Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000 FXUS63 KDTX 041002 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 502 AM EST Sun Dec 4 2016 .AVIATION...
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Southwesterly winds will continue to disrupt the low level inversion enough to cause a brief break in the MVFR stratus across the region. Cirrus debris will start drifting over the region in advance of the trough that will lift through this evening and overnight. Models are in good agreement with a period of IFR CIGS/VSBYS this evening as the deeper ribbon of moisture arrives producing snowfall. Exact onset is still in question but looks good to hold off til after 21Z. Abrupt end to the snow will work through around 06Z. The character of the snow will be wet and with sfc temps in the 30s and relatively warm pavement temps, the snow will struggle to stick to paved surfaces. For DTW... The onset of snow still looks like it will be mainly after 00Z Sun evening at metro, likely not ending until about 06Z. //DTW Threshold Probabilities... * Medium in ceilings below 5000 ft this morning. High this afternoon through tonight. * Low for CIGS/VSBYS to drop below 200ft and/or 1/2SM tonight. * Medium in precip type being mostly wet snow this evening.
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&& .PREV DISCUSSION...
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Issued at 419 AM EST Sun Dec 4 2016 DISCUSSION... The well defined anticyclonically curved exit region of +130 knot Pacific jet streak is currently pushing off of the Rockies and advancing out into the central Plains over Nebraska. MSAS analyses this morning shows that some eastward push of a leeside surface trough has occurred. Meanwhile, water vapor imagery suggests some ongoing deepening of the system with phasing and slight leafing of southern stream moisture over portions of Kansas and Nebraska. As this noted moisture lifts further northward during the next 6 hours, models suggest the enhanced midlevel potential vorticity will react with deep midlevel trough pushing off of the Rockies to cause some spin up and dramatic increase to absolute vorticity that will pivot around the increasingly dominant western Canada lower height anomaly. The significant pivot of the shortwave or extremely rapid change from positive to neutral to negative tilt will occur from 12Z this morning by NE/KS through Chicago around 00Z and through lower Michigan 00-06Z this evening. The 04.00Z numerical weather prediction suite arrived this evening with a few surprises in the solution space compared to output from 24 hours ago. The biggest surprise was a fairly significant uptick in the QPF with this system. Bulk of the model output, chiefly the NAM, is now suggesting that a widespread .20 inch will fall across the southern 2/3rds of the CWA, while the northern 1/3rd of the CWA could see as much as .30 inch. The overall governing dynamics of this event really have not changed much. The event at the big picture will be driven by direct differential cyclonic vorticity advection from the negative tilt shortwave described earlier in this discussion and warm air advection processes. As far as the warm or theta e advection, the nam has certainly evolved much more bullish on the ascent potential. The reason for this is earlier nam solutions were a little stronger with the preceding midlevel anticyclone that pushed across Lake Huron into Ontario today. This stronger localized midlevel cyclone kept system relative flow much more easterly through southeastern Michigan right into the event. Now, models fade out this anticyclonic circulation keeping system relative flow southeasterly today. This southeasterly flow in advance of the system then becomes much more in phase and orthogonal to isentropes in the theta e ridge as it pivots into southeastern Michigan. At first, one thinks this just contributes to more sustained gradual ascent, however, nam cross section views does support a period of upright ascent across the area. However, that is not the end of the story. The regional 10km Canadian was the first to suggest a solution that carries a dynamic lift response farther south through the Ohio River Valley. The cause of this southern solution is that a favorable jetlet structure develops on the forward jet streak over the Ohio River Valley. Models are now suggesting that favorable left exit region dyanmics develop on this jetlet which also becomes increasingly phased with the deep midlevel trough. Best nose convergence off a low level jet would occur down there as well, with the best of the large scale moisture transport almost releasing due eastward, and getting pinched off or occluded before it surges northward in and ahead of the trough. The experimental HRRR has really latched onto this solution. Additionally, the 06Z 4km WRF is onto it as well. What happens over southeastern Michigan is more of straight inverted trough, convergence, response in the presence of high quality, deep, moisture. There is at least, qualitatively, support for this solution in the deterministic NAM mass fields with a dominant midlevel low ejecting eastward across northern Indiana to the Lake Erie/Ohio shoreline this evening. Out of respect for the very high quality of moisture, direct CVA, and more organized southeasterly system relative flow, the decision was made to increase QPF to around .20 inch for all areas. The second surprise of the 00Z suite was to push the timing back again to the 22-05Z timeframe. Bottom line, expecting a mix of light snow and rain to overspread western lower Michigan today, before advancing into southeastern Michigan during the late afternoon hours. Surface temperatures will rise into the upper 30s today, before wet bulbing down into the lower 30s as precipitation begins. Given the later timing and the overwhelming consensus of the environmental thermal profile suggested in the forecast soundings, think there is little doubt that much of the precipitation over southeastern Michigan will change over and fall in the form of snow. The exception at onset and for a couple of hours will be the higher surface pressure areas south and east of the terrain and the Detroit urban heat island, where surface temperatures will be above 2C with a surface dewpoint at or just above freezing. There a light rain will try to hold on longer. Keying off of the direct CVA, belief is that height of event will occur in the 23-03Z window. Big question outstanding is what snow ratios can be with surface dewpoints then actually rising to the 32F as the heart of the inverted surface trough settles in. In-house probabilistic snow guidance is slightly more favorable now for .2 of an inch per hour rates for a 3 or 4 hour period but not more than that. Given a stable warm near surface sounding with lack of low static stability through the DGZ, still expecting needles and columns with a best case snow ratio of 8:1. Summing it all up, 1 to 2 inches of a wet snowfall will be possible in grassy areas with less than 1 inch for Detroit. Better amounts are expected to the north, more because of slightly cooler thermal profile than any max bullseye in qpf. Surface high pressure will grip southeastern Michigan on Monday as shortwave ridging amplifies over much of the Great Lakes. Preference sides again with a cloudier forecast given time of year, westerly flow, and moisture content progged under an inversion at 900 mb. Temperatures will be right at average. The next system will arrive Tuesday afternoon as the Mexico cutoff low gets kicked northeastward. This is one that is really going to warrant some attention over the next couple of days as there is some dramatic differences between the thermal profile of the NAM (cold) and the ECMWF and GFS (warm). The nam if it were to verify suggests an accumulating snowfall for all of the southeastern Michigan. One wonders if the nam is its normal self in failing to activate the frontal surface down lower in the column. The other solutions have a more definitive sharp northern cutoff to the precipitation shield. With the warm origin of the system appearing to be at least preserved in the ECMWF soundings and GFS soundings, will defer to the previous forecast calling for a predominate cold rain Tuesday afternoon. MARINE... Light easterly flow early this morning will increase slightly out of the southeast today in advance of an approaching surface trough. Winds will veer to the southwest behind the trough tonight. Lack of stronger pressure gradient will keep wind gusts below 20 knots through the event though the trough will present a period of light rain/snow this evening and overnight. Southwesterly flow will continue into Monday while decreasing as high pressure builds back in from the south.
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&& .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...NONE. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE. && $$ AVIATION.....DRK DISCUSSION...CB MARINE.......DRK You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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