Area forecast discussion National Weather Service Detroit/Pontiac Michigan 342 am EDT Tuesday may 21 2013 Short term...today and tonight Line of thunderstorms currently over northern Illinois is slowly but surely marching northeast during the early morning hours. The main question this morning is how well this activity will hold together as it tracks through Southeast Michigan. Hi-res model runs are in general agreement in bringing the system through between 09-13z. That is where the agreement ends as the hrrr lifts the best thunderstorm activity north through northern lower Michigan while the WRF maintains the line of strong thunderstorms as it tracks through Southeast Michigan...mainly north of the M-59 corridor. The evolution of the jet stream would validate the hrrr solution as it lifts the 100+ knot jet north over the northern Great Lakes...taking the upper level divergence afforded by the right entrance region of the jet with it. However lower levels lend credence to the WRF solution as the main shortwave driving the northern edge of the convection is prognosticated to go directly through Southeast Michigan with the nose of the 35 knot 850 mb low level jet pointed over Southeast Michigan. Despite the loss of upper level jet support...the lower level dynamics point to an active morning across Southeast Michigan with the line of convection expected to survive the trip and track through the area. This activity will still have quite a bit of instability to work with as MUCAPE values remain at 1000 to 1500 j/kg during the early morning hours. Severe weather threat from this mornings convection will mainly be a wind threat...as upstream observations show little threat from large hail...although small to near severe hail is a possibility. After this initial activity the picture becomes very muddled as the nwp models keep showers and thunderstorms going for much of the day. This scenario is certainly plausible as another jet streak lifts from the southwest and makes its way over the Great Lakes region. As a result the area will be in the path of several shortwaves rotating around the main middle level low pressure system taking up residence over the Missouri Valley. Without a key feature indicating there will be a substantial break from the action...do not plan on deviating much from the previous forecast high probability of precipitation through the daytime hours. Severe weather potential this afternoon will hinge on the ability to get a break in the convection. This window will not have to be large...as evidenced yesterday...with a brief break allowing temperatures and convective available potential energy to rise significantly in a short period of time. Models show MUCAPE values ranging from 500 to 1000 j/kg with a fairly steady stream of convection during the day. Afternoon sunshine could easily boost these values to around 1500 j/kg given the late may sun angle and the very warm air in the low levels. In addition to the instability...0-6 km bulk shear values are a good 30 to 40 knots today...meaning the potential for strong to severe thunderstorms producing damaging winds and large hail are a possibility. Closer inspection of the hodograph and low level cape show isolated tornadoes will also be a threat...especially if we can get some sunshine...as 0-3km cape values reach 150 to 200 j/kg and storm relative helicity values get to 150 to 200 m2/s2. All of this activity makes forecasting high temperatures today quite challenging. Even with all of the activity we should be able to get into the low 80s away from the Great Lakes...however a few middle 80s are a distinct possibility if the aforementioned break in the activity occurs. Overnight models are hinting at a break in the convection as the secondary jet stream moves off to the north with negative vorticity advection prognosticated at the 500 mb level as the surface low lifts over the Soo. Even with these factors there is abundant low level moisture across Southeast Michigan...so a completely dry forecast is not in the cards and neither is a continuation of the high probability of precipitation from today. With all these factors in mind will maintain Chancy probability of precipitation during the overnight hours. Abundant cloud cover and the region still in the warm sector will once again lead to a mild night with overnight lows expected in the 60s. && Long term...Wednesday through Monday Upper level troughing will keep mention of showers and thunderstorms around into Thursday which is when the main trough axis passes east of the area allowing heights to build. Meanwhile...complex surface low pressure system will work through lower Michigan on Wednesday and Thursday before a strong Canadian high pressure system drops into the Great Lakes and expands across much of the eastern Continental U.S. For the Holiday weekend. This will lead to a dry but cool weekend. First off we will address convective potential for Wednesday. The GFS poorly initializes the surface low over the northern plains so will lean more toward the European model (ecmwf)/NAM/Gem regional. The upper level low that has been sitting over the northern plains for a few days now will phase with a northern stream trough digging south through Manitoba/Ontario on Wednesday. This will lead to a more progressive system finally. With the closed nature of the upper low...a few surface lows have been rotating around each other the last two days. As the wave opens further and the upper jet turns west/east across northern Michigan...the main surface low will release and track across northern lower Michigan through the day Wednesday. This will be a broad weakening low but will present isentropic ascent and will keep the Theta-E ridge up into Southeast Michigan. Focus will be placed on a southern stream shortwave that will round the base of the trough and target lower Michigan Wednesday afternoon. Model consensus shows a surface low clipping extreme Southeast Michigan. This will open the door for additional heating and moisture into that area during peak afternoon heating with the eastward tracking upper level jet more squarely overhead. Convective parameters are on the decline from Monday and Tuesday but there will still be 500 to 1000 j/kg of tall skinny MUCAPE with a cold/cool front passing through the area in the evening. Overall Wednesday looks like it will be composed of widespread showers and thunderstorms with a chance of organized convection across the extreme southeast portion of the County Warning Area. Upper level trough axis will be over western lower Michigan Wednesday night providing broad area of lift aloft with the back end of the surface low still lingering. This will keep showers and thunderstorms over much of lower Michigan through the overnight. The axis will sweep through Southeast Michigan early on Thursday leading to a clearing trend in the showers through the day from northwest to southeast. Main baroclinic zone thats been lingering across the northern Great Lakes will surge south on Thursday with strong Canadian high pressure right behind it. With building heights aloft...clouds will clear out but that will not be realized in temperatures. 850 mb temperatures in the low single digits will only allow high temperatures to reach around 60f both Thursday and Friday with lows around 40. A slow warming trend through the weekend will only allow temperatures to rebound to near 70 for Monday and Tuesday. Though it will be cool...at least it looks to stay dry thought the Holiday weekend. && Marine... Modest southerly winds will remain in place today and 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 1151 PM EDT Monday may 20 2013 //discussion... The region of thunderstorms across Illinois will lift north-NE during the early morning hours. Additional convective development is possible out ahead of this cluster within a secondary surge in moisture...traversing Southeast Michigan in the 09z to 15z time frame. This will support the inclusion of some thunderstorms in the tafs around daybreak. The coverage of morning convective activity and extent of cloud cover which may linger into the afternoon raises considerable uncertainty with respect to the chances and timing of thunderstorms later in the day Tuesday. For dtw...given the eastward extent of convection ongoing across Illinois...chances of some thunderstorm activity in and around metropolitan appear high enough to add their inclusion in the tafs during the 11 to 14z time frame. Although timing of additional thunderstorms later in the day Tuesday remains highly uncertain...there is the potential for several rounds of thunderstorms to impact metropolitan on Tuesday. //Dtw threshold threats... * medium confidence in thunderstorms Tuesday morning and afternoon. && DTX watches/warnings/advisories... Michigan...none. Lake Huron...none. Lake St Clair...none. Michigan waters of Lake Erie...none. && $$ Short term...kurimski long term....drk marine.......kurimski aviation.....SC You can obtain your latest National Weather Service forecasts online at www.Weather.Gov/Detroit (all lower case).