Area forecast discussion National Weather Service Detroit/Pontiac Michigan 350 PM EDT Tuesday may 21 2013 Short term...through tonight Southeast Michigan remains firmly positioned within deep middle level southwest flow working around the eastern extension of the prevalent closed low still hovering over eastern South Dakota. Focus through the evening remains on convective potential and any corresponding severe risk. Defined low level instability gradient and better lapse rate environment that emerged earlier today under full sun modulating now with the progressive passage of a remnant mesoscale convective vortex. Recent area observations and Storm Prediction Center mesoanalysis show the effective downturn in the ambient instability (mlcape less 500 j/kg) prompted by some degree of boundary layer drying and slight warming through the middle levels. 18z DTX upper air sounding sampling the lead edge of this transitioning environment shows a fairly shallow unstable layer... marked by weak capping. The presence of just some agitated high based cumulus and scattered showers thus far suggests formidable middle level warmth does exist to hinder the potential for better updraft growth. Other area of focus exists within the combination of the enhanced low level convergence zone slipping off the Lee of Lake Michigan and the eastward propagating some degree of moisture pooling along this boundary is supporting convective development. There is at least a low probability for this activity to attain enough forward momentum to work into northern sections of the area yet today. All this points toward retaining just a low pop through the early evening period. Should a more robust updraft manage to work through the cap...an isolated severe wind/hail risk does exist through this evening given sufficient deep layer shear to retain a more organized structure. General subsidence in the wake of the mesoscale convective vortex will provide a period of dry and relatively stable conditions heading into tonight. However a renewed corridor of Theta-E advection working along the eastern edge of the upper jet axis over the Midwest will work overnight. This process may provide enough middle level destabilization to support scattered shower/thunderstorm development...especially if arriving attendant to a weak pv filament and/or remnant mesoscale convective vortex streaming out of the middle MS valley. Forecast lows for tonight that of general airmass persistence... readings again in the low to middle 60s. && Long term... A strong northern stream trough currently dropping south through Hudson Bay will lead to a pattern shift over the next couple of days...as the large upper level low centered over eastern South Dakota should be able to be picked upped and absorbed with the previously mentioned northern stream trough. Meanwhile...more of a short term pressing matter is the potent shortwave coming out of the Panhandle of Texas...which will be catapulted to the northeast and absorbed within the northern plains circulation. Good height falls spreading into the Great Lakes region tomorrow...with the maximum center tracking through Southern Lake Michigan northeast into Saginaw Bay...which should match up fairly well with the main surface low track. The 12z UKMET is the strongest/deepest with the surface wave...with the regional Gem a bit weaker...but still in the same Ball Park. The wind fields tomorrow will be conducive for severe weather (bowing line segments)...as 850 mb winds advertised around 40 knots...with even more impressive 55-60 knot 700 mb jet slicing through Southeast Michigan during peak heating. Low level wind shear (0-1 km) also looks pretty good with 25 knots to possibly as high as 30 knots...leading to a tornadic threat...along and south of M-59 which looks to be the prime location of the triple point. As usual...instability is always the deciding factor. Good isentropic ascent in the morning coupled with solid instability (showalter index of -2 to -4) will likely lead to showers and thunderstorms in the morning hours...which will tend to limit our diurnal instability. However...any breaks/lull in activity or advective process (surface dew points potentially climbing into upper 60s) as we head into the afternoon...could be sufficient to generate convective available potential energy on the order of 1000 j/kg or better...which will probably be the marker to produce severe weather with the previously described amount of wind shear. Also of note...is the majority of cape is in the low/middle levels (850-500 mb)...also a positive for severe weather and isolated tornado. Still...widespread convection through the day over Southeast Michigan or farther south over the Northern Ohio valley could limit the instability to the more manageable 500 j/kg or less...preventing organized severe storms/stronger winds from descending to ground. Upper level trough axis tracking into the central Great Lakes on Thursday...with another cold front swinging through early in the day should be should be sufficient to touch of showers with decent trailing 850-700 mb Theta-E ridge axis moving through and equally important strong and sharp dry push into northern lower Michigan by days end. Much colder air will be filtering in through the day...as 850 mb temperatures forecasted to fall to near zero toward 00z Friday...thus highs should not be much higher than the mornings low (upper 50s). Northwest upper level confluent flow then looks to be dominating our weather Friday and into the weekend...allowing a strong high for (late may standards) to build into the Great Lakes region...into Monday...per 12z Euro. Best radiators (thumb region...etc) stand good chance of falling into the 30s at night with frost possible. Assuming there is not a huge cumulus up during the daytime hours...would expected maxes to reach and exceed 60 degrees as we mix above 850 mb with late may isolation. && Marine... Modest southerly winds will remain in place through Wednesday as a low pressure system slowly lifts east out of the upper Midwest. A warm and unstable environment will bring a chance of thunderstorms during this time. Winds will shift to the north across all waters by Thursday morning...as the low and trailing cold front push off to the east. These conditions will push winds and waves close to small craft criteria on Thursday over Lake Huron. && Aviation...issued 117 PM EDT Tuesday may 21 2013 //discussion... A deep southwest flow will remain in place across the region through the taf period. While this will leave the local environment relatively moist and unstable...prospects of thunderstorm development remain quite ill-defined. Recent observational/ satellite trends and model guidance would suggest any potential development late this afternoon will tend to remain widely scattered in coverage. A deepening mixed layer will lead to some southwesterly wind gusts in the 20 knots range through peak heating. For dtw...a low probability does exist for thunderstorms to impact the terminal late this afternoon and evening. However the potential now appears too limited given the question of coverage to include a specific mention. //Dtw threshold threats... * low confidence in thunderstorm occurrance from middle afternoon into the evening. && DTX watches/warnings/advisories... Michigan...none. Lake Huron...none. Lake St Clair...none. Michigan waters of Lake Erie...none. && $$ Short term...mr long term....sf marine.......mr aviation.....Mr You can obtain your latest National Weather Service forecasts online at www.Weather.Gov/Detroit (all lower case).