The NWS forecast discussion:
.SHORT TERM /Tonight through Monday Night/... Issued at 347 AM EDT Sat Nov 4 2017 Primary focus for the short term remains squarely on the threat for severe weather and locally heavy rainfall Sunday afternoon and evening as low pressure and a strong cold front swing through the region. Thick low level clouds will likely persist across the forecast area tonight as model soundings maintain a subtle yet shallow inversion. Convective potential increases once again after midnight in response to a low level jet surge spreading over the area and a subtle surface wave tracking from central Illinois northeast into north central Indiana in the vicinity of the warm front. Model soundings show a developing area of weak elevated instability accompanying the surface feature...primarily focused over northwest portions of the forecast area. CAMs beginning to capture the idea of convection increasing in coverage overnight with the potential for a few lightning strikes. Just looking at the thermodynamic setup...a few of the stronger storms could throw out small hail as well. Will raise pops to likely over essentially the northwest half of the forecast area from 06-12Z with scattered pops elsewhere. Showers and storms may linger past daybreak Sunday over northern portions of the area as continued low level forcing and broad ascent support potential for additional convection. The focus will shift to the potential for severe storms to impact central Indiana...primarily during the afternoon and evening as low pressure rides along the frontal boundary from the central Plains into northern Indiana Sunday night. Enhanced low level flow will enable a progressively deeper layer of moisture and a broad elevated mixed layer to advect into the Ohio Valley through the day. Despite cloudy skies...the airmass should have no problem becoming modestly unstable over most of the forecast area...with values peaking between 1000-2000 j/kg by late day. Dynamics aloft remain very good...with diffluence noted aloft over the region and a dual upper jet structure that would support enhanced convective potential focused over northern portions of Indiana and Illinois. BL shear and storm relative helicity values are strongly supportive of the potential for severe storms as well. The primary tornado threat will likely align during the afternoon and early evening in the vicinity of the warm front and immediately ahead of the surface wave track. Confidence continues to increase in the warm front setting up north of the forecast area over northern Indiana. The greatest threat initially as convection develops during the afternoon and migrates into the northern Wabash Valley may end up being large and perhaps significant hail as mid level lapse rates of 7-8C and higher are present over the region. Wet bulb zero values between 8 and 9kft and the potential for nearly 10kft of the mid levels within the hail growth zone both support the large hail concern during the afternoon and early evening. While supercellular structures are likely initially...support pointing towards convection eventually aligning into a QLCS by early evening with the primary severe threat shifting from large hail to damaging winds as convection moves through the forecast area. Isolated tornadoes remain a distinct possibility over central Indiana into Sunday evening...especially considering the potential for quick spinners to develop along the QLCS. That being said...the continued presence of a subtle veer-back-veer wind profile and the lack of an appreciable backing flow in the near surface layer both suggest the tornado threat will be mitigated to some degree. To recap...all modes of severe weather are on the table...but the primary threat over central Indiana may be initially large hail transitioning to damaging winds as the QLCS progresses across the forecast area through the evening. An increase in the low level flow with 850mb winds peaking at 50-55kts Sunday evening will maintain convection into the night with intensity only decreasing slowly as instability is gradually lost. The presence of the stronger low level jet support the potential for heavy rainfall and localized flooding Sunday night with max precip water values near the climatological peak for early November. Rain and lingering storms will be out of the forecast area prior to daybreak Monday with the cold front settling over the Tennessee and lower Ohio Valleys. A surface wave will drift into the Ohio Valley along the boundary Monday night...providing a renewed threat for showers focused over the southern half of the forecast area. Temps...even with no sunshine expected Sunday...it will be unseasonably warm as temperatures should warm into the lower and mid 70s with dewpoints into the 60s providing a distinctly muggy feel to the air. Much cooler air will spread across the region in the wake of the cold frontal passage with highs in the 50s. Lows tonight will remain quite mild with the southerly flow in the warm sector...but expect a return to lows primarily in the 40s by Monday night. Overall...a general model blend matched up with low level thermals nicely. &&
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