Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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
44
45
246 FXUS63 KFSD 292335 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 635 PM CDT Mon Apr 29 2024 .KEY MESSAGES... - Strong to severe storms are expected tomorrow afternoon between 1 pm through 8 pm. Storms will be capable of large hail up to golf ball size initially before growing upscale into a line, transitioning the hazards to damaging wind gusts up to 70 mph along with an isolated tornado. Brief heavy rain is also possible which could result in minor flooding. - A second round of heavy rainfall is possible Wednesday night where rainfall amounts may exceed a half an inch of rain. However, there remains uncertainty in where the heaviest rain will fall. - Seasonal temperatures with highs into the 60s and possibly 70s and lows down to the 40s will continue through the weekend. && .DISCUSSION... Issued at 257 PM CDT Mon Apr 29 2024 Upper level low pressure system continues to push off to the northeast this afternoon. This is resulting in low level stratus finally lifting off to the northeast with the system and bringing sunshine back to parts of the area. West/northwest winds have gusted to around 20-25 mph but will cease as mixing wanes this evening. However, winds will turn to out of the southeast during the overnight hours and become a bit elevated. Despite clear skies tonight, the southeast winds will keep low temperatures just about seasonal with lows falling to the upper 30s to mid 40s. A few hi-res guidance members have been pointing to the potential for fog development across parts of eastern South Dakota, southwest Minnesota, and northwest Iowa. However, think that this is a low probability (<20% chance). Will monitor this potential over the coming hours. A stronger upper level wave will push into the Northern Plains tomorrow, setting the stage for strong to severe storms. As the exit region of the upper jet pushes into the Northern Plains, southerly flow will commence in response to the jets indirect thermal circulation which will advect warmer and higher moisture air into the forecast area. High temperatures are expected to warm to the mid 60s to mid 70s, warmest across northwest Iowa. Dew points will moisten to the 50s, highest along and east of I-29. A surface low residing within the left exit region of the jet will drag a cold front through the forecast area which will serve as the focal point to initiate convection. Looking at the environment, instability up to around 1,500 J/kg is expected with sufficient 0-6 km shear values up to 40-50 knots. Soundings do show weak capping in place due to the warm air advection (WAA) via the southerly flow. This should keep convection limited to the afternoon where the lift from the coupling of the cold front and jet streak circulation overcome the weakening cap and initiate storms. There remains some uncertainty in where storms develop though as latest 12z guidance has been coming in faster with the cold front. With the front passing through at a faster rate, this may limit severe storm chances to a few hours during the afternoon across parts of southwest Minnesota and northwest Iowa. However, there is still the remaining possibility that storms develop right along I-29 during the early afternoon hours and race eastwards through the rest of the afternoon hours. As such, think the timeframe between 1:00 pm to 8:00 pm remains valid. In terms of storm mode, storms look to develop as individual cells (potentially supercells) and pose a large hail threat with hail up to golf ball size. Storms will quickly grow upscale into a line of storms and transition the hazard from large hail to damaging winds. 0-10 km shear values up to 50-60 knots will support a severe line of storms but 0-7 km theta-e difference in only marginal, down to -5 k. Despite the weaker theta-e difference, the strong 0-10 km shear will support strong wind gusts up to 70 mph. 0-3 km shear will be sufficient up to 30-40 knots will be oriented more parallel to the line of storms, but still indicative of a low tornado threat. Brief heavy rain is also possible with the storms but given fast storm motions, think any potential flooding threat will also be low and confined to minor flooding. After storms push out of the area, quiet conditions are expected for the rest of the evening and overnight hours. Low temperatures will fall to the upper 30s and 40s overnight. Wednesday will be quiet during the daylight hours as large scale descent resides over the area. Highs will be a bit cooler, only warming to the 60s during the afternoon hours. An active jet aloft will continue to provide rain chances Wednesday night. The previous surface front will stall southeast of the forecast area but the elevated front may serve as another focus for overnight showers and storms as WAA strengthens in response to a strengthening low level jet (LLJ). Instability looks minor at only a few hundred J/kg so severe weather is not expected. However, heavy rain is possible as precipitable water (PWAT) values will reside in the 90th percentile per NAEFS ensemble. The GFS and Euro ensembles keep the highest probabilities for exceeding a half an inch of rain over central Iowa. However, the Canadian ensemble is an outlier as it shows a broad 50-80% chance for exceeding the same amount of liquid QPF over the forecast area. Cluster analysis also shows similar probabilities for exceeding a half an inch of rain from the most favored cluster. Will continue to monitor this potential. Low temperatures will fall to the 40s overnight. Chances for rain and storms look to continue for the rest of the work week and weekend as an active pattern continues aloft. Too far out to say if any of these chances for rain could coincide with the potential for strong to severe storms. High temperatures look to remain in the 60s and possibly 70s for the weekend with low temperatures falling to the 40s overnight. && .AVIATION /00Z TAFS THROUGH 00Z WEDNESDAY/... Issued at 628 PM CDT Mon Apr 29 2024 Lingering MVFR stratus across southwestern MN and northwestern IA continues to move east through 30.03z. Otherwise, will be watching for fog developing overnight from roughly KBKX to KLYV to south of KSPW, which could lead to MVFR/IFR visibility. As of now, models largely keep this development to the east of KFSD, but will continue to monitor. Expect northwest winds to become light and for some, variable between sunset and daybreak. Winds start the day southeasterly with gusts around 20 knots, but switch back more westerly behind a cold front. This cold front will also bring the chances for showers and thunderstorms. Recent guidance has sped up the progression slightly, so tried to narrow the window as much as possible for KFSD and KSUX. Showers/storms start off as single cells, but will form a line as they track east. Should be west of the IA/MN Hwy 60 corridor by 30.23z to 01.01z. MVFR/IFR conditions are possible with thunderstorms. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...Meyers AVIATION...SG