524 FXUS63 KDMX 210907 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 407 AM CDT Thu May 21 2020 .DISCUSSION.../Today through Wednesday/ Issued at 404 AM CDT Thu May 21 2020 Key Points: 1.) Today-Friday: Cloudy, below normal temperatures to continue. 2.) Friday PM-Saturday: Warmer, with widespread showers and thunderstorms likely. 3.) Sunday-Monday: Humid, strong/severe storm with moderate/heavy rainfall potential. ----------------------------------------------------------------- 1.) Today-Friday: Cloudy, below normal temperatures to continue. High confidence the proverbial broken record of cloudy, below normal temperatures continues for today and likely tomorrow as well. Since we are well-versed in this pattern, will not get into much detail other than to mention widespread drizzle added through this afternoon across most of our area. 2.) Friday PM-Saturday: Warmer, with widespread showers and thunderstorms likely. High confidence continues that the pattern we have been in all week long will come to an end Friday PM-Sat AM, as the closed upper low responsible for all of this will propagate into the northeastern CONUS by 06z Sat. As we have seen the past several nights, deep longwave trough to be in place across the intermountain west by Friday PM. For the past several nights, models have been showing a shortwave ahead of the longwave trough entering western Iowa sometime Friday PM. The 00z operational GFS/NAM/ECMWF in better agreement with handling this feature. Interestingly, the GFS won out as non-GFS models have effectively converged towards the solution it had been running the past few nights. With strong anticyclonic flow from 850mb-300mb off the east coast of Florida, the Gulf of Mexico is wide open for moisture to be transported to Iowa... as simplistically evidenced by sfc dewpoints reaching into the 60s. Expect some shift in the precise location/timing of the shortwave, but setup still in place for overturning nocturnal convection focused along the leading boundary, highlighted by strong frontogenetical forcing at 950mb. PWATs approach 1.5 inches during this timeframe. Forecast sounding not the most efficient ever... but also not dry/inefficient. As this shortwave rapidly propagates to the Great Lakes region by the afternoon, dry air entrainment and subsidence quickly is advected across Iowa. This will likely clear out the state, allowing for strong insolation to heat things up during the daytime. With 850mb temps slated to be in the +12C to +15C range, feel confident going on the upper end of guidance for max temps. 3.) Sunday-Monday: Humid, strong/severe storm with heavy rainfall potential. For Saturday night-Sunday morning... Strong kinematic forcing ushered into Iowa as 50kt LLJ that reaches across eastern Nebraska behind a band of strong theta-e advection that cuts from NW to SE across iowa. Expect minor shifts in the location/timing of this system between now and Friday night. At this time, NW Iowa focus for at least moderate rainfall potential. As with Friday night, not enough instability for severe threat. Sunday PM... Will be interesting to watch how convective debris clears out through the morning hours. Though, this should be an advection-driven event, so will not necessarily need clear skies for convection to initiate. Clearing would certainly support stronger storm potential. Captured decently well on the 950mb height field, strong height falls centered in northern KS at 12z Sun, should race northwestward and make it into NW Iowa by 00z Mon. Obvious warm sector surge ahead of this system, with attendant boundary/cold front draping from N-S across west-central Iowa. Dynamics of the boundary seem to fall apart after 00z Mon, so with loss of daytime heating, severe threat will likely wane. During severe window, continuing with yesterday's messaging that lack of deep-layer shear will be a negative for long-lasting supercells, but multicell clusters and a few supercells could develop early for a conditional severe weather threat. Through this timeframe, the GFS is especially trying to hint at a potential for moderate-heavy rainfall. PWATs look to reach 1.5-1.75 inches, with warm-cloud depths reaching 3500m and a wide swath of 0- 6km mixing ratios in the 6 to 8 g/kg range. Heavy downpours certainly plausible. Pending how much rainfall occurs prior to this, the ground should fairly readily absorb this rainfall. Will continue to monitor. && .AVIATION.../For the 06Z TAFS through 06Z Thursday night/ Issued at 1138 PM CDT Wed May 20 2020 Another night of stratiform cloud cover across Iowa. Ceilings will drop to IFR at most locations for a few hours. MCW and ALO may be able to avoid the IFR ceilings, but the line will be sitting very close by. Drizzle and patchy fog are also possible this overnight, which may reduce visibility to 3 or 4 miles. However, once again the main limiting factor will be ceilings. && .DMX WATCHES/WARNINGS/ADVISORIES... NONE. && $$ DISCUSSION...Kotenberg AVIATION...Krull