410 FXUS63 KGRR 061919 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 319 PM EDT Mon Sep 6 2021 LATEST UPDATE... Synopsis/Discussion .SYNOPSIS... Issued at 318 PM EDT Mon Sep 6 2021 - Mostly clear and pleasantly cool tonight - Strengthening storm brings severe storm threat Tuesday - Cooler weather to follow into Friday then warming again && .DISCUSSION...(This evening through next Monday) Issued at 318 PM EDT Mon Sep 6 2021 - Mostly clear and pleasantly cool tonight The surface high and shortwave ridge pass over Michigan tonight. This will mean mostly clear skies with light winds. Temperatures should fall into the mid 40s north to lower 50s south early Tuesday morning. Actually there is strong warm air advection above the boundary layer tonight. In the layer between 1000 ft AGL and 7000 AGL temperatures rise 4 to 6F between 8 pm and 8 am tomorrow morning. Due to the lack of wind in the boundary layer the warming will not mix to the ground. If there would have been better mixing air temperatures would have actually risen overnight. - Strengthening storm brings severe storm threat Tuesday The long advertised Canadian storm system remains on track to bring the risk of strong to severe storms Tuesday afternoon. Really there has not been much overall change in the model forecast for this event. SPC has put all of our forecast area in Slight Risk for Severe storms tomorrow. They are giving us a 15 pct chance of damaging winds, a 5 pct chance of large hail and and 2 pct chance of tornadoes. This will largely be an afternoon event for our area. These numbers the chance of that event within a 25 miles of a given point. We still have a 130 knot upper level jet digging into the back side of the deepening upper trough (closed upper low) during the day tomorrow. In response we have strengthening low level jet, which is actually aimed at the Holland to Muskegon area during the early afternoon (typically the low level jet is aimed away from us in most events). There is an excellent coupling of the upper divergence and low level convergence through the entire afternoon, but it is best during the mid to late afternoon over SE Lower Michigan actually. The 1000 to 850 moisture transport vectors are also aimed at us and are also impressive. This system will have a narrow area of significantly higher surface to 850 mb level dew points just ahead of the surface cold front. We will be deep into the warm air so we will not have an issue with layered clouds blocking surface heating during the mid to late morning. That will allow surface temperatures to surge into the mid 80s near and east of US-131. Due to cooling at high levels and warming at low levels during the midday hours the instability will increase significantly. There is between 1500 to 2500 j/kg of MU cape for this event. We also have 40 to 50 knots of deep layer shear. There is enough low level helicity for isolated tornado if we can get discrete storms to develop (likely at first). Overall the largest threat will be straight line winds. Another aspect to this is that this system will have strong pressure gradient with is so we will have strong surface winds outside of the thunderstorms. Since this is a warm advection event, the surface wind gusts will be less than they would have been if this happened on the cold side of the system but none the less we should have gusts in the 30 to 40 mph range in the afternoon. Once the front comes through in the evening there should be fairly quick clearing as the dry slot moves into our area. It will get cooler too. - Cooler weather to follow into Friday then warming again As we have been saying, there is enough cold air on the back side of this system for lake effect rains showers and water spouts. Since we will be in the deep cold air through Thursday that means cool temperatures, mostly cloudy skies and some light rain showers. A secondary shortwave helps this cause Wednesday then an much stronger one helps the cause on Thursday. The next upstream system from the Pacific will push a upper level ridge in our direction so it should start warming up on Friday, and who knows maybe we could see highs in the 80s by Saturday once again. Summer type weather is not over yet as the MJO is going into phase 4 and that is warm for our area. I would expect a significant, and extended warm period here by next week. && .AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon) Issued at 146 PM EDT Mon Sep 6 2021 VFR will prevail through the forecast period. Winds will continue to gust towards 20 knots across the area before quickly weakening this evening, becoming calm overnight. Winds will then shift southerly early Tuesday morning while building. Gusts towards 25 knots or greater will be possible Tuesday afternoon with sustained winds around 10 to 15 knots. Very shortly after 18z Tuesday, a line of showers and storms will arrive from the northwest starting at MKG then will move southeast through Tuesday afternoon and evening. && .MARINE... Issued at 300 AM EDT Mon Sep 6 2021 Winds and waves will decrease this morning and we expect to let the Small Craft and Beach Hazards Statement expire at 8 am. Winds and waves will increase again on Tuesday and severe thunderstorms are possible in the afternoon. Conditions will remain hazardous through Wednesday as strong winds and high waves continue. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. LM...None. && $$ SYNOPSIS...WDM DISCUSSION...WDM AVIATION...Thielke MARINE...Ostuno