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
876 FXUS63 KARX 031130 AFDARX Area Forecast Discussion National Weather Service La Crosse WI 630 AM CDT Wed Jul 3 2024 .KEY MESSAGES... - Seasonable temperatures today. Most areas will remain dry, however an isolated shower or storm could develop this afternoon. - Shower and thunderstorm chances increase through the day on July 4th and continue overnight and into Friday. 24hr probability for 1" or more of rain at least 35 to 45%. Severe weather potential will be conditional on the instability. Current risk level is 1 of 5. - Seasonably cool for the weekend into next week. Rain chances increase again for Sunday, however the probability for .5" or more is 20% or less. && .DISCUSSION... Issued at 400 AM CDT Wed Jul 3 2024 Overview: Water vapor satellite imagery, 500mb heights, and lightning showed a large 500mb trough from the Northern and Central Rockies into the Upper and Mid-Mississippi River Valley. A large area of high pressure was over the Mid-Atlantic and Southeast U.S. States. Clusters of storms were noted over eastern Colorado, parts of the Southern Plains and a MCS (mesoscale convective complex) over central Missouri. By 08Z, the cold front had pushed east of PVB, however dewpoints were still in the 60s for much of the local area. The deep moisture with 2.2"+ precipitable water axis was in the vicinity of the outflow boundary/convective complex from central MO into central IL. 24-hr Rainfall amounts across the region per the widespread and impressive. MRMS radar estimate showed .5-2.5" covering a large portion of MO/IA/central and southern WI and northwest/wcentral IL...with pockets of 2.5 to 4". LSE missed their daily rainfall record of 2.05" with 2.01" and RST just missed their daily rainfall record of 1.34" with 1.32". Here at the NWS office, we received 2.24". In the wake of the rainfall, low clouds and fog developed with dense fog especially for parts of northeast Iowa and ridgetops. Seasonable temperatures today. Most areas will remain dry, however an isolated shower or storm could develop this afternoon: Some fog will linger into the early morning hours until the westerly winds can scour out the low level moisture. A series of troughs move through the flow today and there are hints by the various hi-resolution models/HREF that an isolated shower or storm could develop. Some weak moisture transport, temperatures warm into the 80s, steepening lapse rates and weak vorticity advection occurs. Most areas remain dry, but did include some low pops during peak heating to account for this. Shower and thunderstorm chances increase through the day on July 4th and continue overnight and into Friday. 24hr Probability for 1" or more of rain at least 35 to 45%. Severe weather potential will be conditional on the instability. Current risk level is 1 of 5: Return flow increases across Minnesota tonight north of the warm front as weak shortwave trough energy works east in the westerly flow ahead of the approaching 500mb trough. Several of the hi- res cams hint at isolated precipitation, thus have some low pops to cover this. During the day Thursday the 4th of July, the 500mb trough closes off over South Dakota, shifting eastward across Minnesota. This slow-moving storm system continues over the Upper Mississippi Region into Friday, gradually exiting the area Saturday morning. Although precipitable water values increase to the 1.6 to 1.8" range, the very high 2" plus PWAT value appear to hold farther south across MO/IL. In addition, forecast soundings generally show lower warm cloud depths of 3.5km vs. 4.5km with the Tuesday system. The EPS/GEFS forecasts show a bit of spread with the 03.00Z EPS having nearly half the members with less than 0.10" and nearly half with over 1" for a 24 hour period by Friday morning. The slow storm movement and storms repeating over the same area could lead to greater totals though. The HREF mean does not go out far enough yet to cover the event. Will continue to mention the potential for locally heavy rain and the potential for localized flash flooding due to the saturated soils and 1-2"+ local amounts. The bulk of the 1"+ area is mainly over southeast MN and western and central WI, however pockets could occur farther south. With the closed area of low pressure aloft and the triple point, warm front/cold front, there is a conditional severe weather threat depending on the instability present. Forecast soundings show some potential for strong to severe storms with favorable shear, however instability may be limited. CSU machine learning probabilities are increased south of the local area and we are currently in a marginal risk; risk level 1 of 5. For now, this seems reasonable, however increased instability would increase the potential. Seasonably cool for the weekend into next week. Rain chances increase again for Sunday, however the probability for .5" or more is 20% or less: Saturday night into Sunday and into early next week, the unsettled weather pattern continues with a ripple then a deepening trough working into the area. At this time, rain amounts look to be more of the nuisance variety versus the heavier rains we`ve seen of late. && .AVIATION /12Z TAFS THROUGH 12Z THURSDAY/... Issued at 630 AM CDT Wed Jul 3 2024 VFR conditions are expected throughout the TAF period with few to sct high middle and high clouds. An isolated shower or storm this afternoon/evening cannot be ruled out however confidence is low that these will manifest (10-20% chance). Winds will decrease heading into the overnight hours to under 5 kts with some guidance trying to develop some fog over the area during the morning hours tomorrow. With questions remaining for cloud cover and shower trends tomorrow morning, will hold off on fog mention in the TAF for now. && .HYDROLOGY... Issued at 400 AM CDT Wed Jul 3 2024 Flooding continues on the Mississippi river with 24hr rainfall heaviest across parts of northeast Iowa into southwest WI, resulting in higher river levels at McGregor and Guttenberg. Some flooding was also noted on the Kickapoo Tuesday evening at Ontario with rises at Readstown and La Farge. Additional rainfall with locally heavy rain possible Thursday afternoon/night into Friday. With soils being saturated, we`ll continue to evaluate the need for a Flood Watch where the flooding risk is greatest. && .ARX WATCHES/WARNINGS/ADVISORIES... WI...None. MN...None. IA...None. && $$ DISCUSSION...Zapotocny AVIATION...Naylor HYDROLOGY...Zapotocny