Area Forecast Discussion
Issued by NWS La Crosse, WI
Issued by NWS La Crosse, WI
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229 FXUS63 KARX 200912 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 412 AM CDT Thu Jun 20 2024 .KEY MESSAGES... - Multiple rounds of heavy rain expected Friday and Saturday. How the bands line up will determine the local flooding threat. Confidence is lower on how the later storms Friday night/Saturday evolve. - A few storms Friday and Saturday may be severe, with damaging winds the main threat. - A brief respite from the active pattern for Sunday and Monday, but more storms loom for Monday night into Tuesday. && .DISCUSSION... Issued at 400 AM CDT Thu Jun 20 2024 Today - Tonight: Seasonably Cool, Scattered Showers/Storms The early morning 08Z surface analysis shows the region trapped between a 1030-mb high pressure cell centered over the MN/Canadian border and a larger, retrograding synoptic high pressure ridge extending from the Mid-Atlantic to the Southern Plains. A ribbon of higher theta-e air straddles these two ridges from Kansas northeastward into the southern Great Lakes region. Weak isentropic ascent over this airmass has fueled weak, disorganized convection across Iowa and Wisconsin over the last 24 hours. Rain rates on the whole have not been overly high, but the training trajectories of the storms have led to 1-2 inch totals in a few locations. The upstream pattern over the High Plains begins to amplify this afternoon and heights build over the southern Missouri River valley. These two factors combined should nudge this theta-e ribbon northward every so slowly today. Synoptic forcing remains weak and output from the various convective models shows scattered, disorganized convection at best throughout the day. Therefore, have toned back PoPs for today, which may be still too aggressive based on the latest HRRR runs. Bottom line, today looks more dry then wet. Low clouds streaming northeastward from the KS/NE region reach the region this morning and linger for at least the morning hours. Highs today will hinge on high quickly clouds can clear, but given the lack of surface flow, expect similar temperature values to yesterday. Friday - Saturday: Heavy Rain/Severe Weather Details The most impactful period in the forecast exists from Friday morning through Saturday afternoon, with multiple rounds of heavy rain and severe weather on the docket. Surface to 500-mb moisture transport increases over Nebraska and South Dakota tonight in response to the upstream troughing digging over the Four Corners. This results in a large MCS developing along this moisture convergence axis and propagating ENE along the warm front lifting through Iowa and southern Minnesota. The 00/06Z CAM suite show this MCS bowing out and becoming more orthogonal to the flow as it reaches central Minnesota. While rainfall amounts of 1-2 inches are certainly within the realm of possibilities, the orientation of this first complex is not favorable to a widespread flash flood threat. Given the expected storm evolution, cannot rule out pockets of damaging winds as it arrives in the morning on Friday. The strength of this initial convective cold pool will play a huge role in how far south the near-surface baroclinic zone shifts for the daytime Friday into Friday night. Theta-e advection will be increasing during the afternoon on Friday along the boundary, resulting in a west-to-east line of convection developing by the mid- afternoon hours. The steering flow will be slightly offset north of the lower tropospheric boundary orientation, so there is some uncertainty in the degree of training with these cells. A second MCS rolls out of South Dakota early Saturday morning and once again drops a quick 1-3 inches of rain with its passage. Again, damaging winds would also be possible with this early morning convection. Finally, with the upper level trough ejecting through the Dakotas and Minnesota during the day on Saturday, one last round of storms develop for Saturday afternoon. How quickly the morning convection can clear will drive the degree of destabilization and severe weather threat. All modes of severe weather appear on the table at this point, with any tornado potential likely focused on the residual outflow boundary laid out from the morning storms. The environment will certainly be supportive of heavy rainfall with record PWAT values of 2 inches expected from the EPS by Saturday morning. The EPS Extreme Forecast Index shows this axis of heaver QPF with EFI values of 0.8 to 0.9, but the axis is shifted farther north than what the initial runs of the CAMs are indicating, likely owing to the global model not correctly assessing the cold pool strength of Friday morning`s storms. Total QPF values could exceed 5-6 inches by the time the event ends Saturday evening, but where this falls and how widespread these values are will be hashed out in future forecast updates. There you have it. Three rounds of storms in 36 hours. The evolution and tracks of Friday night and Saturday`s storms will be heavily influenced by the storms preceding them. The short/medium range guidance are fairly well locked in on the initial heavy rain axis early Friday laying out north of I-90, with the second round of heavy rain being favored to fall more along I-90 Friday night into Saturday morning. It remains too early to determine where the third and final round of heavy rain will fall on Saturday afternoon. Where these axes of heavy rain line up will see an increased risk for river and flash flooding. Sunday - Monday: Brief respite from the Storms With the main energy associated with the weekend upper level trough lingering along the Canadian border and zonal flow upstream, the cold air advection behind Saturday`s front will not last long. Scattered showers in central WI and daytime cumulus will be the main sensible weather for Sunday before the 1010-mb surface high pressure cell slides through Sunday night. Monday morning will start on the cooler side, with progressive flow ushering the upper tropospheric shortwave ridge axis east of the region by Monday afternoon, allowing warm, moist air to stream northward once again. Monday Night - Mid-Week: Another Round of Storms, Then Cooler A pair of shortwave troughs ripple along the MN/Canadian border Tuesday and Wednesday and will serve as the focus for shower and thunderstorm development. The best potential for severe weather will be with the first round of storms Monday night into Tuesday before the warm sector is shunted south of the region. It remains too far out to nail down the details in the convective evolution and threats given the differences in the wave characteristics and timing amongst the global ensembles (resulting in broad lower end PoPs in the forecast). Additional showers may move through Wednesday with the second shortwave. However, there is higher confidence in longwave ridging building west of the region for mid to late week that will bring quieter weather for the Wednesday to Friday timeframe. The exact placement of the ridge axis will drive the temperature forecast, but indications are for near or even slightly below normal temperatures for that period. && .AVIATION /06Z TAFS THROUGH 06Z FRIDAY/... Issued at 1055 PM CDT Wed Jun 19 2024 VFR ceilings fl100 tonight deteriorating Thursday to MVFR with scattered showers and a few thunderstorms. The latest surface analysis had surface high pressure over the TAF sites with dewpoints in the 50s and the 60s dewpoints off to the south and the surface front over parts of IL. The showers tonight are generally expected to remain to the south tonight, however after 09Z we see the 925mb flow becoming more southerly with increasing moist low level flow returning. This should favor showers re-developing. Included VCSH after 14 or 15Z at the TAF sites with ceilings becoming MVFR 16-18Z. The short term models are on the dry side at the TAF sites, but scattered showers seem reasonable for Thursday. Thunder will also be possible, however due to low confidence on timing did not include at this time. && .HYDROLOGY... Issued at 400 AM CDT Thu Jun 20 2024 Multiple rounds of heavy rain are forecast for Friday morning through Saturday afternoon. Total QPF values may locally exceed 5-6 inches with more widespread amounts of 2-4 inches becoming increasingly likely. Given the breaks between each round of storms, any major flash flood threat will occur where the axes of heavy rain overlap. These rains on top of already saturated soils will result in river flooding concerns along the Mississippi River and its tributaries. Many locations along the Mississippi River will exceed flood stage towards next weekend (60-90% chance) if rainfall unfolds as forecast. Should the heavy rain line up over the smaller river basins, there is a 10-20 percent chance that these basins could see periods of flooding at moderate flood stage. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Skow AVIATION...Zapotocny HYDROLOGY...Skow