586 FXUS63 KDMX 092112 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 412 PM CDT Tue Oct 9 2018 .SHORT TERM.../Tonight through Wednesday/ Issued at 400 PM CDT Tue Oct 9 2018 Deep longwave trough currently extending over the intermountain west all the way down to west Texas. All models in agreement placing a shortwave over eastern CO/western KS, with its sfc reflection over northern KS. While this system was in KS, rainfall rates were around 0.25 to 0.5 inches per hour. Excellent model agreement with this sfc low slightly deepening as it propagates NNE towards Iowa... making it into NW Iowa by 06z Wed. Severe Potential... Primary svr threat for this evening will be tornadoes. Given "mediocre" lapse rates of around 6 to 7 C/km through sunset, rapidly declining shortly thereafter... plus, abundant moisture in low-levels + tall skinny cape profile, am thinking any hail threat would be confined to daylight hours...with most to all hail melting before it reaches the sfc. Damaging wind threat (aside from those associated with tors) minimal as DCAPE is only a few hundred J/kg and best 0-3km Theta-E difference slug SE of the DMX CWA. For tor threat, high BL moisture levels help bring the LCLs down to 500M to 750M or less, which is plenty low. MLCAPE values low, but sufficient in the 500 to 1000J/kg range in a tall, skinny setup. Sfc to 3KM CAPEs are around 300 to 500 J/kg. 0-6km Bulk shear ranges from 40 to 60 kts across Iowa, which is plenty sufficient. Hodographs are extremely impressive as they are very sickle-shaped. Hodograph analysis suggest fast moving storms the N/NE in the 35 to 45 mph range. Watching a boundary/def zone oriented SW-NE across central to southern Iowa. Storms that can isolate align themselves more perpendicular to the low-level shear vector could very easily produce tornadoes. Any tornadoes that form will likely be of the short-lived nature, but cannot rule out one or two slightly longer- lived tors. As the sfc low propagates into NW Iowa around 03-06z Thu, will have to watch for tor potential... but best parameters, helicity, etc. will be well east of the DMX CWA. Flooding/Heavy Rain Potential... Good news is that stronger storms will be very fast moving, as mentioned above. Bad news is strong signal for heavy/efficient rainfall producers. PWATs still around 1.75 inches give or take, with warm cloud depths approaching 12 kft. LLJ nears 50 kts in NW Iowa during the 03-06z time frame. With the fast storm motions, flash flooding potential generally mitigated to any areas of training storms... which doesn't seem too likely. Areal flood watch left in place, as the long-duration of this rainfall overnight would be the trigger. Tomorrow... System continues to slowly propagate to the NE. By tomorrow afternoon, most precip should be NE of the DMX CWA. Strong CAA infiltrating behind the cold front attendant to this sfc low + cloud cover means that temperatures will likely steady to fall during the afternoon behind the fropa... a raw, fall-like day. .LONG TERM.../Wednesday night through Tuesday/ Issued at 400 PM CDT Tue Oct 9 2018 Our recent active, wet pattern will come to an end for the remainder of the period with our weather influenced by iterations of a persistent central CONUS mean trough. The main weather stories will be cool weather and several additional precip windows, but they will be minor with no additional heavy rains anticipated. The growing season will likely come to an end in some locations due to frost and/or temperatures just below freezing. At onset Wednesday evening strong subsidence behind our departing system will move across the state with clearing and cold advection. Brisk NW winds will follow with gusts of 30-35 mph possible north during the evening. Surface high pressure will then settle into the MO Valley Thursday Night with a look ahead to frost potential. Have mentioned areas east but the window may be small due to cloudiness advancing eastward ahead of the next system. With this system and others the models are all in good agreement with the long wave evolution but of course differences with the short wave specifics. This wave in question will dip from the western Canadian provinces through the MO Valley. The forcing with this system will be deep, with both kinematic and thermodynamic support, but low level moisture will be somewhat limited. This will restrict any PoPs to southern Iowa and a lack of moisture in the very lowest levels may preclude any precip at all. There is a token mention of light rain, and even light snow due to evaporational cooling if anything could reach the ground, but any precip of either type would be minor. This will lead to another shot of frost potential Friday Night, but this window is small as well and will be a race with increasing low level southerly flow. The next waves will pass through the northern stream of the trough over the weekend, but recent trends for us seem drier with spotty phasing of lift and adequate moisture, especially in the lower levels. There are PoPs for light rain central and south into early next week, and possibly brief light snow if something could overcome the low level dry air with evaporative cooling and favorable temps, but the overall trends appears to be diminishing chances if trends from the 12z models continue. && .AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/ Issued at 114 PM CDT Tue Oct 9 2018 Messy TAF forecast continues. Expecting Thunder at all sites this afternoon through early tonight...timing dependent upon site. Vsbys will likely be 1 mile or less during heavy downpours. Given the speed of storms, periods of highly reduced vsbys would less than 1 hour. Precip to eventually move east of TAF sites by tomorrow afternoon. && .DMX WATCHES/WARNINGS/ADVISORIES... Flood Watch through Wednesday morning for IAZ004>007-015>017- 023>028-033>039-044>050-057>062-070>075-081>086-092>097. && $$ SHORT TERM...Kotenberg LONG TERM...Small AVIATION...Kotenberg