Area forecast discussion National Weather Service Detroit/Pontiac Michigan 714 PM EDT sun may 19 2013 Aviation... //discussion... Ridging at all levels will maintain mainly VFR conditions, clear skies, and light southeast winds through Monday morning. However, increasing low-level moisture will pose a risk for fog development after about 07z tonight...particularly for the Detroit area including kdtw/kyip/kdet where there will be a contribution of cool & moist low-level air from Lake Erie. Should fog form, visibilities in the MVFR range can be expected along with the lake contribution delaying burn-off until at least 12z Monday morning. By late morning...diurnal mixing will allow southeast winds to gust up to 20 miles per hour as a scattered cumulus field develops around 4kft...becoming broken around 4-5kft by late afternoon as showers and thunderstorms push into the area. Confidence in showers and thunderstorms affecting the area after 21z remains rather high, but confidence remains low with regard to the potential impact for any one location. Will therefore only introduce showers to the forecast for Monday afternoon, except for the addition of thunder to kmbs where the probability is highest. //Dtw threshold threats... * medium confidence in ceilings below 5kft after 21z Monday * low confidence that a thunderstorm will impact kdtw airspace && Previous discussion...issued 407 PM EDT sun may 19 2013 Short term...tonight Upper level ridge axis will slide over lower Michigan this evening/tonight providing an expected dry night. There is a warm front which lifts north...but much better defined to our west. A weakening surge of 925 mb Theta-E arrives toward 12z Monday...with computed lifted indices of -7 c...but that layer looks to remain unsaturated. If activity does materialize...it would more than likely be after 12z anyway. With the increase in low level moisture...already reflected in this afternoon dew points pushing into the lower 60s (south)...comes the potential for fog...especially with cooling influence of Lake Erie. On the flip side...Southeast Michigan has had an extended period of dry weather...and not fully convinced we will see much more than the Standard 3-5 sm fog/haze...as mins expected to only drop back into the upper 50s to lower 60s. Long term...Monday through Saturday A broad upper level trough that initially sets up across the northern plains Monday morning will persist across the northern plains/upper Midwest through midday Thursday. The models have come into an agreement that despite 500mb heights closing off within the center of the trough...behavior of the system will not act like a true cutoff low. Instead...various shortwaves will rotate around the diffuse low allowing for varying bouts of baroclinic like activity. Southeastern Michigan will remain on the periphery of the low...tucked within a burgeoning warm sector. With episodic moisture transport expectations are for an active work week period with a low predictability regarding specifics. Monday...longwave ridge axis is expected to hold across the area throughout much of the day which supports another day of predominately quiet weather. First thing was to incrementally increase high temperatures. A strong Ava signal remains west of the County Warning Area through 18z...before a shearing deformation axis slams into the area between 18-21z. Magnitude of upward vertical velocity forcing will not be that high from the shearing deformation itself...however...the feature should serve as a focus mechanism for a secondary Theta-E surge or active warm front. Difficult this far out to pin down timing and trajectory of inbound shortwave energy (which will emerge out of Oklahoma convection tonight). There is enough model guidance suggesting the shortwave energy may initiate showers and thunderstorms across western lower Michigan before setting a course for the Saginaw Valley between 21-00z. The Theta-E surge behind this warm front should be significant...surging above 1.30 inches by Monday evening. Typical surface moisture biases exist in the model soundings but steepness of 700-500mb lapse rates suggests for potential of MLCAPES of 2000 j/kg. 0-6km bulk shear of 30-40 knots will support mesocyclone and supercellular behavior...good for potential of damaging wind gusts and large hail. Latest swody2 expanded the slight risk into much of southeastern Michigan. Exit region dynamics...responsible for the secondary warm front and moisture surge late Monday...will lift northward causing a wave break and energy to spill over the ridge axis on Tuesday. This dampening is important as it should allow precipitation activity to sink into the ridge a little more and cause activity to focus over the area. Tried to offer some timing resolution...but the whole Monday night through 00z Wednesday timeframe will require a high likely pop. Confidence is high that semich will reside in the warm sector which requires a forecast of low to middle 80s. Understand some hesitation there...with overall clouds limiting insolation. However...given late may any breaks will cause temperatures to soar. The potential will continue for strong to severe storms on Tuesday...but guidance is too muddied at this time to offer much insight. Wednesday...last of the upper level jet energy should lift out of the trough and cause remnant midlevel low circulation to become absorbed in westerlies to the north. The surface reflection is expected to meander over the state in time for the afternoon. We will need to monitor this outcome as it could provide some shear focus to augment the environment for strong to severe thunderstorms. However...at this time thermal forecasts suggest that instability will less than both Monday and Tuesday. There is a strong signal the upcoming Holiday weekend will be one dominated by a stable Canadian anticyclone. What has been typical for this Spring...quiet but very cool conditions for Memorial Day weekend. Marine... Weak southeasterly flow over the lakes will provide favorable marine conditions through tonight. A low pressure system over the Central Plains will drift toward the Great Lakes laying out a warm front across central Michigan. This front will stall over the area through midweek as the low tracks along it. Winds south of the front will remain southerly while winds to the north will back to the east and increase slightly. This will lead to higher waves over Northern Lake Huron while the southern Basin...Lake St Clair and Lake Erie keep waves generally under 2 feet. This low will bring scattered thunderstorms Monday through Thursday. Winds will shift to the north across all waters by Thursday morning...as the low and trailing cold front push off to the east. && DTX watches/warnings/advisories... Michigan...none. Lake Huron...none. Lake St Clair...none. Michigan waters of Lake Erie...none. && $$ Aviation.....Jvc short term...sf long term....cb marine.......sf You can obtain your latest National Weather Service forecasts online at www.Weather.Gov/Detroit (all lower case).