Area Forecast Discussion
Issued by NWS Marquette, MI
Issued by NWS Marquette, MI
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312 FXUS63 KMQT 202231 AFDMQT Area Forecast Discussion National Weather Service Marquette MI 631 PM EDT Mon May 20 2024 .KEY MESSAGES... - Strong storm system will bring rain and thunderstorms late Tuesday into Tuesday night. - Windy conditions develop behind the system on Wednesday. && .SHORT TERM /THROUGH TONIGHT/... Issued at 148 PM EDT Mon May 20 2024 Light rain showers continue across the U.P. and end from west to east this afternoon as a low pressure continues to travel from southern Wisconsin/northern Illinois to northern Lower Michigan the rest of today through tonight. With the SPC mesoanalysis page showing pathetically low lapse rates and CAM soundings showing almost no CAPE to work with, expect at most just a rumble or two of thunder across the central and east the rest of today. With the light rain taking its time to get into the far eastern U.P. this afternoon, min RHs have dropped down to as low as 40% at the Newberry airport. However, now that RHs are beginning to recover, expect the upward trend to continue. Likewise, with light southerly winds dominating today, the warmest temperatures thus far have been near the Lake Superior shoreline, with some spots even getting into the upper 60s. Expect the U.P. to be pretty rain-free by tonight as very localized and weak sfc high pressure moves across the area. As this occurs, we could see some areas of fog and low-level cloud cover develop across the entire Upper Peninsula; with visibilities dropping down to 1/4 mile or less in some areas, be careful if driving late tonight and early Tuesday morning. With the areas of cloud cover across the area tonight, low temperatures are expected to get to around 50. While there was a low chance earlier of a dying MCS moving into far western Upper Michigan late tonight, the CAMs have trended away from this solution entirely now; at most, we will see some light rain showers move into the far west near Ironwood late tonight/early Tuesday morning now (and even that is a fairly low (10%) chance). && .LONG TERM /TUESDAY THROUGH MONDAY/... Issued at 545 PM EDT Mon May 20 2024 Persistent troughing in the vicinity of Alberta, Canada sets the stage for a deepening surface low tracking northeast from western Kansas to near Duluth, MN by early Wednesday morning. Warm air advection ahead of this system may support a few strong to severe storms late on Tuesday, especially closer to Lake MI. An impressive pressure gradient combined with steep low-level lapse rates south of the low pressure supports potential for a windy Wednesday. Cooler and quieter weather is expected for Thursday into Friday with the next weather maker moving in Friday night or Saturday. Beginning Tue, an energetic and negatively tilted shortwave lifts northeast across the Plains resulting in a sub-990 mb low over MN Tuesday evening which is at or near the minimum of CFSR climatology. This unseasonably deep low pressure is forced by strong upper divergence from a coupled upper jet structure (right entrance region of a 130 kt jet max over northern Ontario and the left exit region of a 110 kt jet max over the Southern/Central Plains). As a result, 12hr 500mb height falls of 150-180m predicted by the models over the Upper MS Valley/Upper Great Lakes region late Tue night/early Wed. This impressive cyclone results in two waves of weather hazards 1) potential for strong to severe thunderstorms along/ahead of the cold front Tuesday evening and overnight then 2) potential for a windy Wednesday as the system departs the area. Regarding the thunderstorm potential, SPC continues to outlook our entire area in a marginal/slight risk for severe thunderstorms. HREF guidance shows a cluster of storms developing near the IA/NE state line Tuesday afternoon that rapidly tracks northeast across the Upper MS River Valley. HREF means indicate a narrow corridor of MUCAPE increasing to 500-1000 J/kg immediately ahead of the cold front with bulk shear increasing to near 50 knots. In general, I`d like to see more CAPE to better balance the strong environmental shear, especially considering the unfavorable diurnal timing around or after sunset. Additionally, E-SE flow ahead of the front is not favorable for severe storms due to the stabilizing maritime influence from the still cold Great Lakes implying exclusively elevated storms. Despite all the above, there`s a chance for a few strong to potentially severe thunderstorms that may pose a threat for large hail or damaging winds especially closer to Lake MI where instability is greater. Moderate to locally heavy rainfall is also possible across Upper MI during the period of strong isentropic ascent/strong 850mb theta-e advection to the e of the low track Tue night. Precipitable water also increases to around 1.5 inches or near the climatological daily max for the day at ~200pct of normal. The biggest factor working against heavy rainfall is the rapid system motion with ensemble means suggesting only localized 24-hour amounts greater than 1 inch. Shower coverage diminishes significantly for Wed as we get into the dry slot and q-vector divergence/subsidence behind the system as it lifts north. Some areas may see dry weather for much of or all of the day, especially e half. The biggest impacts from this system may end up being strong winds. ECMWF EFI/Shift of Tails continues to increase into Wed with Upper MI mostly around 0.9 now, suggesting an unseasonably strong wind event based on last 20 years of model climate. The EPS probabilities essentially shows a 80-100% chance that the entire fcst area will see wind gusts exceeding 40 mph Wed afternoon into Wed evening with a 10-30 pct chance of western U.P. higher terrain areas seeing high wind warning criteria gusts of 50 mph or greater. While winds for this fcst were increased from what NBM indicates, they are not as high as the median from the EPS (40-55 mph) and will likely need to be bumped up more. There is still time to reevaluate for higher winds in later fcsts as wind direction will be important for identifying the windiest spots, but at this time the strongest winds are expected across the far western UP. Continued to mention the potential for higher wind gusts on Wed in the HWO product. As the low lifts across northern Ontario toward James Bay on Thu, isolated showers will remain possible under lingering cool, cyclonic flow. Thu should be the coolest day of the week with highs in the lower 50s to the lower 60s F warmest s central. Another shortwave trough lifting through the Plains into central Canada late week will send a sfc low pressure trough/frontal boundary into Upper Mi late Friday/Fri night which could linger over the area into Sunday bringing yet more opportunities for showers. && .AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 630 PM EDT Mon May 20 2024 KIWD will stay MVFR until early morning when IFR conditions set in through Tue morning. KIWD will see MVFR conditions return by Tue afternoon. KCMX will go from VFR to IFR later tonight and then back to MVFR by Tue morning and then VFR for most of Tue afternoon. KSAW will be MVFR this evening and then briefly VFR before fog and IFR conditions move into the site later tonight and lasting into Tue morning. KSAW will then have VFR conditions in the afternoon. && .MARINE... Issued at 545 PM EDT Mon May 20 2024 Weak pressure gradients will keep winds less than 20 knots across Lake Superior through Tuesday morning. However, an unseasonably strong low pressure system approaches Lake Superior Tuesday afternoon and evening resulting in rapidly increasing northeast winds across western portions of the lake. Northeast winds increase above 20 knots midday Tuesday over the far western lake then increase to 35-40 knot gales by Tue evening (60-90% chance). Winds may increase further Tuesday night into Wednesday when our local probabilistic tool indicates a 30% chance of gales exceeding 40 knots over the western and north-central portions of the lake. Confidence is lower that gales will occur across the eastern half of Lake Superior, but a considerable pressure gradient combined with uncertainty in the low pressure track indicates it`s too early to drop the gale watch. Low level stability further reduces forecaster confidence as a very warm air mass flows across the still cold lake water implying strong gusts at elevated platforms. Despite the strong stability depicted over the lake on model soundings, the strong pressure gradient should be sufficient for gales near the sfc of the lake especially at higher elevation observing platforms. In short, decided to continue the Gale Watches for LSZ162 21Z Wed-00Z Thu, for LSZ263-264 02Z Wed-06Z Thu and for LSZ265>267 06Z Wed-21Z Wed. It is worth noting that some of the models that track a deep low just w of Lake Superior into northern Ontario show a potential for higher end westerly gales across the lake Wed into Wed night, especially across the w half. Will be something to monitor. At any rate, west-northwest winds should gradually diminish to 20 to 30 knots late Wed night into Thu and fall generally to 20 knots or less late Thu night into Friday while gradually becoming easterly. && .MQT WATCHES/WARNINGS/ADVISORIES... Upper Michigan... None. Lake Superior... Gale Watch from Tuesday afternoon through Wednesday evening for LSZ162. Gale Watch from Tuesday evening through late Wednesday night for LSZ242>244-263-264. Gale Watch from late Tuesday night through Wednesday evening for LSZ245>251-265>267. Lake Michigan... None. && $$ SHORT TERM...TAP LONG TERM...EK AVIATION...07 MARINE...EK