550 FXUS63 KIWX 131708 AFDIWX Area Forecast Discussion National Weather Service Northern Indiana 108 PM EDT Mon Sep 13 2021 .SYNOPSIS... Issued at 356 AM EDT Mon Sep 13 2021 Warm conditions will continue today as high temperatures range from the mid 80s to lower 90s. There is a low chance for a thunderstorm into this evening across southern Lower Michigan, but thunderstorms will become likely late Tuesday afternoon into Tuesday night as a cold front approaches. Some of these storms may be strong to severe with gusty winds and locally heavy rain. && .SHORT TERM...(Today and Tonight) Issued at 356 AM EDT Mon Sep 13 2021 Short term forecast highlights will continue to center on the continuation of above normal temperatures and the potential of isolated convection across mainly southern Lower Michigan today and tonight. Stalled sfc frontal boundary near MI/IN stateline will move very little today and may be focus for some additional low cloud development early this morning. These low clouds should mix out by mid morning. Already beginning to see some renewed convective development across southern Wisconsin in response to backing of low/mid level winds along this boundary across the MS River Valley downstream of next fast moving, low amplitude upper level short wave tracking across the Corn Belt. Based on orientation of 850-750 mb positive theta-e advection axis from northern Iowa to southwest Lower Michigan this morning, not completely out of the question an isolated shower could graze far southern Lower Michigan, but stubborn mid level inversion should keep best chance of realizing this elevated instability just north of the local area. For this afternoon, the low amplitude short wave will reach the western Great Lakes allowing better northward push of the low level warm front, with expectation that better convective coverage will remain across central Lower Michigan. Cap may be weak enough across far southern Lower Michigan for an isolated storm during peak heating, but confidence remains very low given weak capping likely persisting. If cap can be overcome, steep mid level lapse rates could allow for an isolated strong storm to develop. However, once again this potential appears maximized across central Lower Michigan. A south-north temperature gradient is expected once again today as near sfc warm front not likely to completely clear southern Lower Michigan counties until this evening. Have not made many changes to previous forecast for high temps today ranging from low-mid 80s far north to around 90 south. For later tonight, attention will turn to evolution of a more vigorous upper level short wave trough which is currently progressing through the northern Rockies this morning. A downstream 40+ knot southerly low level jet will lead to strengthening low level moisture convergence across the Upper MS Valley with main convective threat across these locations tonight. Track of this short wave and accompanying advective forcing will keep organized convective chances north of the area, and will just continue to carry slight chance PoPs southern Lower MI until near sfc front retreats well to the north late evening/overnight. && .LONG TERM...(Tuesday through Sunday) Issued at 356 AM EDT Mon Sep 13 2021 A broader, but disjointed eastern Pacific upper level trough will track across south central Canada/central Plains early Tuesday afternoon. with the mid/upper level trough axis approaching western Great Lakes early overnight Tuesday night. Positively tilted nature of this trough will increase frontal forcing Tuesday afternoon, with main source of uncertainty in location and timing of convective initiation being the influence of overnight upstream convection on frontal placement/timing. Good upper support should be in place by early Tuesday evening as frontal forcing allows for strengthening of broad right entrance region of a 110+ upper jet streak across the Great Lakes. With this positive tilt, better shear profiles could slightly lag better instability axis from central Illinois to southwest Lower Michigan late afternoon. Steeper mid level lapse rates on the order of 7 deg C/km could compensate for this slightly lagging shear for isolated-scattered severe threat by late afternoon/early evening. An initial gusty wind/isolated hail threat should transition to a strong/severe wind threat in the evening as large scale lift gradually weakens the elevated lapse rates. While deep layer shear values will be on the marginal side, somewhat impressive 15-20 knot 0-1km shear values are expected late afternoon/early evening. Currently not expecting appreciable tornado threat as this better low level shear should be in place at a time of maximum LCL heights. Most of forecast area has been placed in SPC Day 2 Slight Risk, with greatest potential looking to be in the 21Z- 04Z window. In addition to severe potential, unidirectional boundary parallel flow could result in some training Tuesday evening that could produce some locally heavy rain amounts. Cold front continues to slowly sag southeast into Wednesday as the upper level wave dampens across the eastern Great Lakes. Overall trend of past several cycles has been to a little more progression of the slow moving front, with perhaps a quicker reduction in PoPs from NW-SE on Wednesday. Temps should fall to more seasonable levels for Wed-Thu, with moderating trend back into the mid 80s for Friday into next weekend as large scale mid level height rises commence. && .AVIATION...(For the 18Z TAFS through 18Z Tuesday Afternoon) Issued at 100 PM EDT Mon Sep 13 2021 VFR conditions to prevail throughout the 18z TAF issuance. Models do hint at the potential for shallow ground fog around 11z to 13z Tuesday morning given increasing low level moisture. At this point have decided not to include in prevailing conditions given low confidence of occurrence. Gusty winds develop around midday Tuesday as stronger 850mb winds start to mix to the surface. Any convective concerns look to develop beyond this forecast window. && .IWX WATCHES/WARNINGS/ADVISORIES... IN...NONE. MI...NONE. OH...NONE. LM...Small Craft Advisory from 6 AM to 5 PM EDT Tuesday for LMZ043- 046. && $$ SYNOPSIS...Marsili/CM SHORT TERM...Marsili LONG TERM...Marsili AVIATION...CM Visit us at www.weather.gov/iwx Follow us on Facebook...Twitter...and YouTube at: www.facebook.com/NWSNorthernIndiana www.twitter.com/nwsiwx www.youtube.com/NWSNorthernIndiana