Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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644 FXUS63 KGRR 130144 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 944 PM EDT Sun May 12 2024 .KEY MESSAGES... - Small Window for Aurora This Evening Before Clouds Move In - Shower and Thunderstorm Chances Tonight through Monday Night - Rain Ends Tuesday - Additional Chances of Rain Late Week| && .UPDATE... Issued at 935 PM EDT Sun May 12 2024 As expected, a few thunderstorms have moved out of Wisconsin and across Lake Michigan. The first of these storms has just move onshore near Ludington. Additional showers and embedded storms are expected over the next several hours, generally along and north of the I-96 corridor. Small hail and gusty winds are the main concerns with some of the stronger storms this evening. && .DISCUSSION... Issued at 334 PM EDT Sun May 12 2024 - Small Window for Aurora This Evening Before Clouds Move In For those looking to view the Northern Lights tonight we remain in an active period as a X class flare (the highest level, although there are varying degrees of X class flares) came off the sun at midday. kP index values and magnetometers from across the mid latitudes show limited activity as this time. With additional flares inbound though, aurora remains a possibility tonight. The issue is like Friday evening, clouds are pushing in from the northwest after dark. There may be a window from roughly 1000pm to midnight or so where clouds have not yet obscured the sky. That window only becomes relevant though if we increase aurora activity from current relatively low levels. Stay tuned...or look skyward. - Shower and Thunderstorm Chances Tonight through Monday Night The main focus of the next day or two is the chances for showers and thunderstorms tonight, Monday and Monday night. Two main upper shortwaves of note during this time frame, one is passing well to our north across Ontario. The other lifts into the Plains from the Central Rockies tonight. A cold front associated with the Ontario wave will push into the area from the north tonight. As the Ontario low pulls away to the east the boundary stalls over the area on Monday as the plains system approaches. It is not until Monday night into Tuesday before the boundary sags south of the area. Assessing thunderstorm chances each period beginning with tonight...showers and storms will rapidly increase this evening over Wisconsin and Northern Lake Michigan where a Slight Risk has been issued by the Storm Prediction Center (SPC). Storms will try to advect in across Central Lower Michigan this evening after 800pm and continue roughly through about 300am. The 3km NAM tries to push MUCAPE values into the area that are over 1000 j/kg this evening. That may be a bit high. HRRR MUCAPE is much lower, on the order of a few hundred joules. We are in a marginal risk from the SPC, but feel severe weather is a low risk in our forecast area tonight. Storms will be very much diurnally driven and with limited CAPE to work with in the first place it will be tough to produce strong storms. Cannot rule out some small hail if a stronger storm can survive into our area given steep mid level lapse rates. Updraft helicity swaths tell the tale tonight with best chances at severe in Wisconsin and weakening storms as they move our direction. The HREF has a good flare up of convective activity in our area Monday afternoon, especially near and south of Interstate 96. We are likely looking at around 1,000 j/kg of MUCAPE tomorrow with potentially a bit more if we can develop breaks in the cloud cover located near the stalled front. Deep layer shear is only 20-30 knots where storms will be located so organization and severe threat remains on the low side. The front remains in place for Monday night slowly sagging south with time. Once we get beyond the evening hours, where some embedded thunderstorms remain, we transition into more of a synoptic scale rain. The Plains upper low and its surface reflection will be moving into the Ohio Valley Monday night. Regarding rainfall...the HREF probability matched mean is showing all areas seeing 0.25 to 0.75. Pockets of 1.00 to 1.50 inches will be possible in heavier swaths. We are not expecting any significant hydro concerns with this rainfall. - Rain Ends Tuesday Rain chances will end from north to south as the day progresses on Tuesday. This will be driven by a mid-level trough driving the frontal boundary responsible for our precipitation south of the state, with northerly flow behind it. This northerly flow keeps West Michigan in the 60s for highs Tuesday. Surface ridging and a return to southerly flow Wednesday causes highs to return to the 70s. The surface high pressure system paired with upper-level ridging and the exiting of low-level moisture means dry conditions are favored Wednesday and much of Thursday. There may even be decent sunshine given the lack of moisture. - Additional Chances of Rain Late Week Precipitation returns to the forecast late Thursday into Friday as a mid-level wave approaches the state. Confidence in precipitation is higher than 24 hours ago with the Euro/EPS trending towards the GFS/GEFS suggesting troughing arriving by Friday. Ensemble forecasts suggest that a surface low develops in association with this trough but the position and strength of this feature varies. Given the favorable synoptic setup for rain but still spread in where the best rainfall axis will be, feel the NBM broad 40-55 percent PoPs are reasonable. As the amplitude and tilt of the trough and associated location and strength of the surface low refine in the coming days, POPs may increase across the region, particularly where the synoptic setup provides the best setup for rainfall. Signals for additional precipitation exist into the weekend, but considerable spread remains in the mid-level pattern in whether troughing persists into the weekend or a period of ridging takes over. Given the weaker signal for precipitation under the less certain synoptic conditions, will leave the broad slight chance to low end chance (25-35 percent) chances of precipitation for Saturday in the outgoing forecast. && .AVIATION /00Z TAFS THROUGH 00Z TUESDAY/... Issued at 635 PM EDT Sun May 12 2024 VFR conditions are expected at all the terminals tonight. However low level wind shear is forecast late this evening and overnight as west southwest winds at 2 kft agl ramp up to around 40 kts. Scattered showers will develop overnight but conditions should remain VFR. There is not enough potential for isolated thunderstorms to warrant inclusion in any of the terminal forecasts overnight as the better chance for convection will remain well to the north to nw of all the terminals. Scattered showers will redevelop on Monday particularly in the afternoon and evening but conditions at all the terminals should remain VFR. && .MARINE... Issued at 935 PM EDT Sun May 12 2024 No significant changes to forecast compared to previous discussion. Small craft advisory winds continue near Big Sable and Little Sable points this evening, but have started to trend downward. A few thunderstorms are likely this evening, which will bring local changes to expected winds, but overall southwest winds will continue through the morning before shifting around to the northwest at less than 15 knots tomorrow. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...None. MARINE...Small Craft Advisory until 2 AM EDT Monday for LMZ848-849. && $$ UPDATE...AMD DISCUSSION...Duke/Thomas AVIATION...Laurens MARINE...AMD