Area Forecast Discussion
Issued by NWS Duluth, MN
Issued by NWS Duluth, MN
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649 FXUS63 KDLH 191721 AFDDLH Area Forecast Discussion National Weather Service Duluth MN 1221 PM CDT Thu Sep 19 2024 .KEY MESSAGES... - Showers and thunderstorms linger through the current overnight hours into mid-morning. - This afternoon into the evening, a second round of thunderstorms are expected to initiate along and ahead of a cold front, with scattered severe storms (Level 2 out of 5) possible. Large hail, damaging winds, a couple tornadoes, and locally heavy rain may occur. - Seasonable temperatures return through the weekend and into next week, with additional shower and storm chances (30-50%) on Saturday and periodic low-end rain chances (20-30%) next week. && .DISCUSSION... Issued at 401 AM CDT Thu Sep 19 2024 Current Conditions - This Morning: Elevated (i.e. not surface-based) scattered showers and storms have developed overnight in a warm air advection regime in conjunction with forcing for ascent from a 40 kt low-level jet over central and north-central MN. This activity is occurring in the warm sector of a mature, vertically stacked low-pressure system centered over far southeastern Saskatchewan with a cold front draped south across the central Dakotas. The storms are in an environment of effective bulk shear around 30 kt and some MUCAPE around 1000-1500 J/kg, leading to a mixed mode of pulse/clustered storms. While severe weather is not expected the remainder of the current overnight hours, a few of these storms could produce small hail (pea size to half-inch) and some locally gusty thunderstorm winds to 30-40 mph. While showers and storms should continue into the morning hours as convection moves northeast through north-central and parts of NE MN, the low-level jet will be weakening. Therefore the converge and intensity of storms should reduce into mid to late morning. Lingering low and high level cloud cover in the wake of this morning precipitation may lead to some impacts on the severe weather potential this afternoon and evening, primarily when storms would initiate. Severe Storms this Afternoon - Evening: As the aforementioned vertically-stacked low continues trekking northeast into southern/central Manitoba this afternoon and evening. As it does, the cold front will advect east into north-central MN by the afternoon and north-eastern MN by early evening. While broken cloud cover will likely be ongoing through much of the day, CAM guidance and model soundings depict an atmospheric environment with steep low-mid level lapse rates and strong moisture advection of PWATS of 1.5-1.7 inches near the daily maximum values of sounding climatology for mid- September should still be sufficient to produce 1500-3000 J/kg of MLCAPE, albeit in a somewhat narrow corridor. Combine these parameters with 30-40 kt of 0-6km bulk shear mainly perpendicular to the cold front, and this sets the stage for strong to severe storms to develop around the 3-5 PM timeframe (as CAMs still suggest) as initial discrete/supercellular storm modes before transitioning into several clusters/broken line of storms during the evening. Given this expected time of storm development and the cold front position at that time, the model trends continue to point to storms developing east of Brainerd to International Falls. For specific severe storm hazards, the initial supercell mode would briefly support hail up to around ping-pong size hail or slightly larger during the late afternoon before growing upscale into the broken line/clusters of storms into the evening, at which point damaging wind gust threat will be increasing and the large hail threat will be decreasing. There is also the potential for a tornado or two during the late afternoon into early/mid evening for storms that remain surface-based due to 0-1km storm relative helicity of 100 to locally 200 m2/s2. This severe threat should diminish into the mid to late evening hours as storms track east into the tip of the MN Arrowhead and north-central WI with the loss of daytime heating and waning instability. In addition to the severe threat, the high moisture environment will also support the potential of locally heavy rain due to quick downpours/high rain rates. While storms will be progressive enough to limit training storms and or any more widespread heavy rain threat, localized rainfall amounts of 1-2" are possible, with the highest amounts possible in NW WI where a WPC Marginal Excessive Rainfall Outlook (Level 1 out of 5) is forecast. Rain should exit east of the Northland Thursday night. Friday - Next Week: Despite cyclonic flow behind the cold front on Friday, a drier airmass in place should keep precipitation chances at bay for the Northland. Ensemble guidance does show some good agreement in a mid/upper-level trough skirting the International Border/clipping the Northland on Saturday into Saturday night, bringing a cold front eastward across the Northland with it. Some shower/storm chances (30-50%) are forecast along and ahead of the front. There is some uncertainty as to how much instability/daytime heating will occur ahead of the front, but Saturday afternoon and evening may need to be watched for a low-end severe threat (5% severe chance). Temperatures on Friday and Saturday will still be above-average for late September, with widespread highs in the 70s to near 80 degrees. A more seasonal airmass is forecast in the wake of Saturday`s cold front, with widespread highs in the 60s and nightly lows in the 40s on Sunday through at least the middle of next week. During this timeframe, there will also be a somewhat active upper-level pattern of a series of shortwave troughs that could keep rain chances periodically in the Northland next week (20-30% chances for now), though there will be dry time between rounds. This pattern doesn`t look like a washout by any means, but global ensembles quickly diverge with regards to what the upper-level pattern will look like mid-late next week, so more specific details are fleeting at the moment. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 1220 PM CDT Thu Sep 19 2024 Bit of a mix of conditions early this afternoon with an area of MVFR stratus moving along a frontal region while clearing skies persist outside of that. Should see showers and thunderstorms ignite along this front late this afternoon and push east. Brainerd is not likely to see this activity as the front is already past the terminal. A similar situation for INL though that terminal may be on the edge of some thunderstorm activity. Expect strong to severe thunderstorms as the front passes with brief reductions in visibility due to heavy rain. Hail and strong winds are also possible with passing thunderstorms. The front should clear HIB and DLH by 01-03Z and HYR by 03-05Z. Have tried to add tempo groups for the most likely thunderstorm time at these sites. Conditions improve back to VFR for all MN terminals post front. At HYR, MVFR to IFR fog is possible before VFR conditions return around sunrise. && .MARINE /FOR NEAR SHORE WATERS OF WESTERN LAKE SUPERIOR/... Issued at 401 AM CDT Thu Sep 19 2024 Southeasterly winds along the South Shore are expected today will become onshore for the Twin Ports and North Shore, with some gusts to and become onshore for the Twin Ports and North Shore again today, with some gusts of 10-15 knots possible for the North Shore and 15-20 knots for the Twin Ports and portions of the South Shore. Rain showers and thunderstorms are likely today first during the late morning and early afternoon, and then the better potential in the afternoon to evening. The afternoon to evening storms could produce strong wind gusts and large hail. Breezy southwesterly winds develop for Friday behind the cold frontal passage late Thursday, with gusts up to 15-20 knots before turning light Friday night and backing to the southeast. For the open water discussion, refer to the NWS Marquette Area Forecast Discussion at weather.gov/mqt. && .DLH WATCHES/WARNINGS/ADVISORIES... MN...None. WI...None. MARINE...None. && $$ DISCUSSION...Rothstein AVIATION...Levens MARINE...Rothstein