Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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483 FXUS63 KDMX 260454 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1154 PM CDT Thu Apr 25 2024 ...Updated for the 06z Aviation Discussion... .KEY MESSAGES... * Quiet to round out daytime hours today, giving way to first round of shower/storms tonight and an overall active period through the weekend. * Best severe weather threat W/SW Friday evening, with strongest threats for large hail and a few tornadoes. Primarily W/SW Iowa late afternoon/evening, then lifting NE. Currently SPC Enhanced Day 2 west of I-35. * Severe potential continues Sat/Sun, and at least partially dependent on previous rounds of convection. Including hydro threat Sat evening/night. Current SPC Slight Day 2 (Sat), SPC Day 4 15% (Sun). WPC ERO Slight Sat. * Quieter to begin next week. && .DISCUSSION... Issued at 257 PM CDT Thu Apr 25 2024 The calm before the storms today, so to speak. Aside from a few sprinkles/light rain W/SW this morning, dry conditions have prevailed across the state with highs into 60s and breezy SE winds. Overall dry conditions will continue into the evening before western trough exerts increasing influence across the Plains with surface low beginning to move off the Colorado Front Range. This will see southerly flow, moisture advection, and broad lift increase. While the surface front will lag behind, strong low to mid-level theta-e advection will yield elevated thunderstorm activity at/above an 850- 800mb warm nose during the overnight and into Friday morning before activity subsides as it slides eastward into less supportive air mass. Continue to expect these storms to remain sub-severe, though effective shear may be just supportive enough to allow a few storms to flirt with impactful hail. Nominally bolstered with RAP/HRRR soundings in western areas depicting up to around 1500 J/kg elevated CAPE. Any strong/severe concern would be over western areas with initial storms as upscale growth is expected with widespread lift ongoing and less supportive air mass east. During this time, surface winds will remain breezy, and increasing gusts, as surface low moves out into the Plains. Expect gusts into the 30s mph to continue overnight tonight. Activity will linger into the Friday morning hours, and slide eastward, leaving in its wake cloudy skies. Additional/continued theta-e advection may result in scattered non-severe shower/storm activity through the remainder of the morning and into the afternoon, as well as helping re-prime the environment for severe potential later Friday, as seen in the SPC Day 2 Enhanced broadly west of I-35. Primary window of concern Friday will begin around 21z in/around the Iowa-Nebraska border, and storms will move northeastward into the evening. Strongest threat does continue to look to be in the aforementioned Iowa-Nebraska border area as broad CAM guidance highlights a corridor of SBCAPE values of 2000-3000+ J/kg with nominal (~25 J/kg) to no CIN. Hodographs continue to strongly point towards organized convection with sweeping low level hodographs yielding 0-1km streamwiseness values around or in excess of 90% and moderate streamwise vorticity values of around 0.25/s. Wind fields above about 2km take on a less supportive overall structure, but should be strong enough to keep these storms vented. Add in CAMs consistently depicting 0-1km SRH values in excess of 200 m2/s2, and it becomes pretty apparent that a leading threat may be tornadoes, in conjunction with large hail. Threat will be greatest with initial storms, which are expect in eastern NE/KS areas around 21z, where above mentioned environment will be most supportive. As activity translates eastward late afternoon into evening, guidance is consistent in the environment slowly degrading with less supportive mid-level lapse rates eroding SB/MUCAPE. Even so, effective inflow wind fields will remain supportive of rotation, so tornadic activity and some hail cannot be ruled out as storms approach I-35 corridor around/after 00z. These storms will be moving relatively fast with progs of 40 to 50 kts. All in all, SPC Enhanced area appears to be in a good spot, if not a hare too far east. Phew. That`s not all though with additional strong to severe storm potential expected into Saturday. Some of this will depend on where the surface boundary ends up laying across the state, with a SW to NE orientation expected as the upper level trough and surface low pass to the NW/N of the state early Saturday. In conjunction, another western trough will be inducing surface low development in the Colorado Front Range, and help initiate another round of convection over Central Plains. That activity may drift up into SW portions of the CWA in/around the warm front and would pose the greatest risk for traditional severe weather, with hail and tornadoes the primary risks with any supercellular/organized activity. Aside from that, additional activity in/along the surface front expected as LLJ further enhances Saturday night. Storm motion vectors roughly parallel with the front raise the eyebrow for potential hydrologic issues. Some synoptic and CAM guidance have pointed toward the potential for some areas picking up 3 to 5 inches or more. As hinted, location remains in flux with some uncertainty in final location of boundaries. SPC Day 3 Slight and WPC highlighting southern areas remain reasonably placed at this point in time. Another round of convection will move through Sunday as the second western trough and surface low eject and move northeastward across the Plains. Suffice it to say that additional severe weather potential exists with low tracks moving across western into central Iowa mid-day into the evening. Have not dove into substantial detail here with previously mentioned two rounds of greater focus and cascading effects from those rounds likely to affect Sunday potential to some degree. But again, do not sleep on Sunday potential, as the existing Day 4 SPC 15% is more than warranted, even with a quick casual look. Quieter to begin next week, with scattered/sporadic precipitation activity hinted late-end of next week. && .AVIATION /06Z TAFS THROUGH 06Z SATURDAY/... Issued at 1152 PM CDT Thu Apr 25 2024 Showers and thunderstorms will move into central Iowa over the next several hours and will impact most sites. Periodic chances for the showers and storms will exist through the period and highlighted times of greatest chances. Cigs will continue to lower and will become widespread IFR to local LIFR by mid Friday morning. Areas of fog are possible and may reduce vsbys at times. Breezy southeast winds are expected and will be gusty on Friday. A few periods of low level wind shear are also expected. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Curtis AVIATION...Donavon