Area forecast discussion National Weather Service northern Indiana 814 PM EDT Monday Jun 17 2013 Synopsis... issued at 425 PM EDT Monday Jun 17 2013 A weak cold front will approach the region this evening and bring a chance for showers and thunderstorms overnight and again Tuesday evening. High pressure will then build over the Great Lakes by midweek with a return to dry conditions. && Update... issued at 812 PM EDT Monday Jun 17 2013 Updated mainly to freshen zone forecast product wording... but also bumped up probability of precipitation a bit northwestern areas this evening due to scattered showers over and approaching this area. As mentioned in aviation discussion... instability currently limited over our County Warning Area and Prospect for destabilization slim with mesoscale convective system over the lower Ohio Valley preventing much Theta-E advection ahead of the front... so thunderstorm coverage only expected to be isolated to scattered in our area. Steep low level lapse rates and fairly dry low levels will support gusty winds and possibly small hail if thunderstorms do occur through the evening before diurnal cooling stabilizes low levels. && Short term...(this evening through tuesday) issued at 350 PM EDT Monday Jun 17 2013 Perturbed west-northwest flow aloft will persist this period between an eastern Canada negative height anomaly and an intermountain west/Southern Plains upper ridge. A sheared pv filament embedded in this flow over Wisconsin will translate through the central Great Lakes this evening...forcing a backdoor cold front southeast through the area tonight. This cold front will get a little extra boost from Cool Lake Michigan waters...forcing it into far northwest zones by 00z. Isolated to scattered convection may fire by the way 23-02z along this boundary and then slowly work southeastward overnight. Kept coverage/probability of convection low (20-30 percent) with deeper moisture locked up south of the area and stronger support aloft passing north and east of the iwx County Warning Area. Moderate deep layer shear (40-45 knots) and weak instability (500-1000 j/kg of mlcape) may support an isolated wind/hail threat if convection fires early this evening across northern zones. Frontal slope will sink just south of the Highway 24 corridor by 12z Tuesday...and then clearing our southeastern zones by midday. Low level cool/dry advection will be more impressive over the northern half of the forecast area...especially downwind of Lake Michigan into Northwest Indiana and SW lower Michigan...with guidance backing off somewhat on this cool/dry push into southern areas. 12z guidance also came in a little stronger with a more pronounced pv anomaly (across the northern plains as of this writing) prognosticated to drop southeast through the Northern Ohio valley Tuesday afternoon. This upper level support per DCVA/7-3h q vector convergence forecasts...coupled with modest bl destabilization (maybe 1000-1500 j/kg of cape to work with) and a possible low level focus along a leftover inverted-like trough may combine to help generate scattered showers/storms Tuesday afternoon...best chances south of Highway 30 in Indiana. Bumped up probability of precipitation a bit but held shy of likely with deeper moisture and more unstable air suppressed south of the area along the primary baroclinic zone. There does appear to be a marginal hail/wind threat with sustainable updrafts given prognosticated weak to moderate instability and near 40 knots of effective deep shear. && Long term...(tuesday night through monday) issued at 350 PM EDT Monday Jun 17 2013 A weak upper level trough will be moving across the southern part of the area early in the period. NCEP models have been trending up the chances of precipitation over southeast areas as the upper level trough combines with a weak front and moves southeast. Added a chance for showers and thunder before midnight Tuesday night. Bulk shear and cape should not be sufficient for severe storms. This system should move out of the area fairly quickly...with dry weather and moderating temperatures through the end of the work week as an upper level ridge builds into the area. Have trended temperatures warmer over the last several days of the this period. CR allblend looks too cool and appears to have ingested some of the cooler model data. Overall...favor the GFS with an upper ridge extending into the forecast area over the weekend. 850 mb temperatures over 20c support highs around or topping 90 degrees. CR allblend has trended shower/storm chances lower and appears to be on track as middle level cap should become fairly substantial. Subsidence from the upper level ridge should also inhibit storm activity. && Aviation...(for the 00z tafs through 00z Tuesday evening) issued at 744 PM EDT Monday Jun 17 2013 Cold front dropping southward across southern Michigan/NE Illinois this evening with scattered rain showers/ts in vicinity of the front. Frontal passage expected at sbn by 05z and at FWA by 08z. Left ts out of tafs for tonight as instability over northern Indiana weak at this time and potential for destabilization limited with nightfall approaching and mesoscale convective system over lower Ohio Valley preventing much in the way of Theta-E advection in advance of the front. Instead... continued with vcsh with frontal passage at FWA and forecast tempo -shra this evening at sbn due to proximity of rain showers to the terminal at this time. Moistening of boundary layer behind the front could lead to some MVFR visibilities toward morning and perhaps some stratus closer to Lake Michigan at sbn. Although front will be south of the terminals on Tuesday with northeasterly surface flow... a fairly strong vorticity maximum evident in water vapor loop over ND this evening is expected to move into Indiana Tuesday afternoon. This could trigger some ts over the area Tuesday afternoon as it has upstream this afternoon. && Iwx watches/warnings/advisories... in...beach hazards statement from Tuesday afternoon through late Tuesday night for inz003. Michigan...Beach hazards statement from Tuesday afternoon through late Tuesday night for miz077. Ohio...none. Lm...none. && $$ Update...jt synopsis...ng/kg short term...steinwedel long term...skipper aviation...jt Visit US at www.Weather.Gov/iwx (all lower case) Follow US on facebook...twitter...and youtube at: www.Facebook.Com/US.Nationalweatherservice.Northernin.Gov www.Twitter.Com/nwsiwx www.Youtube.Com/nwsnorthernindiana