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Displaying AFOS PIL: AFDDMX Received: 2017-09-20 20:03 UTC
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789 FXUS63 KDMX 202003 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 303 PM CDT Wed Sep 20 2017 .SHORT TERM.../Tonight through Thursday/ Issued at 303 PM CDT Wed Sep 20 2017 As of 20z a cold front was slowly sagging east through the CWA, evident on radar and surface obs stretching northeast from near Creston, through the DSM metro, to just west of Waterloo. Northwest winds behind the front have ushered in a much drier airmass with dewpoints falling into the 50s. Warm and humid conditions continue out ahead of the front. Increasing instability and convergence along the frontal axis has triggered a few updrafts in northeast Iowa. A strong mid-level cap will likely limit the intensity and coverage of this activity through early evening. Hi-res models advertise more convective potential after 00z further south along the front into south central and southeast Iowa. Again, widespread coverage should be limited by the cap and weak height rises aloft. The unstable airmass and 20-30 kts of effective layer shear may be enough to support a few strong to severe storms, capable of producing hail and strong wind gusts. The cold front stalls out over our southeastern zones tonight. Rich theta-e low level flow lifting over the boundary will maintain the potential for a few showers and storms through Thursday morning, mainly south of Interstate 80. The boundary retreats northward as a warm front tomorrow morning in response to mid-level height falls over the high Plains. The reprieve from the unseasonably high humidity will be short lived as upper 60s/low 70s dewpoints advect back northward through the state. A hot and humid afternoon is expected with heat indicies in the low to mid 90s. Despite the development of a highly unstable airmass, a strong cap and lack of focus for convergence should suppress any late day convective potential. .LONG TERM.../Thursday night through Wednesday/ Issued at 303 PM CDT Wed Sep 20 2017 First half to two thirds of the longer term will be dominated by the amplifying upper pattern. The deepening western will lead to an large upper ridge across the eastern United States. Will see surface front enter the Plains on Thursday and becoming nearly stationary as upper flow becomes nearly parallel with the boundary. This remains the case until Monday when the boundary finally slips eastward into the state. In the meantime, deep southerly flow is expected across Iowa from Thursday night into the weekend. Being in the warm sector, forcing will be limited with a very warm to hot airmass in place for the state. Temperatures will be well above normal with highs in the mid to upper 80s with even some 90s on Friday into Saturday and likely Sunday as well. In addition, moisture will flow north into the state with dewpoints into the mid 60s to even lower 70s. Heat index values well into the 90s if not approaching 100 are expected on Friday if not into the weekend as well. The front will begin to approach western Iowa late Sunday and gradually move into the state on Monday. Increased convergence and lift along the front will help to spread showers and storms into the forecast area during the day. Both GFS and Euro have continued to slow the progression of the boundary with thunderstorms chances persisting well into Tuesday before gradually moving eastward by the end of day 7. Temperatures will cool to normal or below by the middle of next week as the boundary passes through the state. && .AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/ Issued at 1230 PM CDT Wed Sep 20 2017 Main concern for the latest TAF package is the potential for MVFR or low cigs and scattered TSRA. As of 17z a low cloud deck was observed on satellite along and ahead of cold front sagging through central Iowa. Cigs between 2000-3000 feet are possible at KDSM and KALO through the next couple hours. -TSRA will be possible in the vicinity of the front late this afternoon and evening, potentially impacting KOTM. Confidence is not high enough in TSRA coverage to include mention in the TAF at this time, but trends will be monitored. Another round of low stratus is likely Thursday morning as the aforementioned front retreats northward. High confidence in MVFR or lower cigs at KOTM and KDSM, however confidence in ceiling height and coverage diminishes further north. && .DMX WATCHES/WARNINGS/ADVISORIES...NONE. && $$ SHORT TERM...Martin LONG TERM...Cogil AVIATION...Martin