Area Forecast Discussion
Issued by NWS Twin Cities, MN
Issued by NWS Twin Cities, MN
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761 FXUS63 KMPX 182053 AFDMPX Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 353 PM CDT Wed Sep 18 2024 .KEY MESSAGES... - More active weather over the next couple of days. Potential for very large hail exists with Slight Risk (level 2 of 5) Thursday afternoon. - Uncertainty remains high with rainfall forecast this weekend into early next week. - Cooler air still anticipated next week, with highs retreating back into the 60s and 70s (close to normal) && .DISCUSSION... Issued at 348 PM CDT Wed Sep 18 2024 Upper low on satellite over eastern Montana will be the key player with our weather over the next couple of days. By Friday morning, this feature will be near western Hudson Bay in northeast Manitoba. This will drive the cold front currently over the central Dakotas across our area on Thursday, moving across western MN in the morning, crossing the I-35 corridor in the afternoon, then washing out overnight in western to central WI. We have a band of cloud cover and a few embedded showers ahead of a lead shortwave approaching central MN this afternoon that will drive some virga/shower potential as it works into western WI this evening. Anything we get out of this lead band is not expected to be severe. The severe potential for today resides with what, if anything, we get out ahead of the front in SoDak where mesoanlysis shows 1500-2000 j/kg of mlCAPE exists. Models continue to struggle with if/where storms will develop in this warm sector this afternoon, but given the instability with 30-40 kts of bulk shear, severe storms will be possible with any activity that develops in that warm sector. The uncertainty in convective coverage this afternoon carries on through tonight with CAMs showing anything from nothing happening tonight, to a shield of showers and storms working across the MPX area tonight through the morning. Given the uncertainty, held PoPs through the night at 50% outside of central MN. Thursday looks to be our turn for severe weather. For mid- September, we have a higher end severe environment forecast to be in place. Mid-level lapse rates are forecast to steepen overnight ahead of the front. By Thursday afternoon, dewpoints in the upper 60s with these lapse rates will support mlCAPE of 2000-3000+ j/kg. For Thursday it`s not a question of if, but when/where storms initiate. Storms are expected to develop between 19z & 21z near the I-35 corridor in an environment supportive of supercells and the potential for very large hail in particular. Storms are expected to become more linear as the threat moves deeper into WI. The Day 2 update did nudge severe probabilities farther east, but an upgrade to an Enhanced risk (level 3 of 5) for the severe risk on Thursday is not completely out of the question given the hail potential in the metro area. Dry and sunny weather is expected on Friday and that sun is why most of our area is forecast to see highs nudge up into the low 80s once again. For the weekend, uncertainty abounds. There will be two waves to watch, a northern stream wave going across southern Canada on Saturday, then a wave coming up out of the 4-corners late this weekend. That northern stream wave will push a cold front across MN on Saturday. WAA ahead of the front is why Saturday suddenly has fairly widespread highs in the 80s once again. The forcing with this front will be well to our north, while the moisture will be locked up with the southern stream wave coming at us for the end of the weekend. As for that second wave, uncertainty abounds for how far north it will track and how quickly it moves, with the large model spread explaining why the NBM continues to have 30/40 pops spread out across Sunday and Monday. With that spread, we`re still in a situation where we could get very little rain, or a good soaking 1-1.5 inch rain event. Where there is more confidence is that between the Saturday front and this system Sunday/Monday, we`ll see highs dip a little bit below normal to start next week. && .AVIATION /18Z TAFS THROUGH 18Z THURSDAY/... Issued at 1241 PM CDT Wed Sep 18 2024 VFR conditions continue through this afternoon, with breezy winds out of the southeast gusting to 20-25 kts. The low-level jet will strengthen across south central Minnesota tonight, which will introduce scattered convection to the forecast. Made slight tweaks to the ongoing PROB30s to keep a 3-4 hour window of -TSRA potential at each terminal. Confidence is highest in convection across the western Minnesota terminals, with lower confidence in whether or not storms will persist into western Wisconsin Thursday morning. Given the position of the low-level jet, have included mention of WS at the Minnesota terminals tonight. Additional chances for -TSRA will likely be added to the end of the 24-hour TAFs in a forthcoming issuance. KMSP...Maintained the PROB30 for -TSRA between 8-12z. Latest guidance remains on the marginal side for wind shear, though decided to keep it in the TAF after collaborating with ZMP CWSU. Convection is forecast to redevelop across eastern Minnesota Thursday afternoon, so have introduced another PROB30 for -TSRA between 20-23z tomorrow. /OUTLOOK FOR KMSP/ FRI...VFR. Wind WNW 5-10kts. SAT...VFR. Chc SHRA/TSRA. Wind SE 5-10kts. SUN...VFR. Chc SHRA/MVFR. Wind NE 5-10 kts. && .MPX WATCHES/WARNINGS/ADVISORIES... MN...None. WI...None. && $$ DISCUSSION...MPG AVIATION...Strus