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
43
011 FXUS63 KARX 010406 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 1106 PM CDT Tue Apr 30 2024 .KEY MESSAGES... - Showers and storms with a decaying severe weather threat from west to east this evening. Large hail and damaging wind gusts will be the primary hazards. - A secondary system will bring showers and storms resulting heavier rainfall beginning Thursday morning into the overnight hours. Rainfall amounts of 1 to 2 inches are possible. Flood prone areas should be alert for localized flooding potential. - Additional chances for showers and storms persist into the weekend and early next week. && .DISCUSSION... Issued at 305 PM CDT Tue Apr 30 2024 Thunderstorms This Evening Through Tonight: We continue to closely monitor the potential for strong to severe thunderstorms west of the Mississippi later this evening, especially in parts of southeast Iowa. At 130pm, day cloud phase satellite imagery shows a swath of showers and isolated storms moving towards northeast Iowa. Convection associated with this first push of preciptiation is located in a low CAPE, high CIN environment and is not expected to produce a severe weather threat. Further to the west near the Nebraska/Iowa border, a ML CAPE pool is building on the order of 1500-2000 J/kg as strong warm air advection works into this region ahead of an approaching cold front. Despite showers and storms in this area earlier in the day, rapid clearing and surface temperature recovery is underway. This is also where satellite imagery shows convective initiation taking place. CAMs suggest initial development along the Nebraska/Iowa border, upscale growth to a lines or clusters of storms quickly moving west across Iowa. These are the storms that pose a level 2 out of 5 (slight) risk locally for severe weather this evening. Confidence for severe weather is much higher to the south and west of the local forecast area, with medium confidence for a decaying severe weather threat locally. Model soundings and guidance show an environment primed with a lot of low-level and deep-layer shear this evening with a deepening shortwave trough sliding from southwest Minnesota up towards northwest Wisconsin. Despite the surface low moving northwest of the area, much of the CAPE associated with this system is progged to advect southeast of our northeast Iowa counties. This leaves much of our forecast area in between these two forcing features muddying our severe weather risk locally. With plenty of shear present, we cannot rule out a few stronger to severe storms, but organized severe weather may be hard to come by without more CAPE to tap into. Large hail, damaging wind, and a tornado or two are all on the table during a narrow time window between about 6 to 9pm. The meager instability present peters out after sunset. Non-severe storms continue eastward across the forecast area late this evening, exiting to the east by sunrise. Given the progressive nature of this system, rainfall amounts will remain under a half inch for most and do not pose a flooding/flash flooding threat. Wednesday - Thursday Night: Heavy Rain & Storm Chances Thursday As we head into the middle of the week, a secondary wave will progress eastbound out of the MT/WY region. With its general pivot northeast as it resides in the Upper Midwest, another round of showers and storms will be likely across much of the area on Thursday. The general surface setup amongst guidance suggests a northward progressing warm sector with strong 850mb moisture transport in the 30.12z NAM/GFS. Consequently, a notable precipitable water surge can be seen in guidance Thursday afternoon with values across deterministic guidance and their members reaching to 1.1" to 1.4" which is well above the 90th percentile for climo and approaching daily maximums for early May. With the higher warm cloud depths approaching 3000-3500 meters or greater, could certainly see some efficient rainfall when also considering skinny instability profiles. As a result, the WPC has a Slight Risk for our entire forecast area for excessive rainfall on Thursday. The 30.12z EC ensemble showing fairly respectable probabilities (50-70% chance) for over an inch of QPF. When considering that 3-hr flash flood guidance is around 1.5" to 2" for many spots, will have to watch for some localized flooding concerns in susceptible locations. Otherwise, there is some marginal severe potential as storms progress into our region on Thursday as well. The 30.12z NAM shows a degrading axis of MUCAPE pushing into portions of northeast IA and towards I-90 into the afternoon hours associated with the aforementioned 850mb moisture transport. As an MCS evolves along the warm front maybe some gusty winds or small hail could materialize. However, the hail threat seems to be a lesser concern with the previously stated deeper warm cloud depths and skinny instability profiles. However, with a stronger low-level jet to around 50-55 kts in place, could not rule out a couple of 50-60 mph wind gusts with any storms that can utilize it. Friday - Tuesday: Additional Shower & Storm Chances Into Next Week Deterministic long-range guidance in combination with cluster analysis into the weekend and early next week signal the possibility that we remain in a somewhat active pattern with additional embedded upper-level waves progressing into our region through the weekend and into next week. As we head into Saturday, the first of these waves pushes into the region as a parent upper low centered across Manitoba will help pivot the weak shortwave into the Upper Midwest. Still some disagreement in the placement of this weak wave so will keep chances for showers and storms low in accordance with the recent NBM for now as much of the 30.12z GEFS/EC ensemble probabilities have very low chances for any significant instability ahead of this wave. So generally not expecting any substantial precipitation with this system. Guidance begins to exhibit a fair amount of spread after the weekend with cluster analysis showing a couple of different scenarios in the 30.00z grand ensemble (EC ens/GEFS/Canadian ens). The majority of the ensembles (roughly 70% of members) show some form of a wave that ejects out of the rockies towards our region to in the later Monday into Tuesday timeframe. The remaining members (roughly 30%) favor keeping a ridge that guidance moves into place on Sunday. As a result, have kept modest probabilities for precipitation across the region during this period in accordance with the NBM. Mode of how showers and storms will develop along with any severe threat is unclear at this time with any surface features and thermodynamics remaining unresolved. && .AVIATION /06Z TAFS THROUGH 06Z THURSDAY/... Issued at 1056 PM CDT Tue Apr 30 2024 Showers and thunderstorms will be exiting the area early in the TAF period with variable MVFR/IFR ceiling becoming VFR. Variable winds with the showers and storms in the area will become southwest, then shift to the west and northwest increasing 10 to 20kts with gusts 20 to 30kts. The northwest winds will decrease to less than 12kts Wednesday night. VFR ceilings fl100-150 will be on the increase Wednesday night. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...KAA/Naylor AVIATION...Zapotocny