Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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006 FXUS63 KARX 012324 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 624 PM CDT Sat Jun 1 2024 .KEY MESSAGES... - Fog looking likely tonight with questions on how widespread, how dense it could be. - Showers and storms overnight Sunday and Monday morning. Daytime hours on Sunday will likely remain dry for most. - Additional storms likely (50-80% chance) on Tuesday afternoon and evening. Monitoring the potential for organized severe weather. && .DISCUSSION... Issued at 236 PM CDT Sat Jun 1 2024 * FOG TONIGHT - how widespread, how thick is uncertain. Dense fog advisory not out of the question. Challenging forecast in terms of fog for tonight and accompanying impacts on the vsby. Morning, early afternoon rain with clearing just in time for sundown, coupled with light winds at the sfc and near sfc, lay the ground work for fog development. Generally, scenarios like this tend to form the fog earlier in the nighttime hours (closer to midnight as opposed to daybreak the next day), potentially lifting earlier as winds start to stir toward daybreak. Also, is it widespread or more confined to the river valleys (soundings look like a typical fall fog setup)? Latest SREF suggests most of the fog will develop from along the Mississippi River eastward with 40-60% chance for 1 mile or less vsbys. HREF paints widespread, but lower impacts fog (mostly 2-4 mile vsby). However, it suffers from differences in the short term models that make it up. For example the FV3 says "no fog" while the HRRR and NSSL have suggest sub 1 mile is more likely. Blend those altogether and you get the HREF. Like the outlay of the HREF and leaning more into the SREF and HRRR for vsby. Outcome would be widespread fog with a lot of 1-3SM mile vsbys, but also increasing chances for 1/4SM dense fog. Still some uncertainties on how this all plays out and will monitor trends closely as we move into the evening. Might need a dense fog advisory for portions (much?) of the area. * SUNDAY NIGHT/MONDAY: Showers and a Few Storms As we head into Sunday night, a shortwave trough will progress through the region bringing convection on the nose of some warm advection associated with an increasing low-level jet. Consequently, expecting showers and storms to pass through the region during the overnight hours Sunday and into Monday morning as shown in many of the recent CAMs of which have been trending later with the bulk of precipitation towards Monday morning. Currently, the setup with this convection is not super impressive with timing during the overnight and morning hours keeping instability fairly limited (200-500 J/kg of MLCAPE). As a result, not expecting any organized strong to severe storms but cannot rule out some gusty winds with the 850mb jet to around 35-40 kts. Additionally, could still be a fairly healthy rain event as well with high probabilities (70-90% chance) for over 0.1" of precipitation and moderate probabilities (30-50% chance) for 0.5" in the local area. * TUESDAY/TUESDAY NIGHT: Warmer & Another Round of Storms, Monitoring for Severe Potential The synoptic setup for Tuesday features a 500mb ridge sitting progressing eastbound ahead of an incoming trough and developing surface cold front ejecting out of the Northern Rockies and Upper Midwest. Subsidence under the ridge during the afternoon and southerly surface flow will allow for temperatures to climb well into the 80s with the 01.00z grand ensemble (GEFS/EC ens/Canadian ens) showing an inter-quartile range for maximum temperatures at La Crosse of 79 to 84 degrees for Tuesday. Consequently, expecting Tuesday to be the warmest day of the forecast period at this time. Additionally, southerly flow ahead of the aforementioned surface cold front will allow for fairly robust moisture advection with the 01.12z NAM/GFS having ample 850mb moisture transport. Therefore, expecting dewpoints and precipitable waters to increase into the afternoon with deterministic guidance showing dewpoints in the upper 60s to lower 70s along and south of I-90. Corresponding instability profiles from the increased moisture would be fairly conducive for convection with the 01.12z GFS having respectable values for MLCAPE (1500-2500 J/kg). The larger point of uncertainty in this instance would be the shear profiles. Currently, deterministic guidance has some differences in the orientation of the aforementioned 500mb trough and how this interplays with the surface cold front. As a result, confidence is low in how bulk shear profiles will manifest ahead of the front. Additionally, the 01.00z grand ensemble (GEFS/EC ens/Canadian ens) joint probabilities for over 500 J/kg of CAPE and 30 kts of bulk shear are relatively low (20-40% chance) for the local area. These probabilities are heavily CAPE driven. Will have to watch how shear profiles resolve in short-range guidance though with the CSU machine learning probabilities painting 15%+ mention in our local area which combined with the favorable instability profiles certainly means Tuesday will need to be monitored in upcoming forecast iterations. Regardless of severe potential, confidence is increasing for healthy rainfall later Tuesday with the 01.12z GEFS/EC ensemble having high probabilities (50-80% chance) for at least 0.5" of precipitation for Tuesday. Furthermore, locally heavy rainfall may be a concern with warm cloud depth approaching 4km and preciptable waters increasing to around 1.75" to 2" in the 01.12z NAM/GFS runs. Storm motions will be fairly progressive so any flooding concerns appear minimal at this time. In any case, have opted to keep likely mention for precipitation chances in accordance with the NBM on Tuesday evening. * LATE WEEK: Trending Cooler and Some Shower Chances With the passage of the previously stated cold front, guidance overall agrees on temperatures cooling off into the later portions of the upcoming week with very broad trough developing within the vicinity of the Great Lakes region. This would allow for northwest upper-level flow allowing for some shower chances and overall pleasant temperatures for this time of year with strong consensus in ensemble guidance for highs to remain in the 60s and 70s across the local area. && .AVIATION /00Z TAFS THROUGH 00Z SUNDAY/... Issued at 624 PM CDT Sat Jun 1 2024 Main taf concerns are IFR/LIFR conditions tonight with the development of fog by 06z across the area. Skies will clear this evening across much of the region. With light winds...less than 10 knots aloft/near surface...an inversion develops and mostly clear skies expected tonight. Fog will develop...especially in river valleys/low lying areas. In addition with recent rain...ridge tops and open areas fog will likely develop and possibly become dense in areas. Have introduced IFR/LIFR conditions at both RST/LSE taf sites after just 06z Sunday. Fog is expected to erode mid to late morning Sunday and conditions will improve to VFR for the rest of the taf period. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Naylor/Rieck AVIATION...DTJ