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Area Forecast Discussion
Issued by NWS Twin Cities, MN
Issued by NWS Twin Cities, MN
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821 FXUS63 KMPX 140805 AFDMPX Area Forecast Discussion National Weather Service Twin Cities/Chanhassen MN 305 AM CDT Wed Aug 14 2024 .KEY MESSAGES... - Widespread rain will expand from west to east today. Heaviest rain is expected late this afternoon into tonight when storm potential is greatest. - Lingering showers are possible to start the weekend before we resume our dry and mild stretch of weather through next week. && .DISCUSSION... Issued at 258 AM CDT Wed Aug 14 2024 We`ve been talking about it, and it`s finally here. The long awaited system after our stretch of quiet weather is on the doorstep of southwestern MN. The main threats from this system will be prolonged rainfall with heavy rates and localized flooding possible at times. There is a marginal chance for severe weather, mainly as damaging winds, but isolated large hail and a tornado or two cannot be ruled out. Let`s break it down... Timing and Coverage... Recent CAMS have started to come into better agreement, with the general idea of light to moderate showery precip blanketing southwest MN by 9-11am. Current radar over eastern Nebraska suggests that both the NAMNest and HRRR may be slightly slow (by an hour or so). The 00z HRW Fv3 seems to have the best handle on timing and coverage so far, so I`ll be favoring that in the discussion here. Showers and a few isolated non-severe storms will look to move into eastern Minnesota by the mid-late afternoon, and western WI by early evening. There continues to be uncertainty with how far north the initial wave of precip will progress. The Fv3 has widespread showers moving into north- central MN during the afternoon, however many of the other models keep precip along and south of a line extending from Stevens Co, MN to Rusk Co, WI. Scattered to widespread showers and storms will continue into Wednesday night as the surface low develops and gradually shifts NE through Minnesota and northwestern WI. Portions of western WI could wake up to widespread showers Thursday morning, otherwise the bulk of the rain should have moved well north and east of the area. Much more isolated to scattered rain is expected on the backside of the low during the day Thursday. Rain chances fall off quickly by Friday AM, as the surface low finally makes its way east further into the Great Lakes region. Heavy Rain and Flooding Potential... Forecast soundings show a deep, saturated warm layer extending from roughly 1-1.5kft to 12-13kft, weak instability at best, and PWAT values approaching 2". These all favor an environment for high rain rates and efficient rainfall production. Storm total QPF values (over nearly 36 hours) continue to stay around 0.75-1.5", with the highest amounts falling late this afternoon into tonight when the storm potential is greatest. The highest probabilities of exceeding 1.5" (roughly 40-60%) land within a line extending from Sioux Falls, SD up through the Twin Cities metro and all way NE to Rice Lake, WI. Severe Potential... Continues to look low, BUT the best timing and location looks to be over southwest MN tonight or southern MN tomorrow morning. While there should be sufficient shear and forcing, the biggest limiting factors will be the lack of instability and the deep saturation throughout the atmosphere (lack of dry air). Some models, such as the HRRR, entertain the idea of a more organized line of convection developing somewhere over the Dakotas that attempts to progress SE along a boundary of 500-1500 J/kg MUCAPE. It is tough to say how far east this line would progress into MN since the CAPE values really start to decline across the border, however if it holds together, there could be marginally severe weather of any threat (wind, hail, tornado). A northward extension of the LLJ overnight into Thursday could also be enough to support isolated storms, mainly over southeastern MN. Weekend and Beyond... This weekend should see a return to pleasant conditions again, with weak N/NW winds, partly cloudy skies, and highs in the low 80s. Both the EPS and GEFS H500 height anomalies hint at a relatively quiet upcoming week, with persistent ridging over the central CONUS. A shortwave or two may be enough for a few tenths of rain mid-week, but nothing too impactful seems to be on the horizon following our system today/tomorrow. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1242 AM CDT Wed Aug 14 2024 Initialized with VFR and light winds tonight, but patchy fog has already developed in western WI (specifically EAU). Fog may be possible at other sites outside of MSP but confidence not high enough to include at other sites at this point. Will continue to monitor through the early morning hours for possible TAF updates. The main focus of these TAFs remains the rain for Wednesday evening through Thursday. Rain will develop from southwest MN and progress eastward. As rain starts, a drop to MVFR is expected and eventually some IFR is expected as well. The coverage of IFR is still uncertain, but confidence has increased such that IFR ceilings were added for western and central MN sites. Rain chances will continue past the end of the current TAFs with degraded conditions likely continuing into Thursday morning. KMSP...Initialized with VFR conditions and light winds which will persist through early Wednesday afternoon. Conditions then deteriorate late Wednesday afternoon onward as the rain shield moves into eastern MN. Still a little bit of wiggle room as to the start time so have maintained the PROB30 mention for a potential of rainfall to move in prior to 00z but confidence increases significantly from 00z onward that rain will move across the MSP airspace. Low confidence on CB/TS so have omitted such mention at this point. Regardless, MVFR conditions at best will persist through Wednesday night, possibly dropping to IFR by Thursday morning. Will see how the model guidance develops such a scenario for later TAFs. /OUTLOOK FOR KMSP/ FRI...VFR, chc MVFR/-SHRA. Wind NW 10-15 kts. SAT...VFR. Wind NW 10 kts. SUN...VFR. Wind NW 5-10 kts. && .MPX WATCHES/WARNINGS/ADVISORIES... MN...None. WI...None. && $$ DISCUSSION...BED AVIATION...JPC