Scientific Forecaster Discussion

000 fxus63 kdtx 251951 afddtx Area forecast discussion National Weather Service Detroit/Pontiac Michigan 351 PM EDT Thu Apr 25 2019 Discussion... Near term / rest of today through tonight As of 350 PM EDT...baroclinic Leaf structure depicted in water vapor imagery associated with a low pressure system across the Ohio/Tennessee valleys continues to expand northward across lower Michigan this afternoon. This feature has been apparent for much of the day as abundant cloud cover across much of the region, with the exception being a sharp gradient north of the I-69 corridor allowing for mostly sunny conditions to largely prevail for the bulk of the day. Despite the cloud cover, temperatures have still been able to warm to around 60, with mid 60s readings where sunshine has been most prevalent. Warm front associated with aforementioned low pressure will continue lifting northward into the Northern Ohio valley before being shunted off to the southeast as the surface low center tracks northeast across west-Central Lake Erie. The low will deepen rather aggressively through tonight from around 1003 hpa to the lower 990s hpa by 12z Friday. This deepening will be in response to phasing of its associated southern stream wave with northern stream energy racing southeast from the upper Midwest. Midlevel height field will take on a neutral to slightly negative-tilt as a result, allowing for enhanced dynamic lift from a combination of initial isentropic lift along the nose of the warm conveyer belt followed by deformation forcing. Dynamic support will also be enhanced by the favorable juxtaposition of a coupled upper jet and left-exit region of the low level jet. Fairly widespread rain will develop from south to north as a result, and with the low center tracking across west-Central Lake Erie, the greatest coverage will be focused primarily south and east of a Howell/Lapeer/Sandusky line. Pwats will increase to just over an inch helping to allow for embedded moderate/heavier rain segments. Cannot totally rule out a rumble of thunder as elevated instability will be limited, but nevertheless efficient, convective rain rates should be realized especially across the Detroit Metro and down into Monroe County. Latest hi-res guidance suggests that the heaviest rainfall totals look to occur just east of the forecast area, but amounts in excess of an inch will still be possible for far southeastern areas, and for more on potential amounts and possible flooding impacts, refer to the hydrology section below. Strengthening north/northwest flow around the backside of the low will result in a breezy night and along with abundant cloud cover/rain will help keep low temperatures in the mid/upper 40s. Short term / Friday through Saturday night The low will deepen further into the mid 980s hpa Friday, allowing for a tight pressure gradient to develop across the region with prevailing northwest flow. Winds will quickly ramp up Friday morning, with gusts around 35 mph possible for the bulk of the day. As rain exits from west to east as the low pressure system departs towards Quebec, sharp height rises aloft and increasing subsidence/dry air will allow for skies to clear out quickly. The increased insolation coupled with the tight pressure gradient will allow for efficient mixing and rapid boundary layer growth to 5-6 kft, helping to keep the gusty conditions persist well through the afternoon. If cloud cover can disperse earlier in the morning, would not be surprised to see a few gusts closer to 40 mph. Despite strong cold air advection in the wake of the low characterized by 850 hpa temps falling from 7-9 c to the negative single digits by Friday night, the deepening boundary layer will still enable high temperatures to reach around 60 for most areas. Mainly clear skies continue Friday night with ridging aloft passing overhead. Winds will only slowly weaken through the night helping to keep low temperatures from tanking, but the influx of low-level cold air will still result in low temperatures plummeting into the 30s. Attention then shifts to late Saturday into Saturday night, as a vigorous and compact northern stream wave dives southeast across the northern plains and upper Midwest towards lower Michigan. This feature will be accompanied by a clipper-type surface low and a bowling Ball of pv energy that will help enhance lift across lower Michigan Saturday evening through Saturday night. After high temperatures only reaching into the lower 50s with the thermal trough passing overhead, temperatures will steadily fall again into the 30s under brisk north to northeast wind and 850 hpa temps dipping below 0 c as far south as the i94 corridor. Cooling will also be enhanced by diabatic effects as the wave tracks across lower Michigan. Model guidance continues to be in very solid agreement with this system, with the track in vicinity of the Michigan/Indiana/Ohio border. Impressive trowal feature wrapping around a closed 850 hpa low will enhance precipitation across Southeast Michigan as initial rain transitions to a wet snow/rain mix. Still some uncertainty as to the degree of boundary layer cooling, with best potential for an almost full transition to wet snow north of the I-96/696 corridor. Despite favorable nighttime passage of the wave, accumulation efficiency will probably be difficult with likely above freezing skin temperatures and snow ratios struggling to reach past 8:1 in system-relative moist airmass. Still, around a half of an inch of quantitative precipitation forecast with the low snow ratios and at least a tenth lost to initial rain should yield 1-3 inches of accumulation, primarily on grassy surfaces. If higher snow rates can be achieved (bulk of the snow falls within a 6hr period), some slushy Road accumulation is also possible, along with isolated totals closer to 4 inches. These totals will primarily be focused between the I-96/696 and I-69 corridors. Less than an inch will be possible north to the M-46 corridor and south to the I-94 corridor. Long term / Sunday through Wednesday By Sunday morning, the system will be pushing away from the area bringing drier conditions to the area by the afternoon. A tightening pressure gradient on Sunday will bring gusty winds during the day before settling down later in the day. After a brief dry period going into Monday, conditions look to become more active through the week. There looks to be chances for precip during the week as a few waves move across the area. Temperatures look to be on the cooler side Sunday night and Monday before warming back into the 50s for highs through the week. && Marine... Light and variable flow late today will become north northeast overnight as strong low pressure lifts to near Lake Erie. Rain will spread north into the area with this low, reaching Southern Lake Huron overnight into Friday morning. Winds will increase late tonight into Friday and back to the northwest as a cold front surges through the area in the wake of this passing low pressure. Small craft advisories will go into effect for Western Lake Erie/Lake St Clair/Lake Huron nearshore waters Friday morning as wind gusts reach 25 knots with a Gale Warning for the open waters of Northern Lake Huron by Friday afternoon expanding south portions of the lake by evening as cold northwest flow expands across the area. A few nearshore zones may very well need to be upgraded to a Gale Warning by Friday evening/night as strongest northwest flow sets up. Another strong low pressure system will cross the Northern Ohio valley Saturday night. Moderate northeast flow will bring a modest increase in waves by Sunday as wind gusts reach 20 knots or so after a period of relative calm in between low pressure systems from late Saturday into Saturday night. This second low will bring another period of precipitation, a combination of rain and snow, especially form far Southern Lake Huron south to Western Lake Erie. && Hydrology... Rain will spread north into the region this evening and persist into Friday morning as strengthening low pressure tracks through the Ohio Valley and then across Lake Erie overnight. Rainfall amounts of between one half and one inch will be possible generally along and southeast of line from Howell to Lapeer to Sandusky with the highest amounts expected from the Michigan and Ohio state line northeast through parts of Metro Detroit into the Port Huron area (where amounts may locally exceed an inch). In these areas, rainfall could lead to some minor urban and poor drainage flooding in typically prone areas. To the northwest of this line, rainfall amounts will average less than one half inch and will quite possibly be one tenth of an inch or less over the Saginaw Valley. Additional precipitation is expected late Saturday through Saturday night as another strong low pressure system tracks west to east through the southern Great Lakes. This precipitation will fall as a combination of rain and snow with several inches of accumulating snow expected in some locations. A liquid equivalent of around one half an inch can be expected with this second storm system with the heaviest swath currently expected between roughly I-96 and I-69. && Previous discussion... issued at 246 PM EDT Thu Apr 25 2019 Aviation... Lower VFR/MVFR cigs can be expected kfnt south this afternoon with lowering cigs/rain then spreading north into the area this evening as low pressure tracks into the Ohio Valley. IFR conditions can be expected at times overnight into early Friday morning kptk south into I-94 where the core of deformation will set up on northwest side of low track. Cigs/vsbys will be progressively higher back into kfnt/kmbs with VFR likely holding kmbs. North flow become northwest around the low on Friday will become rather gusty, particularly by the end of the forecast period as gusts begin to push 30 knots. For dtw...ceilings below 5kft will persist this afternoon and then lower this evening into the overnight as rain spreads north with approach of low pressure. While a few showers may dot the area by early evening, steady rain is expected to hold off until 02z or so. MVFR to IFR will then hold into Friday morning before rain moves east with the passage of low after 14z or so. Northwest winds will begin to gust in the morning and peak after 18z at near 30 knots in strong afternoon mixing. Dtw threshold probabilities... * high for ceilings aob 5000 feet into Friday morning. * Low in northwest wind gusts meeting crosswind thresholds by ~18z Friday. && DTX watches/warnings/advisories... Michigan...Lakeshore flood advisory from 8 PM Friday to 10 am EDT Saturday for miz049. Lake Huron...Gale Warning from 8 PM Friday to 10 am EDT Saturday for lhz363-462- 463. Small Craft Advisory from 10 am Friday to 10 am EDT Saturday for lhz421-422-441>443. Gale Warning from 4 PM Friday to 10 am EDT Saturday for lhz361-362. Lake St Clair...Small Craft Advisory from 8 am Friday to 10 am EDT Saturday for lcz460. Michigan waters of Lake Erie...Small Craft Advisory from 8 am to 10 PM EDT Friday for lez444. && $$ Discussion...irl/sp marine.......dg hydrology....dg aviation.....Dg

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