Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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181 FXUS63 KARX 171045 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 545 AM CDT Tue Sep 17 2024 .KEY MESSAGES... - Above normal temperatures continue through the work week with considerable uncertainty in temperatures from Sunday onward. Today and Wednesday look to also feature drier conditions compared to previous days. - Next shower and thunderstorm potential arrives Thursday and Thursday night. If thunderstorms develop during the late afternoon and evening - a big if - a severe storm cannot be ruled out. - Additional periods will have rain potential from Saturday onward but there is considerable uncertainty surrounding the exact timing of these periods. && .DISCUSSION... Issued at 359 AM CDT Tue Sep 17 2024 TODAY THROUGH SATURDAY: Warmth Continues, Dry Tuesday and Wednesday Overall pattern through this weekend continues to be dominated by the influence of an upper ridge centered over a Texas to western Great Lakes axis. 17.00z guidance is in good agreement that this ridge will be maintained either overhead or just to our east through Wednesday. With an upper low favored to advance from the Great Basin to the northern High Plains, surface pressure gradient should increase as a surface cyclone deepens in MT/ND/SD, driving a light south-southeasterly breeze each of the next two afternoons while these south-southeasterlies should also advect drier air over the CWA. Given the warm temperatures aloft and expected mixing, temperatures at the surface should reach the mid to upper 80s with a few 90s readings Tuesday and Wednesday while RH values drop below 40 percent in some areas east of the MS River. Thursday through Saturday, warm temperatures look to continue but with a slow decreasing trend as heights aloft moderate. Additionally, the turn toward dry conditions looks to end as low level moist advection increases Wednesday night into Thursday as the northern High Plains upper low begins to churn eastward over the southern Canadian Prairies. THURSDAY AND THURSDAY NIGHT: Potential for Showers, Thunderstorms Starting early Thursday, as the aforementioned upper low moves eastward, multiple shortwaves will advance downstream in the south- southwesterly flow aloft over the Upper Midwest. Given the influence of the upper ridge discussed above, primary limiting factor for precip from early Thursday morning through the nighttime hours will be capping. While multiple shortwaves do eject over SE MN during the morning and early afternoon hours, appreciable 850mb moisture looks to hold off until the latter part of the afternoon, so am doubtful any parcels are able to overcome the warm temperatures aloft and have attempted to move PoPs toward the low end of consensus blends during the daytime hours Thursday. For Thursday evening, have largely kept NBM likely PoPs as temperatures aloft slowly moderate and low-level moisture ramps up. Progged 17.00z GFS MLCAPE values reach 1500 J/kg west of the MS River as sfc-6km shear values increase to around 40 knots, so should convection succeed in overcoming the cap, a severe storm or two cannot be ruled out. The broader universe of guidance seems to point to this low end potential as well as reflected in 17.00z CSU-GEFS ML output and LREF joint probabilities. Plenty of details still to be worked out, but will need to keep an eye on late Thursday afternoon and evening. SATURDAY THROUGH MONDAY: High Uncertainty Wide spread is seen across operational and ensemble guidance as zonal flow aloft Friday and Saturday gives way to large scale troughing over the central CONUS. Uncertainty largely surrounds the timing of any shortwaves in the zonal flow aloft and then how an upper low, initially over southern NV early Saturday, rejoins the larger scale flow pattern. Difficult to latch on to any solution at this time, so have retained NBM PoPs which are spread over every day Saturday through Monday. Forecast users are cautioned that, while precip may be mentioned in the forecast for 78 hours, the end result will likely see precip focusing on much shorter periods within this larger time frame. Hopefully details become clearer soon. && .AVIATION /12Z TAFS THROUGH 12Z WEDNESDAY/... Issued at 545 AM CDT Tue Sep 17 2024 Predominant VFR conditions expected through the TAF period, with the exception of some valley fog in the Wisconsin River Valley this morning and Wednesday morning. As far as CIGS go, daytime cumulus clouds will form this afternoon and some mid to high level clouds will move into the area during the overnight period. Southeasterly winds will shift to the southwest this afternoon and be capable of gusts up to 20kts, especially west of the Mississippi River. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Ferguson AVIATION...Cecava