Area Forecast Discussion
Issued by NWS Detroit/Pontiac, MI

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000 FXUS63 KDTX 201957 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 357 PM EDT Fri Oct 20 2017 .DISCUSSION...
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Near Term / Through 6 AM Saturday morning As of 355 PM EDT...A tranquil and unseasonably warm weather day continues across southeast Michigan as surface high pressure remains anchored across the upper Ohio River Valley. Aloft, upper-level ridging is in place. Generally light south to southwest flow continues to prevail with the return flow around the high. This return flow has allowed for a surge in midlevel dry air across the region, which has resulted in dry weather and sunny skies. Temperatures have warmed well above normal for this time of year, with temperatures warming well into the 70s for most areas, and even upper 70s near US 23. Upstream, an increase in high cloud cover was noted across the Mississippi River Valley in association with a deepening longwave trough which will impact the region during the short term period (more in the short term section of the discussion). This cloud cover will begin streaming into the area tonight, and combined with a lingering southerly wind due to a tightening pressure gradient, will help keep low temperatures elevated in the low to mid 50s. Sheltered areas that are able to briefly decouple will be able to see lows dip into the upper 40s. Short Term / 6 AM Saturday morning through Monday Complicated forecast scenario evolves throughout the short term period, as both the northern and southern stream jet become quite active and resemble a more typical transition season energetics pattern. During this period, potent northern stream energy will dive southeastward from the Pacific Northwest, noted by a strong PV anomaly dipping as far south as the Four Corners region. This PV anomaly will help to dig a deep longwave trough across the central CONUS, but much of the corresponding northern stream jet dynamics look to be displaced further north as the initial PV surge becomes pinched off from the larger scale flow. Meanwhile, a surge in the southern stream subtropical jet looks to round the base of the longwave trough, with the 12z guidance now coming into better agreement with a cutoff low developing across the lower Plains out of the remnant northern stream PV lobe, while the northern stream jet remains displaced to the north, and a split-flow regime setting up for the weekend and into early Monday. Initially in this split flow, it looks to be one last dry day on Saturday with gradually increasing cloud cover and temperatures once again reaching well into the 70s with lows Saturday night in the 50s. Then for the latter half of the weekend, a cold front will cross the region late Sunday into Sunday night, with the trend in the past few runs continuing to indicate a slowing in the timing of the front. This front will bring scattered to numerous showers riding a surge of deeper moisture, with rainfall amounts generally on the order of one quarter to three quarters of an inch of rain. Temperatures on Sunday look to spike again into the 70s despite thickening cloud cover and increasing chances for rain from west to east late in the day. Long Term / Monday night through Thursday Forecast uncertainty continues into the extended period as long range guidance continues to struggle handling the evolution of the split-flow regime. While there continues to remain uncertainty in the forecast, there are increasing trends that a strong low pressure system (most long range guidance deepens it to sub-990 hPa by the midweek period!) will develop in the vicinity of southeast Michigan for early next week. This low pressure system looks to develop in response to another surge in northern stream PV energy diving south out of the northern Plains, and interacting with the ejecting upper- level closed low in an active southern stream. Because of the split- flow regime leading up to this event (long range guidance tends to struggle handling this regime beyond 3 days), the degree to which phasing and amplification of the synoptic pattern occurs is still in question, and should hopefully be resolved in a clearer picture in the next 24-36 hours. Will go with a blended approach of the 12z GFS/ECMWF and various ensemble members during this timeframe, with the forecast depicting likely showers Monday night through Tuesday night. Monday night into Tuesday, there are signals for a potential heavy rain/high wind event ultimately depending on the evolution of the low pressure system, and this trend will need to be monitored closely over the coming days. A drying trend in the wake of the system will be possible to end the long term period, but again will hinge on the evolution of the low pressure system. Temperature-wise for the long term period, a trend back towards normal will occur, as much colder air is ushered in within the strong northern stream flow. Daytime highs will return to the upper 40s to upper 50s, with lows in the upper 30s to mid 40s.
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&& .MARINE...
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High pressure continues to dominate the weather pattern with southerly winds 10 to 20 knots through tonight. A trough moving across the Rockies and through the Plains will push a cold front through the Great Lakes late Sunday night. Wind will pick up to around 20-25 kts out of the south ahead of this cold front as surface pressure gradient tightens. Showers will be possible Sunday night through the early part of next week as the next low pressure system impacts the region. Some gusts could reach around 30 knots by Tuesday.
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&& .HYDROLOGY...
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A cold front and upper trough approaching the region at the end of the weekend will bring a chance of increased rainfall. Rain chances begin Sunday night and continue through the early part of next week as the next low pressure system arrives. There is the potential for several inches of rain across the region beginning late Sunday and continuing well into next week. Rainfall amounts will be fine tuned as the event draws nearer as there remains much uncertainty in the forecast beyond Sunday.
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&& .PREV DISCUSSION...
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Issued at 151 PM EDT Fri Oct 20 2017 AVIATION... VFR conditions will prevail for this TAF period under south winds around 5-10 knots. Winds will begin to decrease into the evening. Few clouds except for occasional FEW-SCT high clouds around 25kft. //DTW THRESHOLD PROBABILITIES... * None.
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&& .DTX WATCHES/WARNINGS/ADVISORIES... MI...NONE. Lake Huron...NONE. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE. && $$ DISCUSSION...IRL MARINE.......AA HYDROLOGY...AA/IRL AVIATION.....AA You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.

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