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FXUS63 KDMX 022332 AFDDMX

Area Forecast Discussion National Weather Service Des Moines IA 632 PM CDT Sun Jun 2 2024


Updated for the 00z Aviation Discussion

KEY MESSAGES

- Thunderstorm chances through Tuesday. Confidence in details and severe potential is low however, especially tonight through Monday night.

- Little or no precipitation Wed-Sun with seasonal temperatures and lower humidities

DISCUSSION
Issued at 241 PM CDT Sun Jun 2 2024

Our pattern will undergo a fairly significant change through the period transitioning from one that remains active with lower amplitude flow, to one less progressive with much higher amplitude as a large eastern CONUS closed low and corresponding western ridge keep a drier regime in place for at least several days. The current scenario at mid afternoon is largely inactive, but recent satellite imagery continues to suggest some large scale ascent is still present from lingering synoptic short waves and remnant MCVs, one of which is vaguely noted over the DSM metro. Several CAMs and thus the 12z HRRR are fairly emphatic that peak heating development will occur soon across the NW Iowa and the Siouxland area, although there doesn't appear to be much of a focus for initiation beyond the aforementioned mid level triggers from overnight convection.
There is little noted with regard to surface features, and recent HRRR runs have been more tranquil in those areas, so confidence in that occurrence is diminishing, as is confidence overall. Several missing RAOBs upstream across central and southern Plains were missing model ingest with 12z launches omitted (LBF/TOP/DDC/AMA/DNR). Recent RAP/HRRR runs have placed less emphasis on the NE/KS wind potential, and its remnants into IA overnight, and more emphasis into a MN MCS, which may fit trends as RAP 0-2km moisture convergence trends shifts from the Siouxland area northward. Convective trends still remain uncertain though with lingering larger scale vertical motion still drifting through the MO Valley in a low CINH ~1000 J/kg MLCAPE regime across IA, and effective shear +/- 30kts. Thus the HRRR members increasing convection central/NW later this afternoon and evening can’t be discounted, with broad-brush chance PoPs until increasing overnight when upstream convection may still reach the state, or drop southward from MN.

Moving into tomorrow, behind whatever occurs overnight, there isn't much in the way of low level focus or baroclinicity with Iowa in the generalized warm sector. Some chances will still linger Monday and Monday night with weak waves continuing to traverse the developing southwest flow aloft ahead of the maturing northern Plains upper level trough. This changes by Tuesday however with much higher confidence in evolution, as the model signal has been fairly consistent over the past few days with phased large scale support and low level convergence along the associated surface front later in the day. Although recent GFS and EC deterministic runs have depicted weak 0-6km shear ahead of the boundary, seasonal instability of 1000-2000 J/kg MLCAPEs should be in place still suggesting at least some severe potential with a disorganized wind/microburst environment the most likely concern. Recent GEFS/EC ensemble specific humidity and precipitable water values are anomalously high for early June too suggesting some heavy rain potential. Corresponding recent deterministic runs also continue to reflect healthy 1-2" amounts in or near Iowa with the system passage.

A pattern change can then be anticipated for the remainder of the week and through next weekend. Persistent northwest flow aloft will result in little if any precipitation chances with minimal moisture or forcing. A few weak waves traversing the flow could result in brief and infrequent light precip through the Upper MS Valley at times, and the baroclinic zone through the Plains may be active at times, but generally removed from IA with little anticipated in between. Although temperatures should remain at seasonal levels, northwest flow both aloft and at low levels should keep dewpoints seasonally low with readings no better than the 50s as the stagnant pattern keeps surface high pressure no further east than the MO Valley.

AVIATION /00Z TAFS THROUGH 00Z MONDAY/
Issued at 632 PM CDT Sun Jun 2 2024

VFR conditions will prevail for much of the 00Z TAF period, with two exceptions. First, a period of scattered to numerous thunderstorms is forecast overnight into early Monday morning, moving eastward primarily between about 06Z and 12Z at the terminals. Intermittent MVFR or lower conditions are expected in and around the thunderstorms, however, confidence in timing and coverage is too low to justify TEMPO or prevailing groups at this time and have handled with VCTS for now, expecting refinement of details in later updates. Also, there may be a period of prevailing MVFR ceilings around sunrise Monday, mainly around MCW and have included those in the outgoing TAFs.

DMX WATCHES/WARNINGS/ADVISORIES
None.

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