Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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502 FXUS63 KARX 211128 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 628 AM CDT Sat Sep 21 2024 .KEY MESSAGES... - Showers and storms today into tonight. A few could be strong later this afternoon/evening. Pcpn chances shift southward later tonight and Sunday. - One more warm day today then much cooler (below normal) temps flow in for Sunday, continuing for the first part of the new week. && .DISCUSSION... Issued at 305 AM CDT Sat Sep 21 2024 * CURRENTLY Cluster of showers/storms continue to spark over northern MN, mostly in response to evaluated Fgen with impingement of low level moisture transport. This activity should continue through the overnight, but RAP suggests a weakening in the lift after sunrise, so could see that area of showers wane. Locally small chances could graze far northern parts of the forecast area. * TODAY/SUNDAY: shower & storm chances. Warm today, turning much cooler for Sun. A trough in the northern branch of the upper level flow is on track to slide west-east along the Canadian/U.S. border today. While the bulk of its energy will hold north, short term models all suggest a piece will stray farther south, moving across MN into central WI tonight. To the south, bits of shortwave energy are progged to eject northeast out of trough over the desert southwest, sliding across IA and southern WI/northern IL by this evening. This train of ripples in the flow are expected to persist through Sunday as the parent shortwave lifts over the mid Mississippi river valley but gains more of a positive tilt (some weakening). At the sfc, a cold front will drop southeast across the region this afternoon/evening, accompanying the northern shortwave wave. Ample forcing, although fairly disjointed with no phasing of northern/southern features evident. A broad swath of low level moisture transport will slide ahead of the sfc front with PWs of 1 1/2+". RAP SBCAPE builds to 2+K J/kg in the afternoon, but some low level capping ahead of the front to contend with (per RAP bufkit soundings). The deep layer shear lies westward of the cold front (and west of the instability axis), but soundings suggest enough to help foster updraft development. CAMS models depict a mixed bag when it comes to how/where the convection will spark/develop. Some develop shower/storms by mid/late morning over southern MN, in response to a ripple in the upper level flow pushing out of the northern plains. This results in pcpn chances moving in by late morning, persisting through the afternoon. Meanwhile the HRRR and RAP focus their chances on the cold front, which would delay the showers/storms until late afternoon. However, both models are doing poorly with ongoing convection. It`s a messy setup. Confidence is high for at least scattered convection today into tonight but lower with where/timing for where initiation is more likely. Will lean into the model blend for the moment but expect revisions to the forecast as we move through the day. A few stronger storms will be possible and cannot rule out an isolated severe. Later tonight into Sunday the focus for pcpn shifts to the south and the train of shortwave energy riding along the exiting cold front. Mostly showers as the instability axis will be well south, with chances generally along/south of I-90. Expect the chances to continue to shift south Sun night with the exit of the front and upper level forcing. * NEW WORk WEEK: cool start, return to normal temps toward the weekend? Trending dry. Long range guidance suggests a bit of upper level energy will shift eastward, just south of the local forecast area Tue, while a shortwave trough in northern branch of the flow transitions west-east across southern Canada/northern parts of the region. Locally, saturation is mostly confined to the mid/upper levels, negatively impacting pcpn potential. The Grand Ensemble of models only paints a 0-15% chance for either bit of energy to drop measurable rain across the forecast area. That said, some recent deterministic runs of the GFS take that northern shortwave and dig it over the mid mississippi river valley, developing a closed 500 mb low that would then meander back northeast, impacting the region with widespread rains and more cool air. However, when compared with the GEFS suite of ensemble members there are only 2 to 3 that harbor similar outcomes. EPS members also have only a few in this bucket, with overwhelming support for a drier outcome. WPC cluster analysis - the 4 clusters don`t show much/if any potential for that closed low. For now, will view that possible outcome as an outlier. Going to keep an eye on it, but not putting much stock in it (yet). For temps, GEFS/EPS favoring bringing in the west coast ridge moving into the latter half of the new week. While the spread in temperature outcomes grows, the upper 75% of both models` ensemble members return highs into the 70s - back to the seasonable normals. && .AVIATION /12Z TAFS THROUGH 12Z SUNDAY/... Issued at 628 AM CDT Sat Sep 21 2024 Scattered showers and thunderstorms will be possible this afternoon and evening as a cold front moves across the area. With this front, CIGS are expected to drop to between 4kft and 8kft and southerly winds will switch to northwesterly winds. Precipitation will continue to be possible into the overnight period, especially for areas along and south of I-90. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Naylor/Zapotocny AVIATION...Cecava