Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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816 FXUS63 KARX 191737 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1237 PM CDT Thu Sep 19 2024 .KEY MESSAGES... - Scattered showers and storms this morning, especially west of the Mississippi. Higher chances (70 to 90%) for storms this afternoon and evening. A slight risk (level 2 of 5) for severe weather is in place for much of the area. Main hazards are large hail and damaging winds. - Above normal temperatures continue through Saturday before cooling down to near or slightly below normal temperatures. - Chances of rain for Saturday and Sunday as next system moves into the region. && .DISCUSSION... Issued at 315 AM CDT Thu Sep 19 2024 Today: Scattered AM Showers/Storms, Strong to Severe PM Storms A low pressure system moving into southern Saskatchewan and Manitoba will be the main synoptic focus for today. An axis of stronger moisture transport shifts into the Upper Midwest early this morning as a shortwave riding along the southern edge of the main low pressure system moves eastward. As it does so, this shortwave will strengthen a little allowing for so showers and thunderstorms to form in the early to mid-morning period. Areas along and west of the Mississippi will have the best chance at seeing these showers and thunderstorms this morning. Moisture transport is expected to decrease into the afternoon resulting in the showers and thunderstorms diminishing. After the aforementioned storms diminish, there will be a lull in the action for a few hours. By mid-afternoon, a cold front oriented north to south across Minnesota and Iowa and into the Central Plains, will slide eastward. Ahead of this front, a corridor of 1000 to 2000 J/kg of SBCAPE will exist. Strong deep-layer (0-6km) shear of 35 to 45kts will be perpendicular to the front orientation. This will allow the potential for supercells to form initially in the late afternoon before becoming more linear in the evening. With the initial onset of storms potentially being supercells, the main threat will be hail. As storms become a little more linear and cold pools congeal, wind will become the primary threat for the evening. The tornado potential is more conditional and may occur early on with discrete convection or as storms become more linear, QLCS tornadoes could be possible. With this severe weather potential, SPC has continued the slight (level 2 of 5) risk for severe weather across much of the forecast area. Another threat to watch out for, due to moisture transport picking back up again during the afternoon and the slower storm motions, is the potential for locally heavy rainfall. With how dry we have been, flooding does not look to be much of a concern, however if any impacts are felt, they would be mainly in the urban and small stream locations. The HREF probability matched mean QPF shows a broad area of 0.5 to 1.5" along and north of I-90, with some smaller pockets of 0.5 to 1.5" along the line of storms in northeast Iowa and southwestern Wisconsin. During the overnight, the main low pressure system continues to lift northeastward and the main severe threat diminishes, however showers and storms will continue on and shift eastward and be out of the forecast area between 06 and 08Z. Friday through Middle of Next Week: More Showers and Storms Weak ridging returns for Friday with high temperatures remaining above average in the mid 70s to mid 80s. Heading into the weekend, we get into zonal flow, allowing for a couple of shortwaves to move into the area. The first comes through Saturday bringing about the first chance of showers and storms to occur. The next chance is Saturday evening into Sunday. Looking at ensemble and deterministic guidance, this shortwave appears a little stronger than the first. This has resulted in higher PoPs (50 to 70%) for Saturday night into Sunday morning. By Monday and Tuesday an area of low pressure moves through the Central Plains and into the Upper Midwest. There remains uncertainty in the exact path the storm system takes. Comparing the GEFS and EPS, the GEFS is wetter than the EPS for early next week. This is seen when looking at the 25th and 75th percentiles for 24 hour precipitation. For example, on Sunday afternoon, GEFS 25th to 75th percentiles are roughly between 0.5 and 1.0" whereas the EPS percentiles are between 0 and 0.5". Looking into this more, EPS members have around 20 to 30% of them being dry through the weekend, whereas almost all GEFS members have measurable precipitation through the weekend. After Tuesday, uncertainty in precipitation chances exist as we remain in zonal flow and then northwest flow by the middle of next week. This may allow for weak shortwaves to pass through the area. Due to this uncertainty, most PoPs are between 20 and 30% from Tuesday through Thursday. && .AVIATION /18Z TAFS THROUGH 18Z FRIDAY/... Issued at 1236 PM CDT Thu Sep 19 2024 Thunderstorms will be the main aviation concern this evening with IFR category reductions possible at times. As a result, have included a prevailing MVFR conditions with a tempo for IFR visbys where confidence is highest on timing. Winds may shift to westerly with strong wind gusts being possible as storms move through. With convective initiation still 4-5 hours away, likely will need to make forecast adjustments prior to the 00z issuance depending on convective trends. Storms move eastbound through the region during the evening and eventually exit towards midnight. As a front passes through the region, cooling overnight temperatures and recent rainfall in combination with clearing skies will create a fairly favorable fog/low-stratus setup into the morning hours on Friday. However, there remains some questions with how some stronger winds to around 20 kts in model soundings (recent HRRR/RAP/NAM) just off the surface will impact any fog/low-stratus formation. Consequently, held with the 3SM but will likely introduce IFR/LIFR reductions in subsequent issuances as forecast confidence increases. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Cecava AVIATION...Naylor