Very scientifically interesting AFD from IWX this morning on the lake effect potential.
DISCUSSION... Issued at 440 AM EST Mon Dec 2 2024
Overall no major changes to the forecast thinking from yesterday. Highest confidence in 8+" remains over the eastern half of Berrien County into NW Cass. Lower confidence in warning-criteria totals for St. Joseph IN but latest hi-res suggests far northern portions of the county will get clipped with heavier rates later today. Did delay the start time for areas outside of Berrien/Cass though. Went with advisory for northern La Porte, Elkhart, and St. Joseph (MI) with the lowest confidence on the eastern side of St. Joseph (MI) and the western side of La Porte. No headlines to the south but may have to watch northern Marshall and northern Kosciusko. We will continue to fine-tune the location of the lake effect snow bands through today and some changes to headlines may still be possible.
Details below...
Morning water vapor shows robust shortwave currently diving into the Upper Midwest. Low level wind profiles will continue to slowly veer through tonight as this midlevel trough axis passes and low level lake aggregate troughing becomes better established. LES which is currently hampered by WNW fetch, dry air entrainment, low inversion heights, and overall meager instability will gradually intensify and organize through tonight. Equilibrium levels still expected to climb above 10 kft by this afternoon per latest hi-res forecast soundings. 0-2km delta theta-e values also drop below zero by this afternoon and strong ascent/supersaturation in the DGZ will promote large flakes and high (potentially over 20:1) SLR`s. Of greatest concern though is the increasing potential for meso-low formation over SE Lake MI and into Berrien County this afternoon as suggested by the latest runs of the HRRR and RAP. Already starting to see some hints of this in latest GRR radar imagery given overall weak synoptic flow becoming increasingly convergent over the eastern lake thanks to very warm lake waters/induced troughing. If this verifies...much higher snowfall totals are likely in the 18-00Z time window. Models have been mixed on whether this feature drifts far enough south to impact South Bend before dissipating but latest runs seem to be trending further south and this was the primary reason for going with a warning in St. Joseph (IN).
There may be a brief lull in activity during the evening hours before the low level flow veers further and briefly connects with Lake Superior during the late overnight hours. Even here some models are hinting at a "type-VI" (dominant single band with a meso-low on the southern end) moving down the lake in the 06-12Z period. Instability parameters remain highly favorable but final lake effect band intensity and snowfall amounts will be heavily driven by the degree to which this band can become organized. The biggest negative factor for this event continues to be the highly variable low level wind directions. Stronger synoptic flow is anticipated tonight with only a brief window of favorable down-axis fetch/Superior connection before flow quickly backs once again Tue AM. Significant questions remain if this will be enough time to allow a single band to fully develop, or if (as hi-res models suggest) it is more of a broken/transient hybrid with pockets of heavier snow and possible small-scale circulations that never fully organize into a single continuous band. Either way the chances for heavy snowfall rates to remain stationary in one particular area for a long time remain low and warrant a cautious approach to overnight snowfall totals for now. Additional changes are quite possible as we progress into tonight though.
Continued to make very few changes to the NBM-initialized long term grids. LES will end quickly late Tue AM as flow backs SW and dry conditions will persist trough most of Wed. Highs even climb above freezing on Wed with some brief but modest WAA. This will be short- lived though as another deep upper low dives into the Great Lakes Wed night and sends a strong cold front through the area. Moisture availability is not ideal but expect some SCT rain showers along the front followed but blossoming lake effect snow behind the front. Still plenty of time for adjustments but currently expect strong NW flow will bring a cold (but dry) airmass with limited fetch for significant snow accumulations. LES ends by Fri with a longer period of light WAA expected over the weekend into early next week that will return temps to seasonal norms.
