996 
NZUS93 KLMK 150804
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INZ076>079-083-084-089>092-KYZ023>043-045>049-053>057-061>067-070>078-081-
082-152004-

Public Information Statement
National Weather Service Louisville KY
404 AM EDT Mon Apr 15 2024

SynopsisA low-level frontal boundary will slowly sink southward across the Ohio Valley today as mid-level ridging moves in from the Central US. As a result, a warm and moist environment will exist for strong to marginally severe storms to proliferate along the stalling boundary. Then, mid-level shortwave/upper low progressing from the Desert Southwest to the Mid-Mississippi Valley will start to lift the front northward reducing the precipitation chances overnight.
Model Evaluation/ConfidenceOverall, there is high confidence in thunderstorm chances for late this afternoon and early evening based on the good consistency and model agreement from CAM guidance (HREF and RRFS). As for severe weather chances, the CSU ML algorithm has been highlighting a low chance of severe hail and wind threat around the position of the front with similar, if not lower, chances in the Nadocast ML output. Accordingly, SPC has included most of the forecast area in a Marginal risk of severe weather. 
TodayCurrent satellite imagery shows clear skies across the lower Ohio Valley with surface observations and RGB Airmass satellite product depicting the frontal boundary moving from northern to central Indiana. During the early morning hours, the front will keep moving closer to the Ohio Valley while quiet and mostly clear conditions continue in the forecast area. Rapid boundary layer warming due to unblocked solar radiation will foster cumulus development ahead of the front by midmorning with moderate surface-based instability quickly building amid an 850-700 capping inversion. Dry weather should continue during the first half of the afternoon as the surface front extends around the Ohio River and mid-level subsidence starts to develop a second inversion around 500-mb which will reduce a bit the ongoing impressive mid-level lapse rate. Even with a scattered cumulus cloud layer, highs might overachieve given SW low-level winds and positive upper height anomalies; therefore, raised afternoon temperatures by 2 or 3 degrees. 
Then, active weather will set in during the second half of the afternoon and early evening as convective temperatures are easily achieved and SW winds enhance moisture convergence along and ahead the frontal boundary. Other mechanisms for convective initiation might include outflow boundary interactions and differential heating boundaries amid a thick upper cloud layer moving in from Arkansas and Tennessee. Based on CAM modelling, storm initiation should easily break through the low-level capping inversion realizing MUCAPE around 2000 J/kg or higher. Forecast soundings indicate dry mid-level air entrainment, unusually steep mid-level lapse rates (above 8 degC/km), and enough effective bulk shear (above 25 knots) to highlight an isolated large hail threat, especially if splitting supercell develops based on the straight hodograph profile and marginally favorable upper-level winds. That being said, the main convective mode should be multicell clusters with sub-severe hail and strong wind gusts. It is important to mention that convective coverage is low confidence at the moment as the position of the front and resultant storm initiation varies across CAM models.  
TonightShowers and isolated sub-severe storms will continue for a few hours after sunset, but overall the marginal severe risk (intensity and coverage) will greatly diminish shortly after daytime heating is lost.  



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