Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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170 FXUS63 KARX 161119 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 619 AM CDT Mon Sep 16 2024 .KEY MESSAGES... - Above normal temperatures are expected for much of the week with high temperatures in the 80s for most. - Relatively dry conditions are expected for the first half of the week with increasing chances for precipitation later this week and into the weekend. && .DISCUSSION... Issued at 316 AM CDT Mon Sep 16 2024 Today - Wednesday: Quite Warm With Relatively Dry Conditions GOES-16 water vapor imagery and 16.06z RAP 500mb heights show the general synoptic pattern over the CONUS early this morning with a deepening trough situated over the Western Rockies with high pressure sitting to our east. As we continue through the early part of this week, much of our weather will be dictated by a building ridge and a drier airmass attempting to move into our region from the east, shown in the precipitable waters across the recent RAP/NAM being generally under 1" across eastern portions of the local area early this week. Consequently, looks like a fairly favorable day to dry out the low-levels when considering diunral mixing and reasonably steep hydro-lapse rates at 700-800mb in 14.06z RAP soundings. As a result, should certainly feel a bit less humid in spite of the well above normal temperatures which when given the overall drier setup likely will overachieve the NBM some. Therefore, have trended temperatures towards the upper-quartile of the NBM for today with highs generally in the middle to upper 80s, with an isolated 90 being possible. Similar conditions can be expected for both Tuesday and Wednesday as upper-level ridging remains firmly in place with 850mb temperatures holding steady (around 15-18C) in deterministic guidance through Wednesday. Thursday - Sunday: Precipitation Chances Return As we head into late week, a sharp axis of 850mb moisture transport accompanied with a parent broader troughing regime attempts to nudge its way westward, eventually reaching the western periphery of the local area. As this occurs, a shortwave trough is progged to pivot through north-central MN on Thursday, providing some ascent with a downstream surface frontal passage into Thursday evening. While there is some disagreements between the deterministic models on the exact orientation of this shortwave, the overwhelming majority of ensemble guidance paint high measurable precipitation probabilities (80-90% in the 16.00z EC/GEFS) across southeast Minnesota. Generally noting lower precipitation chances (50-80%), particularly in the GEFS, as you head eastward with weaker lift and more minimal instability. As a result, thinking the NBM precipitation chances for Thursday evening are fairly appropriate with the highest rain chances furthest west. The threat for more organized severe weather with this frontal passage appears to remain west of the region at the current moment with the 16.00z GFS generally waning instability as the front move towards our region. Additionally, with our region currently expected to be further displaced from the parent shortwave and stronger mid-level flow in the current state of guidance, our bulk shear would likely be limited as well. As a result, very low probabilities (under 10%) are present in the joint CAPE/Shear probs (>500 J/kg of SBCAPE and >30 kts of bulk wind shear) in the grand ensemble (GEFS/EC ensemble/Canadian ensemble). The upper-level flow pattern becomes far more uncertain as the weekend approaches with disagreements on how a secondary shortwave will eject towards the region for the weekend and if any upper-level ridging can manifest ahead of it. As a result, noting a large degree of spread in ensemble guidance for high temperatures heading into the weekend with the 10th to 90th range in the grand ensemble across the local area having roughly 15 degrees of spread on Sunday with median temperatures generally in the 70s areawide. Additionally, with the low confidence in how the overall synoptic pattern will pan out, noting more broad brushing of precipitation chances with higher probabilities (50-70%) spread across the local area in the grand ensemble for the weekend. This would suggest that while confidence is lower on exact details for precipitation, there is a reasonable chance that our area will get some rainfall into the weekend. && .AVIATION /12Z TAFS THROUGH 12Z TUESDAY/... Issued at 614 AM CDT Mon Sep 16 2024 VFR conditions appear favored to prevail over the next 24 hours. Valley fog this morning has been limited to just the Wisconsin River valley around OVS so do not expect any to occur at LSE over the next 1-2 hours. Looking ahead to late Monday night, greater than 10 knot wind speeds look to be present as low as 300 feet so fog/stratus appears doubtful at LSE. Otherwise, expect modest winds out of the southeast at all sites through the period. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Naylor AVIATION...Ferguson