Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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292 FXUS63 KARX 272341 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 641 PM CDT Thu Jun 27 2024 .KEY MESSAGES... - Showers with isolated storms tonight into Friday morning. Storm chances for Fri afternoon - evening. Conditional strong/severe threat with a lot of uncertainty. Expect updates/refinement to these concerns over the next 12-24 hours. - Cooler, drier Sat night into Monday. - More rounds of showers/storms Mon N-Tue and again next Thu-Fri && .DISCUSSION... Issued at 225 PM CDT Thu Jun 27 2024 * SHOWERS/STORMS CHANCES TONIGHT/FRI - conditional strong/severe storm risk Fri afternoon/evening Broad area of low level warm air advection, along with the nose of the low level jet/moisture transport and ripples of energy a loft, helping to produce a broad area of showers with a few embedded storms across SD, southwest MN into northwest IA early this afternoon. The lift/fuel will continue to slide eastward to across the local area tonight, with an expansive area of rain reaching the Mississippi River toward midnight (per latest CAMS models). Instability is meager and elevated, but enough for at least low end (10-20%) thunder chances. While there could be pockets of moderate (perhaps heavy) rainfall, overall amounts tonight through Fri morning trending mostly from 1/4 to 1/2" - generally from northwest WI southwest into northeast IA. The rain will continue to ease off to the east over the eastern 1/2 of the badger state for the afternoon. What happens westward of this rain shield is a tricker question to answer. All the meso guidance points to a potentially deep (4kft) stratus layer with the showers, that lingers well into the afternoon. The GFS provides additional support for this. Clouds would keep it cooler, reinforcing the low level inversion and limiting the potential instability. Where the westward edge to the low cloud deck hangs, and how quickly it shifts east aren`t clear. CAPE pool (2k+ J/kg SCAPE per RAP in MN) progged to develop in the clear slot to the west of the clouds during the day. Deeper shear lies to its west, behind a cold front, although 0-3Km shear could be enough to provide some storm organization. There will be various forcing mechanisms that could kick off storms in the warm sector - from a sfc cold front to bits of upper level energy/MCV, cloud-sun thermogradients, left over outflows, etc. Not much clarity in how this will play out. Could be some strong/severe risk where the elements all come together, but hard to pin point where that could/would be. Expect refinements to expectations over the next 12- 24 hours. * DRIER, COOLER SAT NIGHT INTO MON Cooler air set to drop in from Canada post the cold front Sat with 850 mb temps progged to drop from 16 C at 00z Sat to around 7 C by 18z Sun. A sfc high will be meandering across the region Sun while a loft the shortwave ridge axis zips in Mon. Below normal temps should result (5 to 10 degrees on the cool side) with mostly low 70s for highs. Also looking at a break from the rain, at least for a couple days. * MON NIGHT/TUE: next round of showers, storms Long range guidance in solid agreement with driving an upper level shortwave trough from the PAC NW east to the northern plains Mon/afternoon-eve and then across the upper mississippi river valley Tue. Broad area of low level warming and PWs around 2" will provide ample fuel/lift for widespread rain chances ahead of the shortwave and its attendant cold front. How much instability will be around to increase the thunder threat will be dependent on timing - and there are some differences (see next paragraph). Comes earlier - more instability...later, not as much. However, the warm cloud depth (4k+ km per GFS) and the aforementioned juicy PWs will favor warm cloud/heavy rain threat. The system looks to be transitory, helping lower a flash flooding risk. However, depending on where the heaviest rains fall, will work to keep river levels elevated, slower to abate. Some differences in the models with timing - the GEFS generally trending a bit slower than the EC. While some of the EPS suite suggest the related shower/storm threat could push in by Mon afternoon, latest operational run sides with the GFS as does the bulk of its (EPS) members. NBM chances a bit more aggressive/quicker with onset. While slowing the system down looks like a solid trend, will let the NBM detail the timing for now - given the system is still several days out. * NEXT THU/FRI: more rain chances The progressive upper level flow currently favors driving another shortwave trough across the northern CONUS, spinning over over the local area in the Thu/Fri time frame. A lot of consensus in the EPS suite of members while the GEFS has more variability. The projected system would still have a juicy, summery airmass to work on - highlighting widespread rain (potentially locally heavy) chances. Too far out to fine tune any details but another period to keep an eye on. && .AVIATION /00Z TAFS THROUGH 00Z SATURDAY/... Issued at 640 PM CDT Thu Jun 27 2024 A series of weak shortwave troughs will move through the area tonight and Friday. The best chances of showers will be from tonight into Friday morning and during the mid and late afternoon. Ceilings will be VFR tonight and then become IFR/MVFR early Friday morning and then continue through the day. While visibilities might become MVFR at times due to showers on Friday, confidence was not high enough to include them in the TAFs at this time. && .HYDROLOGY... Issued at 401 AM CDT Thu Jun 27 2024 While rivers continue to fall below flood stage, most of this water is still on route to the Mississippi River. As a result, the Mississippi River is expected to keep rising through the next 3 to 5 days depending on location and routing of tributary rivers. While official forecasts from the River Forecast Center do take into consideration all of the water that has already fallen and is on its way into the Mississippi River, it only takes the next 24-48 hours of forecast precipitation. The next chance of precipitation returns late tonight lasting through Friday. While widespread amounts of 0.5" to 1.0" are not expected to immediately or substantially affect ongoing flooding concerns, an additional widespread 1" to 2" of rainfall upstream will prolong residence of elevated river levels. As a result, the Mississippi River levels may initially slightly decrease before increasing again as the additional rainfall flows downstream this weekend into next week. Elevated rivers with ongoing minor to moderate flood stages from Lake City through Guttenberg are expected to persist beyond the next 7 to 10 days. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Rieck AVIATION...Boyne HYDROLOGY...JAR