750 FXUS63 KGRR 100730 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 330 AM EDT Wed Oct 10 2018 LATEST UPDATE... Synopsis/Discussion/Marine .SYNOPSIS... Issued at 330 AM EDT Wed Oct 10 2018 What a difference a day or so will make, when comparing highs in the 80s on Tuesday with temperatures tumbling into the 40s by Thursday morning. By Thursday evening we will see temperatures slipping into the 30s. A strengthening low pressure center moving through the Great Lakes today will be responsible for the falling temperatures. A cold front associated with the low will drive through the area tonight. Temperatures will fall rapidly behind the front. Ahead of the front showers and a few thunderstorms are expected today. Some locally heavy rain is possible. Lake effect rain showers are expected in the colder air from Thursday into Friday. && .DISCUSSION...(Today through next Tuesday) Issued at 330 AM EDT Wed Oct 10 2018 The first order of business is the low pressure system moving through the Great Lakes today and tonight. Specifically, what will its affects on our weather be with respect to...the threat for heavy rain and whether or not we see thunderstorms. Regarding heavy rain there are some factors in favor of it. The first is the fact that precipitable water values are anomalously high. The GFS has PWAT values surging to above 2 inches which is high for August, let along mid October. A front plowing in from the west should focus rainfall as well, but it will transitory as the front will be on the move. The rain looks to mainly fall in a 12 hour period from 200pm through 200am today and tonight. HREF ensemble data shows most amounts should be in the range of a half inch and an inch. The ensemble maximum data does show a few swaths of rain near 2 inches. These higher amounts do not look widespread though. The thought is that we should be able to handle 0.50 to 1.00 inch amounts without too much of an issue. Regarding storms today, the chance for storms overall looks low with the threat for severe even lower. The main issue with seeing storms today is the fact that instability is meager. Mid level lapse rates are very low on the order of 5 to 5.5 C/km. The current thinking is we will see showers press in from the south at midday becoming fairly widespread during the afternoon. A broken line of storms will develop off to our west over IL/WI this afternoon and evening, largely falling apart as it moves our direction. The rain should be sweeping east of Southwest Lower Michigan by around 200am in the morning. The focus will then shift to the cold side of the system as lake effect rain showers appear likely. Lake surface temperatures are running in the low 60f or around +16C. So, all we need is to get to around +3C at 850mb's to produce enough instability for rain showers. We will be cold enough in all areas for lake effect rain by daybreak on Thursday, the cold advection is that strong. The thermal trough at 850mb will be overhead Thursday evening, with delta T's around 22C over Central Lake Michigan. The bottom line is that we should see lake effect rain showers from Thursday right through Friday night. The NAMnest has a signal in its model reflectivity that rain showers may be widespread and possibly extending quite a distance across the Lower Peninsula driven by the wind. Saturday morning we may have an issue with frost and freezing temperatures if the clouds can clear from inland areas especially. Lower 30s certainly looks possible so we may need frost or freeze headlines going forward. The next system of interest will be an upper shortwave moving our direction in northwest upper flow from Saturday night into Sunday. We have rain showers in the forecast during this time frame. Cold air follows in the wake of this system as well, with the lake effect rain shower machine kicking in again for Sunday night into Tuesday. With 850mb temps dropping to around -6C during this time frame cannot rule out a little snow mixing in across Central Lower during the late night and early morning time frame of Monday morning. Bottom line is it is going to feel much more like fall the next 7 days, with temperatures running below normal much of the time. Lake effect rain showers will bring a fall feel as well. && .AVIATION...(For the 06Z TAFS through 06Z Wednesday night) Issued at 151 AM EDT Wed Oct 10 2018 An active next 24 hours is expected as a cold front will approach and push through the region. The TAF period is starting out with VFR conditions at all TAF sites but that will change today as showers spread in from the south midday. Around 15-16z, we expect showers to break out across the area becoming numerous in the afternoon. Ceilings will deteriorate fairly rapidly this afternoon as well, going from VFR at midday to IFR by evening. The low ceilings will sweep in from west to east. At this point have only included rain showers as the threat for thunder at any one TAF site is somewhat low. Thunder is certainly possible although confidence at this point in time is low. The rain will sweep east during the evening with ceilings steadily rising later tonight. Southerly winds of 10 to 20 knots today will shift to the southwest tonight after the cold front moves east. && .MARINE... Issued at 330 AM EDT Wed Oct 10 2018 Will be expanding the Small Craft Advisory for all zones this afternoon and early evening as southerly winds increase ahead of the cold front. Waves will build to around 4 feet near the points as early as 18z today. Also, have opted to join neighbors with a Gale Warning from this evening into the midday hours on Thursday. Winds in the profile are not all that impressive, but the model winds may be underdone, given the gradient in place and the fact that this will be strong cold air advection. Strong cold air advection situations are the ones that normally overperform. Waves look to peak around the 10 foot mark Thursday morning around daybreak. Piers will be dangerous places given the combination of higher than normal water levels and a 35 knot westerly gale. If you are headed to the lakeshore steer clear of the piers. && .HYDROLOGY... Issued at 1254 PM EDT Tue Oct 9 2018 The Kalamazoo and Grand Rivers continue to slowly drop as rainfall from last weekend works its way through the river system. Most small and medium-sized tributary streams are seeing falling water levels as well. The mainstem of the Muskegon river is rising slowly as the 1.0 to 1.5 inches of rain that fell yesterday morning drains into the river. A few showers and thunderstorms are possible across West Michigan this afternoon and evening, but are not expected to result in any flooding. The next chance for heavy area-wide rainfall arrives on Wednesday afternoon and evening as a strong cold front is set to move through our area. At this time it looks like many areas may see rainfall totals around 1 inch. This would cause the larger rivers to rise, but is not expected to cause any flooding on the mainstem rivers. Despite a few days break in the heavy rain, soils remain saturated and the smaller streams may see a quicker water level rise than normal. Sycamore Creek near Holt may rise to near bankfull by Thursday morning in response to Wednesday evening rainfall, but is not expected to reach flood stage. What will also need to be monitored is the potential for urban areas to be overwhelmed by heavy rainfall if the storms linger over the same areas repeatedly. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. LM...Small Craft Advisory from 2 PM this afternoon to 10 PM EDT this evening for LMZ844>849. Gale Warning from 10 PM this evening to 1 PM EDT Thursday for LMZ844>849. && $$ SYNOPSIS...Duke DISCUSSION...Duke AVIATION...Duke HYDROLOGY...AMD MARINE...Duke