Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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969 FXUS63 KDMX 040508 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1208 AM CDT Sat May 4 2024 ...Updated for the 06z Aviation Discussion... .KEY MESSAGES... - Storms and showers arriving after midnight tonight near and east of Highway 71 - severe risk is small hail, gusty winds possible - Stormy weather returns later Monday into Monday night with severe weather possible, perhaps redeveloping later Tuesday - More clouds than sunshine, fleeting showers, and trending towards seasonal levels mid to late next week && .DISCUSSION... Issued at 232 PM CDT Fri May 3 2024 Morning fog was rather impressive from visible satellite this morning highlighting the numerous river valleys across portions of Iowa. This fog was able to fully dissipate between 9 and 10am. Now, upper level water vapor imagery shows a parent upper low that is spinning into the Manitoba province. Dropping through the Intermountain West into the northern/central Rockies around this parent upper low is the shortwave trough that will bring thunderstorms into Iowa late this evening and our forecast area after midnight. An area of instability up to around 500 J/kg of MUCAPE is expected ahead of a cold front along with steep mid-level lapse rates and sufficient shear for storm organization. Forecast soundings show storms will be elevated in nature and thus pose a risk of primarily small to around 1" hail with isolated gusty winds possible. Convective allowing models show a few healthy updraft helicity tracks entering western Iowa, but their strength quickly wanes as they move toward central Iowa. Further, even the more aggressive WRF-ARW and FV3 are showing wind gust potential topping out between 30 to 40 knots. Overall, the marginal risks by the Storm Prediction Center on day 1 into day 2 seems to handle this risk well. Rainfall amounts look to be highest over western into portions of central Iowa, mainly east of I-35 with 12z HREF localized probability matched mean values of half an inch with small areas of 1.25 to 2 inches possible. The water issues over our southeastern forecast area - namely Monroe and parts of Appanoose and Wayne counties including the Cedar Creek basin - may then be spared from a return of water problems. These storms will be over eastern Iowa around midday leaving behind breezy winds from the north-northwest, decreasing clouds, and highs in the 60s in most central Iowa cities. With a clear sky over much of central Iowa, decreasing winds, and the rainfall from Saturday morning, we are looking to be set up for another round of dense fog. However, drier air will be pulled into Iowa and brings into question whether this will negate the other favorable factors. After any fog dissipates Sunday morning, weak ridging will pass overhead and a southern stream wave continues to trend south of the state with the latest NBM PoPs staying south of Iowa. At the same time, a potent shortwave trough over the western CONUS will move eastward toward the region and take on a negative tilt as it lifts northeastward into our region Monday into Monday night. Low level kinematics will be favorable at 40 to 50 knot Monday night in the region with resulting deep layer shear looking healthy for organized convection. This wind field will draw moister air into the region with low 60 surface dewpoints making their way to near if not into at least southern Iowa later Monday. MUCAPE values will be over 1000 J/kg and mid-level lapse rates rates will be steep around sunset Monday, but both are forecast to diminish into the evening to some degree. Forecast sounding continue to show an elevated warm layer that may keep storms elevated; however, given the strong kinematic fields in the 0-3km layer, cannot rule out a tornado risk with any line of storms if the orientation can become favorable to the low level shear vector. So, at a minimum, at least strong storms look favored, especially over the western half of the state Monday night, with severe weather possible. Like yesterday, SPC maintains probabilities largely southwest of our area on Monday and southeast of our area on Tuesday. Latest Colorado State`s machine learning random forest outlook and CIPS Experimental Analog- Based Severe Probability Guidance continues to paint 5 to 15% probabilities of severe over more of central Iowa each day lending credence to the idea of some strong to if not severe storms in portions of our forecast area. The strong to severe storm risk may persist Tuesday afternoon given the SPC, CIPS, and CSU mentioned above, but if things speed up then this may all be pushed off to the east on Tuesday. Wednesday through Friday has broad brushed PoPs in our current forecast as the shortwave trough that will bring our storms chances the first two days of next week will lift up and meander over the Montana and western Dakotas area. This will keep weak shortwaves passing over the state and thus keep varying degrees of clouds, fleeting showers, and temperatures trending closer to normal in the 60s by late next week. Severe weather Wednesday through Friday looks unlikely as the instability will be away from the state. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1205 AM CDT Sat May 4 2024 MVFR to IFR cigs to move in with rain and thunderstorms. Have added thunder mentions to to all terminals as confidence is increasing in thunderstorms for even KMCW and KALO. Highest confidence at KFOD, KDSM and KOTM with vsby reductions and gusty winds also possible for KFOD and KDSM in the next 6 hours. Will continue to evaluate trends. Gusty northwest winds to follow departing rain, easing at sunset. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Ansorge AVIATION...Jimenez