Area Forecast Discussion
Issued by NWS Grand Rapids, MI
Issued by NWS Grand Rapids, MI
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578 FXUS63 KGRR 181917 AFDGRR Area Forecast Discussion National Weather Service Grand Rapids MI 317 PM EDT Tue Jun 18 2024 .KEY MESSAGES... - Small Risk for Stronger Storms into Late This Afternoon - Backdoor Cold Front may drop down from the north Thursday - Heat/Humidity Persists into Saturday - Cooler Weather Behind a Cold Front Sunday && .DISCUSSION... Issued at 240 PM EDT Tue Jun 18 2024 - Small Risk for Stronger Storms into Late This Afternoon Low level convergence has been relatively weak so for and the instability has been building steadily. Surface based instability ranged from around 1000 J/kg south to 2000 J/kg north with ML values about half those values. A mid level vort max was tracking through western parts of the CWA and that feature will push northeast over the next few hours. Based on the instability values and lift, a few storms could get going, primarily over eastern parts of the CWA in the next couple of hours. They could have the potential to produce locally stronger wind gusts, but even that risk looks low given the lack of DCAPE. On the backside of this mid level wave, NVA will generate some subsidence which would limit the storm risk for western parts of the region. - Backdoor Cold Front may drop down from the north Thursday This feature could keep temperatures down a few degrees Thursday, lessening the risk for heat related impacts. Ensemble output from the Canadian and GFS suggest the Grand Rapids area will likely fall short of 90 degrees for high temperatures Thursday but the 12z ECMWF still has central parts of the CWA reaching into the low 90`s. We will not change the headline for Thursday just yet, but confidence on reaching criteria has trended lower. - Heat/Humidity Persists into Saturday No changes to headlines planned. As noted previously, Saturday looks quite hot, perhaps even more so than the preceding days. This will be at the end of a prolonged and deleterious stretch of heat and humidity. Moreover, being the weekend, there will be even more exposure to the risk. All of this supports the notion of extending the heat advisory into Saturday - but as it currently is only Tuesday, we still more time to assess trends. - Cooler Weather Behind a Cold Front Sunday As noted previously, guidance is starting to converge on a solution that brings showers and thunderstorms through the forecast area Saturday night. As is often the case, the best upper PV forcing will be shifted a bit poleward of the best instability, so it`s unclear how strong convection will get. But, as I noted previously, the odds favor at least some strong/severe convection during this time given the pronounced airmass change. Predictability really falls off the shelf after Monday, although generally speaking, we can expect another round of above normal temperatures and precipitation chances right around climatology starting Tuesday. && .AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/... Issued at 146 PM EDT Tue Jun 18 2024 Main changes were to refine timing for thunder threat this afternoon and to include LLWS for MKG and GRR terminals. A north to south axis of showers and thunderstorms may develop in the 18-19Z timeframe, originating from an approximate AZO to GRR line and moving east towards the BTL/JXN/LAN terminals later this afternoon. Detail certainty is lacking so went with VCTS at the mentioned terminals for now. A distinct upper level disturbance swinging through the area will be responsible for this different low level wind profile overnight that justifies some LLWS. Additionally, there is less concern for precipitation overnight compared to the past couple of nights due to unfavorable upper level support. && .MARINE... Issued at 240 PM EDT Tue Jun 18 2024 The southwest winds at 925/850 mb are progged to increase this evening as the pressure gradient at those levels tightens. This will likely result in increasing southerly surface winds especially north of Grand Haven. Based on this we will maintain the headlines for small craft and swimmers in those locations. We will need monitor locations further south as conditions are likely to be become marginally hazardous. && .GRR WATCHES/WARNINGS/ADVISORIES... MI...Beach Hazards Statement through Wednesday afternoon for MIZ037- 043-050. Heat Advisory until 8 PM EDT Friday for MIZ043>046-050>052- 056>059-064>067-071>074. Air Quality Alert from midnight tonight to midnight EDT Wednesday night for MIZ050-056-057-064-071. MARINE...Small Craft Advisory until 4 PM EDT Wednesday for LMZ847>849. && $$ DISCUSSION...MJS/NJJ AVIATION...TJT MARINE...MJS