106 FXUS63 KDTX 101154 AFDDTX Area Forecast Discussion National Weather Service Detroit/Pontiac MI 654 AM EST Thu Dec 10 2020 .AVIATION... Southeast Michigan airspace will reside within the eastern expanse of a large area of very low stratus and dense fog through the morning hours. Recent observational and satellite data point to a high likelihood for LIFR to VLIFR conditions in some combination of low ceiling and visibility restrictions. This moisture will only slowly mix out during the late morning hours, eventually yielding mostly clear skies across the lowest 10k ft for the afternoon and evening hours. Environment does remain favorable for some degree of fog formation again late tonight. Winds will remain light from the south through the period. For DTW...Metro will remain under dense fog and/or very low stratus through late morning. Recent trends suggest visibility below 1/2 mile remains possible through 15z. Improving conditions thereafter with mostly clear skies this afternoon/evening. Potential for additional fog development again tonight. .DTW THRESHOLD PROBABILITIES... * High in ceilings below 5000 feet this morning. * Medium in ceilings and/or visibilities below 200 feet and/or 1/2 mile Thursday morning. && .PREV DISCUSSION... Issued at 413 AM EST Thu Dec 10 2020 DISCUSSION... Ridge of high pressure is now anchored across the Great Lakes, with active subsidence/dry air advection holding clear skies (removed from the ongoing fog) across the cwa. Saturated surface conditions in conjunction with the dry air aloft and calm winds have allowed for widespread dense fog to develop across the Metro area and locations south to the Ohio border, which has prompted the issuance of a freezing fog advisory given the sub-freezing temperatures. In addition to visibility concerns, slick conditions will remain possible through the morning hours mainly for bridges, overpasses, and elevated surfaces. Lingering morning fog will start to lift and erode during the late morning hours given some weak December diurnal heating, but also from return flow from a departing high pressure system increasing gradient winds and boosting waa. This should also aid in boosting temperatures above freezing through the late morning to early afternoon hours. Any stratus that develops as a result will likely mix out leading to periods of sunshine for the afternoon hours. Tricky temperatures forecast with the fog in place, but warm advection should promote a healthy rise of temperatures into the mid 40s for most locations, potentially peaking towards the upper 40s depending on the rate of clearing of fog. All MOS guidance suggests upper-40s to lower-50s is obtainable, capitalizing on the stronger waa, but with this fog in place, will keep highs short of this. Otherwise, dry weather continues through the day with the subtle ridging aloft. Introduced patchy fog again to Friday morning as condensation pressure deficit drops to or below 4 mb given with a subtle uptick in dew points. Soundings indicate another morning with saturated surface conditions and dry air aloft, however, a stream of upper- level clouds ahead of an advancing cold front along with slightly better gradient flow relative to this morning may act to inhibit better fog formation. Attention then turns to the development of allow pressure system around TX. This low will strengthen, as a closed low over the southwest Baja region opens and extends into TX while a NW Pacific wave and jet drives south into TX/OK through Friday. Low pressure will then drive northeast into MO by 06Z Sat before pushing into northern Indiana by 18Z Saturday. This system will produce definitive widespread rain, possibly mixed in with melting flakes/snow at times over the Tri-Cities, to all of SE MI. As noted in the previous discussion, strong convergence has now been observed across deterministic model runs and the majority of ensemble systems, with the EPS now coming more in line with the NAEFS in terms of track of the low pressure system. The convergence noted yesterday holds with the current 00Z package. The EPS ensemble average loosens the influence of the strong 1030+ mb Canadian high pressure over Ontario which provides a more northward track for the approaching low pressure system, set to travel into central lower Michigan late Saturday evening. For Friday evening into Saturday evening... Much less variance is noted within model spread in the cluster phase space analysis, with only minor variance noted with timing/position uncertainty of the trough/ridge couple across the desert SW and continental northeast respectively. Regardless, this uncertainty will have little impact on temperature and precipitation trends for SE MI. Initial precipitation looks to start as early as Friday evening along the leading fgen band/isentropic ascent, centered north of the Metro region. Increasing PW values (.75 to .80 inches, 90th percentile w.r.t climatology) to overspread SE MI throughout Saturday, with overrunning along an elevated front to then produce widespread precipitation through early Saturday morning before the low arrives over the state. Convergence of the track of the low has increased confidence that the thermal profile will support all rain with this event (lows above freezing, highs between 40-50) , with the exception of Bay and Midland County, which could see a mix of rain and snow (lows near freezing). Low pressure will depart east Saturday evening into Sunday evening, with wrap around moisture on the backside of the low providing additional rain chances through Sunday morning. Cold air wrapping around the low will provide the chance to see lingering rain/snow or snow across more portions of the cwa, although precipitation rates should be trending down at this time. dprog/dt continues to lift snow accumulation potential northward into northern lower Michigan, with the chance to see up to an inch or two across northern Bay and Midland Counties by the end of the event, likely reserved for elevated or grassy surface. The cold air wrapping behind the low is expected to near or below freezing throughout the day on Sunday, so residual moisture on roadways may produce slick conditions. Additionally, gusts around 20- 30 mph will be expected with this system given the strengthening pressure gradient. High pressure fills in behind the system early next week and will provide a period of dry weather. MARINE... Ridging aloft will facilitate a temporary relaxation of gradient winds today as surface high pressure builds northeast along the Appalachians. Winds back toward the south in response this afternoon and evening with prevailing speeds mainly below 15 knots. Flow briefly strengthens after sunset due to inbound low pressure dropping southeast along the southern shores of Lake Superior. Expect a northeasterly reorientation of the winds Friday afternoon behind the associated cold front. Fortunately, enhanced low-level jet winds will remain displaced above a sinking inversion layer keeping gusts below 25 knots. A broad low pressure system will bring a rain/snow mix, higher wave action, and potentially gales on Saturday and Sunday. Currently monitoring the chance for gusts to gales over much of Lake Huron, including Saginaw Bay and the Thumb from Saturday night into Sunday morning. Duration and coverage of +34 knot gusts remains in question. Regardless, strong gusts to around 30 knots and max waves in the 6-10 feet range will strain Huron basin travel efforts until the system exits into southern Ontario. HYDROLOGY... A strong low pressure system will travel towards and into Michigan late Friday towards Saturday which will bring widespread rainfall to all of SE MI. Initial rain chances look to start late Friday evening into early Saturday morning north of the Metro area, with rain then filling in across all of SE MI throughout the later morning hours on Saturday. This will be a prolonged event, lasting into early Sunday morning. Elevated instability leading to some thunderstorm chances will be possible late Saturday evening which could lead to localized higher rainfall amounts. PW values will near .80 inches during this event, which falls around the 90th percentile per climatology, brings increased confidence to see totals between .50 to 1 inch by Sunday afternoon. Localized higher totals greater than an inch will be possible with thunderstorm activity. If that materializes, there could be an isolated minor flood threat concern. Otherwise, given the longer duration event and antecedent dry conditions, only localized flooding concerns are expected with the usual trouble spots. && .DTX WATCHES/WARNINGS/ADVISORIES... MI...Freezing Fog Advisory until 10 AM EST this morning for MIZ060-061- 068-069-075-076-082-083. Lake Huron...NONE. Lake St Clair...NONE. Michigan waters of Lake Erie...NONE. && $$ AVIATION.....MR DISCUSSION...AM MARINE.......KK HYDROLOGY....AM You can obtain your latest National Weather Service forecasts online at www.weather.gov/detroit.