Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
516 FXUS63 KARX 141737 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1237 PM CDT Sat Sep 14 2024 .KEY MESSAGES... - Showers continue to move in throughout the morning and into much of the day. Highest confidence (50-80%) for showers and isolated storms is west of the Mississippi River this morning, then shifting north of I-90 during the afternoon. Isolated lingering showers are possible (15-20%) for Sunday and Monday. - Seasonably warm conditions are expected to persist this weekend and into next week with high temperatures generally in the 80s for much of the local area, roughly 5-10 degrees above average for this time of year. && .DISCUSSION... Issued at 331 AM CDT Sat Sep 14 2024 Today: Showers Move Into The Region, Highest Confidence For Showers and Isolated Storms West of Mississippi River The overall synoptic setup currently is dictated by a few key features that can be noted in 500mb geopotential heights across deterministic guidance this morning. Positioned to our south, the remnant upper-low from what remains of Hurricane Francine is meandering through the lower portions of the Mississippi River Valley. Throughout the overnight hours and into the morning, a lobe of vorticity was ejected northward caught in between an upper-level ridge to the east and a broad trough further west. This piece of vorticity combined with tropical moisture advection (precipitable waters of 1.5-1.75" in the 14.03z RAP) from the southeast provided by the remnants of Francine will continue to be the primary instigating mechanism for any showers and isolated storms throughout the day. As this vorticity lobe pushes north, showers west of the Mississippi River this morning will shift north of I-90 during the afternoon. With the main axis of moisture and forcing being focused generally west of the Mississippi River, noting probabilistic guidance (14.00z GEFS/EC ensemble) favoring high probabilities (60-90% chance) for over 1/10" of precipitation across northeast IA and southeast MN throughout today. Overall, the chance for any thunder is on the lower side (20-30%) west of the Mississippi River with instability generally being fairly skinny in model soundings at KRST with MLCAPE values of up to 1000 J/kg during the afternoon hours with virtually no instability present this morning. Furthermore, with the lower convective threat and the overall forcing regime being on the weaker side, noting fairly low probabilities (10-30% for over 0.5" in the GEFS/EC ensemble/HREF) for higher rainfall amounts despite progged precipitable waters being generally above the 90th percentile for KDVN sounding climatology. However, if any storms manage to get going some locally higher rainfall amounts cannot be ruled out. Sunday - Monday: Small Chances For Showers & Storms The forcing regime for any precipitation generally is a bit more questionable with the aforementioned vorticity lobe ejecting north of the region for Sunday/Monday. As a result, the primary instigating mechanism for any showers would be embedded shortwaves around the periphery of the ridge to our east in conjunction with the aforementioned moisture rich airmass in place. However, based on the state of current deterministic guidance this currently is low confidence. With the 14.00z HREF probability matched mean having very low QPF across our region and the overall unimpressive synoptic setup, have kept minimal precipitation chances (15-20%) for Sunday/Monday for now. With slightly more favorable instability present in 14.03z RAP soundings for Sunday (1500-2000 J/kg MLCAPE), would not be able to rule out an isolated non-severe storm if a stronger updraft can be realized with the overall questionable forcing regime. Tuesday - Friday: Continued Warm Pattern As we head into next week, upper-level ridging generally is favored to build back into our region with a broad trough situated across the Rockies. As a result, expecting this warmer pattern to persist with 850mb temperatures in the 14.00z EC/GFS of 16 to 18 degrees C. This in combination with the NAEFS percentiles generally keeping 850mb temperatures in the 90th percentile would suggest temperatures will likely remain above average through next week. Consequently, forecast high temperatures with the national blend through Friday generally remain steady state with high temperatures generally in the 80s through Friday with only around 2 to 3 degrees of inter- quartile spread within the 13.12z grand ensemble. Precipitation chances are dependent on if our region can eventually be pushed into the southwesterly flow regime off to our west. Different schools of thought on how that will transpire by the second half of the week with the 14.00z EC/GFS trying to pivot pieces of shortwave energy west of the region for the second half of the week. However, respective ensemble members in the GEFS/EC ensemble do not all jump on this bandwagon with medium confidence (40-60% chance) for measurable precipitation in portions of southeast MN/northeast IA. Consequently, still a fairly lower confidence period that will likely need more adjustments in upcoming forecast cycles. && .AVIATION /18Z TAFS THROUGH 18Z SUNDAY/... Issued at 1237 PM CDT Sat Sep 14 2024 Showers will slowly lift north/east of the RST/LSE taf sites this afternoon/early evening. MVFR conditions are expected early in the taf period at RST taf site. With daytime heating...this should allow for clouds to become scattered west of the Mississippi River Valley and conditions to improve to VFR by middle of the afternoon. VFR conditions should prevail for both of the LSE/RST taf sites the rest of the taf period. Wind speeds will remain less than 10 knots. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...DTJ AVIATION...Naylor