DISCUSSION
Issued at 311 AM CDT Wed Sep 18 2024

Early morning upper level water vapor imagery shows three prominent areas of low pressure/shortwave troughs. The first is an upper low over the northern Rockies with a plume of moisture associated with convection to the east from the northern into the central Plains. The second is descending down the West Coast. Both of these are within the mean longwave trough over the western US. To the east, a low continues to spin over the Carolinas region, which will begin to drift to the northeast and move along the Eastern US Seaboard through week's end. It's slow departure will result in the a slow eastward progression of the western US trough as we move through the rest of the week into this weekend resulting in a transition to a more active weather pattern with rain and storm chances.

Filaments of high clouds from the convection associated with the first low are starting to move into western and central Iowa early this morning. This low will occlude as it moves over Montana and the stronger QG convergence will remain west of the state. This leads to the biggest forecast change in the near term - that is slowing down the arrival of rain chances while also lowering their chances today into early this evening. Despite clouds increasing today, highs will still reach the middle 80s, which is a touch lower than yesterday.
Breezy winds from the south and later southeast will prevail as well with seasonably high humidity. As this occluded low slowly drifts northeastward into southern Canada, its surface front along with the stronger low level QG convergence will move into Iowa later tonight into Thursday. This forcing and more so a strong low level jet upwards of 40 knots pointed into Iowa should help for scattered thunderstorms to develop, particularly toward sunrise Thursday. SPC maintains a marginal risk over northwest Iowa with instability and shear marginal in our service area keeping chances for severe weather minimal. These storms may fester through the morning as they move off to the east. The higher chance for more widespread storms and severe weather will be late Thursday afternoon into early Thursday evening along the cold front. The convergence along the front may be somewhat weak with a more parallel warm sector wind vector with a more perpendicular cold sector wind vector. Still, convergence will be relatively maximized along it and this should help storms to form. Entrainment CAPE values upwards of 2000 J/kg, deep layer shear around 30 knots, and favorable (e.g. 7-8C/km) low and mid-level lapse rates for parcel acceleration will result in a severe risk - namely damaging wind gusts and hail. Downdraft CAPE values are north of 1200 J/kg with forecast sounding at MCW showing plenty of dry air in the mid-levels and to a lesser degree sub- cloud. The lapse rates raise concerns for large hail, though the freezing level around 13kft is a bit high and given warm subcloud air, this may aid in some melting as the hail falls. While upper level winds may allow storms to remain somewhat separate, low level SRH values and streamwiseness are not impressive for tornadoes unless winds can locally back to increase the curvature. SPC continues and slightly expanded the slight risk over northern Iowa, which lines up with where the better parameter space for severe weather resides.

These storms will exit to the east toward or a little after midnight Thursday night/early Friday morning while the front will settle near the Iowa-Missouri border or over northern Missouri. Friday continues to look like a minimum in rain chances as large scale subsidence slides across the region. Meanwhile, the mean longwave trough with the new upper low at its base will begin to slowly progress eastward across the Southwestern US toward the central US. As our flow becomes southwesterly, this will funnel in higher than normal precipitable water values to the state later Saturday through Sunday night with values of 1.5 to 1.75 inches. This flow should also help to lift the front back into the state. Details regarding timing and location of rainfall do still remain uncertain and this results in widespread, prolonged rainfall chances in our forecast this weekend into early next week. Both the deterministic ECMWF and GFS bring a surface low somewhere over Iowa late Sunday or Monday and preceding this low's arrival, there will be shower and storm chances. It is interesting to note that the experimental AI models (e.g. AIFS, Graphcast) are focusing their rainfall either south of the state or over the southern half of Iowa.

While hard to pinpoint anything down with regards to any severe weather chances this weekend into early next week, locally heavy rainfall is possible given the favorable warm rainfall environment.
The 12z ensemble probabilities show up to around a 30% chance of 1 inch of rainfall in 24 hours on Sunday into Monday. Given our dry antecedent conditions with no more than a half inch of rainfall over the last 14 days per radar estimates and CoCoRaHS gauge data and NASA SPoRT upper level relative soil moisture no higher than 30%, much of this rainfall will be able to infiltrate outside of any hydrophobic areas or rates. Latest experimental HEFS (Hydrologic Ensemble Forecasting Service) data supports this with reasonable worst case output showing just a few gages in the Cedar Basin reaching action stage in the next 10 days.



AVIATION /06Z TAFS THROUGH 06Z THURSDAY/
Issued at 1036 PM CDT Tue Sep 17 2024

VFR conditions prevail through the current TAF period. A few showers and storms are possible over western and into north central Iowa tomorrow, mainly after 18z. Spatial coverage is expected to be limited, and the probability of categorical impacts at any terminal is low. Thus opted to leave any mention of shra/tsra out of the TAF at this time.

DMX WATCHES/WARNINGS/ADVISORIES
None.