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Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
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331 FXUS63 KDMX 031923 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 223 PM CDT Wed Jul 3 2024 .KEY MESSAGES... - Scattered thunderstorms expected during the day on Thursday, with some potential for severe weather and locally heavy rainfall. - Rain chances are lower Thursday evening, but a few isolated storms will still be possible around sunset and into the early nighttime hours. - Cooler on Friday, with highs only in the 70s and scattered showers and storms north and east. Rain and thunderstorm chances then return periodically from late Saturday into next week. && .DISCUSSION... Issued at 223 PM CDT Wed Jul 3 2024 A surface high pressure area is currently sliding across the region, resulting in relatively light winds and only high clouds streaming across Iowa. Meanwhile, the frontal boundary that crossed Iowa and brought storms to the area yesterday is now stalled across central Missouri and is triggering additional rainfall there. Tonight, a 500 mb trough will swing from eastern Montana down into the Dakotas and Nebraska, causing steering flow to turn toward the southwest over Iowa and resulting in modest warm air/moisture advection aloft, and also push the old frontal boundary up into northern Missouri by sunrise Thursday. The aforementioned trough will push fairly quickly eastward across Iowa during the day on Thursday, with a ribbon of enhanced moisture and instability streaming up just ahead of the boundary. A corridor of MLCAPE in excess of 1000 J/KG is predicted by most high-res models, which combined with the broad forcing associated with the mid-level trough will be more than sufficient to trigger convection as the boundary moves through. Timing is a bit difficult to pin down, but the most likely time for frontal passage/storms will be from around midday in our far western counties to mid/late afternoon in our far eastern counties. In terms of severe weather potential, deep-layer bulk shear of around 40-50 KT definitely supports some threat of organized updrafts and large hail, however low-level helicity and lapse rates are somewhat less conducive to a wind or tornado threat. Still, if gaps in cloud cover permit sufficient insolation to strengthen near-surface instability in the vicinity of the boundary, then all modes of severe weather could be possible in a brief window of opportunity. Overall, however, the severe weather threat appears fairly marginal and is well covered by the current SPC outlook. PWATs of 1.25-1.50 inches along the boundary will also support a transient heavy rain threat, but with the storm coverage expected to be scattered and the system progressive in its eastward movement, the heavy rain threat is marginal as discussed in the hydrology section below. Once the primary trough and line of storms clears out to the east by late in the day Thursday precipitation chances will diminish, but lingering instability behind the boundary combined with a subtle secondary impulse aloft may lead to redevelopment of isolated storms during the evening. Thus for Independence Day festivities around sunset the overall odds of conducive weather are favorable, but there is still a low threat of rain that will need to be monitored. By Thursday night the 500 mb trough will have closed off as a low over Minnesota, which will then sink slowly southeastward into Wisconsin by Friday. A developing precipitation shield around this low may spread another round of rain/storms into our northern and northeastern counties during this time, but severe weather and heavy rain are not expected given the synoptic scenario and environmental parameters. Any such lingering rain should clear out of our northeastern counties by late Friday, but the modest northwesterly surface flow and cloud shield around the low will keep temperatures unseasonably cool, with Friday afternoon highs only in the 70s across the service area. A weak ridge will then move by to the south providing a break Friday night, but then a mid- level shortwave will approach Saturday and Saturday night, followed by a deeper layer trough building down across the north central and northeast U.S. from Sunday into early next week. This will lead to persistent rain and thunderstorm chances during the extended forecast period, beginning late Saturday and continuing at intervals through the remainder of the forecast. Any details of severe weather potential during this time remain unclear, but the overall pattern supports a generally low threat level. Still, this and any heavy rain potential will be monitored closely in the coming days. && .AVIATION /18Z TAFS THROUGH 18Z THURSDAY/... Issued at 1211 PM CDT Wed Jul 3 2024 VFR conditions will prevail through the TAF period. Only high clouds are forecast through tonight, then later on Thursday morning scattered showers and thunderstorms will move into the area. Some terminals may be periodically affected right around or shortly after 18Z Thursday, but will account for that in subsequent TAF issuances. && .HYDROLOGY... Issued at 332 AM CDT Wed Jul 3 2024 Most rivers across northern Iowa continue to slowly fall today, but with a minor secondary crest forecast in parts of the Cedar River basin in the coming days. Meanwhile the water is flowing southward and Saylorville Lake continues to rise, but the forecast crest has eased just a bit to around 878 feet. Fortunately we have had a break from rain today, but additional storm chances in the coming days mean continued vigilance against renewed flooding. The next round of showers and thunderstorms is expected during the day Thursday and possibly into the evening, however those appear more progressive and thus with a lower probability for heavy rainfall accumulations. In addition, the storms should be more scattered and not concentrated along any one river basin. All in all, concerns for additional/renewed flooding remain relatively low through the end of the week. && .DMX WATCHES/WARNINGS/ADVISORIES...None. && $$ DISCUSSION...Lee AVIATION...Lee HYDROLOGY...Lee