Area forecast discussion National Weather Service Detroit/Pontiac Michigan 1149 PM EDT Sat may 18 2013 Aviation... //discussion... A region of very dry low level air circulating around the high pressure system to the east of the region has held surface dewpoints in the 40s across much of the area this evening. The low level flow will turn more toward the southeast during the course of the morning which will advect the middle to upper 50s dewpoints now over north central Ohio into Southeast Michigan. It now appears that the bulk of the surface moisture advection will occur after daybreak. This supports the removal of any mention of haze in the tafs. Otherwise...an upper level ridge will maintain benign aviation weather conditions through the afternoon. //Dtw threshold threats... * none && Previous discussion...issued 338 PM EDT Sat may 18 2013 Short term...this evening and tonight Downstream amplification of the eastern North America ridge will occur tonight as strong jet energy digs through The Four Corners. The main consequence of this during the short term is the apex of the ridge will sharpen and lift into James Bay guiding the consolidated jet core along with it. The net result is for the pesky altostratus that has been harbored within the shearing deformation axis over northern lower Michigan...Saginaw Bay...thumb to exit with time. Meanwhile...water vapor imagery shows smaller wavelength anticyclonic motion over portions of northeastern Illinois and Lake Michigan folding over. This highlights the increasing ridging aloft through tonight. Some modification to the air mass in combination with a trend very late for some increasingly southerly flow suggests a good 2 to 3 degrees warmer tonight. Lows should dip into the lower 50s north...middle 50s south. Long term...Sunday through next Saturday Upper ridge will hold over Michigan for one more day tomorrow. Subsidence under the ridge should prevent showers/thunderstorms from developing...although we will build up some instability during the day as the ground heats and lapse rates steepen. Southeast winds will reduce potential for low-level convergence near the lakeshores (no convective trigger)...and will also hold temperatures closer to the shorelines into the 70s. Low 80s are expected well inland as good insolation combines with rising temperatures at 925-850mb. Frontal boundary by this point will also be poised north of Lake Huron...having little impact on weather for Sunday but will become a factor in our weather through most of next week. Upper wave off the Pacific northwest coast will become absorbed in the broad upper trough now along and west of The Rockies. Models show this system closing off over the Central Plains on Monday...then very slowly working east into the Great Lakes/Ohio Valley by Thursday. Arrival of this system has been hard to predict as it will either cut-off from main flow or remain under very light steering flow. Models continue to display some timing differences with its entrance and exit into the area...in part due to timing/placement issues with another upper low dropping through eastern Canada. Overall confidence in the forecast Monday afternoon through Thursday is not high...other than to say rounds of showers and thunderstorms will likely roll through and affect everything from cloud cover to temperatures. Chances for precipitation will start to increase late Sunday night and Monday morning as a weak low-level jet noses towards Lake Michigan. Weak push of Theta-E advection coinciding with this could trigger a few showers/tstorms...and have maintained low probability of precipitation through the first part of the day on Monday. Models then in great agreement with clipping shortwave energy up through lower Michigan as it rides southwest flow preceding the large low pressure system. Model timing of this wave varies from late morning through early evening. Consensus does bring it through during the late afternoon...right at peak heating...and will continue to anticipate a fairly good coverage of showers/thunderstorms developing. Chances may be better further north over central Michigan...as the frontal boundary north of Lake Huron starts to drop southward. The front will then become quasi-stationary over lower Michigan (best support for either central or northern lower) Tuesday through early Wednesday...while additional upper shortwaves ejecting out of the low and mcvs from upstream convection ride over the front or within the warm sector and trigger additional rounds of convection. The front should lift back north late on Wednesday as the low approaches and crosses the region...before dropping back south as a strong cold front on Thursday. Temperatures will be very uncertain with rounds of showers/thunderstorms and cloud remnants from upstream convection crossing overhead. Assuming a pretty dry start to the day...temperatures on Monday should warm into the 80s. Remained a little conservative with forecast highs the low/middle 80s. If convection waits until very late afternoon or evening to fire...some locations could see upper 80s. Severe weather may become a concern Monday through Wednesday. Model sb cape values of 2500-3000 j/kg may be realized on Monday in a "fresh atmosphere" that has not been worked over...assuming no upstream remnants arrive early in the day. Stronger storms may also fire overnight and especially Tuesday as the front looks to sink slightly southward into the area...and upper jet support becomes more favorable. Model sb cape values continue to look conducive for strong/severe storms Wednesday as the low approaches and the front lifts back north. Shear values do not look overly impressive any of the days however. Locally heavy rainfall will also be a threat with precipitable water values running between 1.25-1.50+ inches during this entire timeframe. Quiet weather is then expected towards the end of the week as high pressure builds over the area...but temperatures will run much cooler with maximum temperatures only in the 60s. Marine... Easterly flow will continue through tonight as we remain under the influence of high pressure over Canada. The favorable fetch will allow waves to be slightly high over Western Lake Erie...but a decreasing trend will keep them below Small Craft Advisory criteria. Winds will veer to southerly by Sunday night before increasing next week as low pressure lifts out of the plains. This low will bring a chance of thunderstorms Monday through at least Wednesday and will be accompanied by a modest increase in southerly winds by Monday afternoon. A middle-week wind shift to northerly can then be expected as a cold front pushes across area waters. && DTX watches/warnings/advisories... Michigan...none. Lake Huron...none. Lake St Clair...none. Michigan waters of Lake Erie...none. && $$ Aviation.....SC short term...cumulonimbus long term....hlo marine.......cb You can obtain your latest National Weather Service forecasts online at www.Weather.Gov/Detroit (all lower case).