Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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932 FXUS63 KDMX 041745 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 1245 PM CDT Tue Jun 4 2024 ...Updated for the 18z Aviation Discussion... .KEY MESSAGES... - Few early day shower or storm chances mainly south and east (<20-30%). - Scattered strong to severe storms and locally heavy rain remain possible this afternoon into evening as a front moves through the state. Main threats remain large hail and strong wind gusts, though localized flooding could develop. - Mainly dry with seasonal temperatures and lower humidities Wednesday into the weekend. Breezy conditions expected Wednesday & Thursday. && .DISCUSSION... Issued at 345 AM CDT Tue Jun 4 2024 Today: Upper-level trough that will help drive our convective potential later today is moving into the Northern Plains early this morning and is expected to amplify through the day today. An accompanying cold front will extend to the south of this system and looks to be nearing the Missouri River around 18Z (around midday to early afternoon). Ahead of this front, we will be in a weak warm air advection regime with a favorable stream of moisture that will help send dew points further into the 60s and send our MLCAPE values towards the 1000-2000 J/kg range this afternoon. As a result, it will certainly feel warm and humid today as highs reach well into the 80s. Ahead of the front and upper trough to the northwest, a MCV rotating over eastern Kansas is expected to lift through northern Missouri into the southeast to eastern half of Iowa and has the potential to bring some early day showers or storms. Despite having the moisture to work with, this MCV is fighting some larger scale subsidence so it remains unclear on the coverage of any early day activity, but general expectation given current trends and CAMs will be spotty showers or storms at best this morning southeast with a few models suggesting more widespread development over eastern Iowa early this afternoon which is generally not supported in forecast soundings in our eastern forecast area given the dry sub cloud layer. What is known about this MCV is it has brought cloud cover to roughly the southeast half of the state which adds some significant caveats to our severe threat later today with the approaching front. Many models suggest this cloud cover will stick around through much of central and eastern Iowa with any clearing generally over our far western CWA which could severely limit our instability and heating into this afternoon. With already marginal bulk shear values in place of 20 to maybe 30 knots at best, any lesser instability environment will continue to limit the extent and ceiling of our severe threat. Increasing convergence near the front should help kick off storms with any storms that fire ahead of the front likely to be slow- moving with mean wind speeds around 10 to 20 knots. These storms could certainly pose a heavy rain threat given their slow movement but could also produce marginally severe hail or gusty winds, especially when they collapse due to evaporative cooling and the generally expected pulse nature unless clearing occurs sooner and a better environment can be realized. Along to just behind the front is where the better shear parameter space exists which should help make storms more progressive helping to limit the hydro threat, but could still bring a threat for strong wind gusts depending on storm organization which as mentioned previously looks messy, and maintenance is certainly in question. All of this said, the overall environment looks only marginally supportive of a severe threat this afternoon with hail and wind in addition to heavy rain in any initial storms ahead of the front and potentially strong wind gusts with the main line, especially early, since storm intensity seems to wane with time as storms move eastward. Given the messy environment, low shear and helicity values the tornado threat looks very low today, but won`t go as far to completely rule a brief tornado in case a better environment can materialize if cloud cover moves out faster and in turn heating/instability values increase. See the hydro section for some additional information on the hydro threat today. Wednesday and Beyond: Any lingering storms push out of the area by late tonight/early Wednesday morning with a mostly dry forecast prevailing through the end of the work week. A few sprinkles may be seen north on Wednesday as a quick-moving shortwave moves to the north of the area in our otherwise northwest flow behind the departing trough/low to the east. This northwest flow looks to continue for the next several days which may bring some low end additional opportunities for a few sprinkles or showers at times but the overall forecast will feature highs in the 70s to low 80s with less humidity through the end of the week and weekend. An increasing pressure gradient behind the departing system will make for some breezy days on Wednesday and Thursday, especially north. && .AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/... Issued at 1243 PM CDT Tue Jun 4 2024 A few showers and local MVFR cigs are across central Iowa early this afternoon. A few thunderstorms may develop over the next few hours but the areal coverage will be low. A line of thunderstorms will enter western IA by mid to late afternoon and move east across the area and should impact most sites in front of a boundary, which will also bring a wind shift to more west/northwest. Gusty winds will be possible with the storms. VFR conditions will follow. Expect breezy to gusty southwest wind by late Wed morning. && .HYDROLOGY... Issued at 345 AM CDT Tue Jun 4 2024 As mentioned in the previous hydrologic discussion, main concern in terms of river flooding remains with the single flood warning at Estherville with RFC contingency forecasts and HEFS output mainly suggesting within bank rises on area rivers. Although heavy rain is possible today with pwat values of 1.5 to 1.75", warm cloud depths of 11-12kft, and 850-300 mb flow of 25-30 knots, the progressive nature of storms near to behind the front should help preclude any hydro issues unless storms move over the same exact location of any pre-frontal storms which are expected to be slower moving, as alluded to in the discussion. HREF 24-hr probability-matched mean QPF values are generally only as high as 1-2", with a few isolated areas in eastern Iowa with values of 3"+. Will certainly monitor the situation today, especially if any early day showers can manifest east, but overall weaker trends in system and less priming from the events earlier in the week should help mitigate. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...KCM AVIATION...Donavon HYDROLOGY...KCM