Flash Flood Guidance
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Issued by NWS
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537 AWUS01 KWNH 211855 FFGMPD WIZ000-MNZ000-IAZ000-220030- Mesoscale Precipitation Discussion 0298 NWS Weather Prediction Center College Park MD 254 PM EDT Tue May 21 2024 Areas affected...Southern MN...Northern IA...Western WI Concerning...Heavy rainfall...Flash flooding possible Valid 211855Z - 220030Z SUMMARY...Blossoming strong thunderstorms containing efficient hourly rainfall rates up to 2"/hr may causes areas of flash flooding, particularly in areas with sensitive and saturated soils. DISCUSSION...A dynamic synoptic scale pattern is fostering excellent upper level ascent through two primary methods. The first is a negatively tilted 250-500mb trough approaching from the Central Plains and the second is the region`s placement beneath two divergent 250mb quadrants (right entrance region via a jet streak in southern Canada, left exit region via a jet streak in the southern Plains). At the surface, a strengthening surface low and its associated warm front are lifting north through IA and heading for southern MN this afternoon. Visible satellite imagery showed brief clearing over northern IA and southern MN in wake of this morning`s rainfall, helping to increase MLCAPE in the region by 200-600 J/kg over the past few hours. Meanwhile, the cold front swinging east through eastern NE is responsible for sparking storms that are quickly tracking northeast. Adding to the excessive rainfall threat this afternoon is an exceptional IVT topping 750 kg/m/s at 18Z, easily topping the 99.5 climatological percentile according to NAEFS. PWs will surpass 1.5" this afternoon within the highlighted region through 21Z, which is also around the 99.5 climatological percentile. As peak heating continues, lifting mechanisms via surface based heating and mesoscale triggers (the warm and cold fronts) will ignite numerous heavy thunderstorms over northern IA that are forecast to race into southeast MN and western WI. Projected soundings via the 16Z HRRR this afternoon suggest MLCAPE is likely to surpass 500 J/kg and low-mid level RH values averaging around 90%. In addition, warm cloud layers not only feature some of that available instability but are also as deep as 10,000ft in most cases. These storms are likely to generate exceptional downpours with rainfall rates topping out around 2"/hr at their peak. One deterring factor for flash flooding is that storm motions should be progressive, but the areas that see excessive rainfall longest will be closer to the developing triple point this afternoon. These areas would receive heavy rainfall closer to the warm front initially, then as storms along the cold front approach from the southwest. The 12Z HREF depicts moderate chances (40-60%) for 3-hr QPF > 3-hr FFGs in central MN between 21-00Z this afternoon, which would include the Twin Cities metro area. NASA SPoRT-LIS 0-10cm soil moisture is above normal (70-90%) in central MN, making those areas particularly susceptible to flash flooding. In summary, flash flooding is possible in parts of the Upper Mississippi Valley this afternoon and evening with storms likely impacting the afternoon rush hour. Mullinax ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...ARX...DLH...DMX...FSD...MPX...OAX... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 46029172 45179115 44349149 42439313 41579503 41879601 42889647 44339584 45749369