Area Forecast Discussion
Issued by NWS Twin Cities, MN
Issued by NWS Twin Cities, MN
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392 FXUS63 KMPX 150843 AFDMPX Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 343 AM CDT Sat Jun 15 2024 .KEY MESSAGES... - Widespread showers and thunderstorms likely today with locally heavy rainfall of 2"+ possible. - An intense heat wave will result in periods of hot and humid conditions and multiple rounds of thunderstorms, with the potential for severe weather and heavy rainfall. && .DISCUSSION... Issued at 339 AM CDT Sat Jun 15 2024 This is a very active forecast with three main stories to unfold: multiple chances for severe weather coming up, anomalously high rainfall totals leading to potential flooding, and our first summer-like heat wave. Severe Weather: There will be on and off chances for thunderstorm development through Tuesday, with the potential for some of these storms to be severe. With our setup today, we will see the current line of storms over eastern Nebraska push eastwards into Iowa. A broad shield of light precip will move through north of the stronger, convective storms into the southern half of Minnesota. As the rain gradually slides into Wisconsin, a couple hours of clearing could be enough to foster scattered thunderstorms over southern and east-central Minnesota in the afternoon/evening. The latest CAMS favor storm development around 5-7pm. These would gradually weaken and last through early tomorrow morning. Heading into tomorrow, a frontal boundary will track across MN/WI during the evening/night time frame. A strengthening LLJ, plenty of available moisture, and an EML with steep lapse rates along the stationary front, will lead to a conditional risk of strong to severe thunderstorm development. The primary threats would be damaging wind and large hail. On Monday and Tuesday, northwestern and central Minnesota will see additional chances for t-storms. Strong instability aloft will again be favorable for large hail in the strongest storms, and any organized clusters could produce damaging wind gusts. Rainfall Totals: Following collaboration with neighboring offices and the WPC, we have highlighted a broad area of slight/marginal risk for heavy rain leading to flooding concerns across portions of Minnesota/Wisconsin for the next five days of the Excessive Rainfall Outlook. Take your pick of any ensemble or deterministic model output, and you`ll find PWAT values of 2+ inches as the LLJ helps crank the Gulf moisture into the northern Great Lakes region later today. This is expected to continue throughout much of this week, with little time for relief from persistent rains. The latest suite of ensemble runs are largely in agreement of a maximum QPF trail stretching from eastern NE/SD up through northwestern WI throughout the next 7 days. Values in excess of 8-10" could be possible depending on the exact location of the heaviest rainfall totals each day. To put that into perspective, that would be nearly a 1/3 of our average annual rainfall... in one week. Because of this, flooding will be a big concern especially given the already wet/active spring we`ve had. Heat Wave: There is growing confidence in heat risk concerns heading into next week as an upper-level ridge builds eastward, bringing anomalously high h500 heights to the eastern half of the CONUS. Surface temperatures on Sunday will creep into the lower 90s, with dew point values in the low-mid 70s. This will result in heat indices up to 95-100 degrees for much of the area. Given the fact that it will be the first hot and humid day of the year, we want the public to be aware and prepared. The Excessive Heat Watch for the Twin Cities metro highlights the area that will most susceptible to heat impacts due to the urban effects. Little relief will follow with overnight lows in the low 70s, and highs reaching the mid-upper 80s both Monday and Tuesday most likely. && .AVIATION /06Z TAFS THROUGH 06Z SUNDAY/... Issued at 1215 AM CDT Sat Jun 15 2024 Mid-high level cloudiness will continue to push in from the west overnight, as our rainmaker approaches NE from Nebraska this morning. Expect light rain associated with the initial precip shield to move into SW Minnesota early this morning (around 12z), and then gradually make its way northeastward throughout the morning. Winds will increase out of the southeast after sunrise and will generally persist at 10-15kts, gusting up to 20-25kts into the afternoon. Additional showers and thunderstorms will be possible heading into the afternoon/evening, which could result in lower visibilities at times. KMSP...No major changes to the forecast from 00z. Light winds from the southeast this morning, with increasing speeds heading into the afternoon. Light showers will reach the terminal by around 14z, becoming more persistent by about 16z. Rain will prevail much of the day, with an increasing chance for embedded t-storms to develop during the evening. Things will start to taper off heading into tomorrow morning, but lingering rain is likely until about 10/11z. /OUTLOOK FOR KMSP/ SUN...VFR. Slight chc TSRA/MVFR. Wind S 10-15 kts. MON...VFR with MVFR/TSRA possible. Wind SE 10-15G20 kts. TUE...VFR with MVFR/TSRA possible. Wind S 10-15G25 kts. && .HYDROLOGY... Issued at 315 PM CDT Fri Jun 14 2024 The extended period of heavy rainfall brings with it a potential return to river flooding in the 5-10 day period, extending to the end of the month on the mainstems. HEFS graphics show the potential threat nicely, and also show the degree of uncertainty with large ranges even within the blue "most likely" band by day 10. Factors that will determine flood severity (minor vs. moderate or worse) include how fast the rain falls...a number of 6-12 hour periods with less than an inch adding up to 3-5 inches is better than a couple of 2-3 inch events in a short period. We will also need to be wary of a boundary setting up with training storms moving from southwest to northeast over the same area. In summary, this scenario bears watching closely over the next week, with lots of uncertainty along the way. && .MPX WATCHES/WARNINGS/ADVISORIES... MN...Excessive Heat Watch from Sunday afternoon through Sunday evening for Anoka-Carver-Dakota-Hennepin-Ramsey-Scott- Washington. WI...None. && $$ DISCUSSION...BED AVIATION...BED HYDROLOGY...CCS