direct route to Iowa as surface high pressure ridged into central Texas and a tropical system in the western Gulf, focus much of the moisture advection into southeast Texas with any remaining moisture taking the long detour route into Kansas. Those two areas certainly have very heavy rain potential so how much of a rich moisture source will remain available later tonight into early Wednesday. Still expect a fairly widespread 1-3 inches of rain with potential for higher amounts in some areas. Several guidance sources are targeting heavy rain into the Des Moines Metro area. No hydro headlines for this period as some uncertainty remains but this still may turn into flooding/flash flooding into some areas especially parts of central Iowa. The boundary will being to lift back northward on Thursday which will bring addition storm chances to the region. Friday should be mostly dry and warmer again. Another system moves through later Saturday and Saturday night which will bring renewed storm chances. The high temperature forecast Friday through early next week is currently dominated by the NBM, which is in the 75-90th percentile range and is likely running to warm in most cases through this period. With the attention on the near term, no changes were made to these temperatures at this time. && .AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/... Issued at 1140 PM CDT Mon Jun 17 2024 VFR conditions are expected to prevail through the night, before low cloud cover develops in the morning hours. These may lead to a brief periods of MVFR conditions. Gusty winds also pick up in the morning hours, with gusts up to 30 to 35 kts possible through the day. Cloud cover will continue to increase through the day, with showers and thunderstorms becoming likely in the evening and overnight hours, especially at KFOD, KMCW, and KDSM. Thunder will be possible during this period, but will be better refined in future TAF issuances. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Donavon AVIATION...Dodson