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
812 FXUS63 KFSD 190218 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 918 PM CDT Wed Sep 18 2024 .KEY MESSAGES... - Shower and thunderstorm chances (30%-40%) increase overnight for areas mainly along and south of Interstate 90. The strongest storms may be capable of producing hail to the size of quarters. - An active pattern aloft will lead to renewed shower and thunderstorm chances (40%-60%) this weekend with the highest chances occurring on Saturday and Saturday. - Confidence continues to increase in cooler stretch of temperatures starting as early as Sunday. && .UPDATE... Issued at 918 PM CDT Wed Sep 18 2024 Showers and thunderstorms have largely failed to materialize this evening within broad area of increased instability ahead of an approaching cold front - likely due the lack of substantial forcing. That said, focus has shifted to the overnight hours when some guidance suggests additional thunderstorms may develop in response to an increasing 60 kt 500 hPa jet streak and 40-50 kt 850 hPa jet. In particular, multiple runs of the HRRR have shown thunderstorms first developing near the central SD/NE border region after midnight and then pushing east mainly along and south of I-90 through the overnight hours. Additional storms may develop across eastern NE and move north. With the loss of daytime heating, the near surface layer is quite stable, but model soundings do show roughly 1000-1500 J/kg of elevated CAPE. This could allow for the strongest updrafts to produce some isolated large hail up to the size of quarters. To account for this possibility, introduced 30-40 percent POPs overnight, but some adjustments are likely over the next several hours. && .DISCUSSION... Issued at 211 PM CDT Wed Sep 18 2024 REST OF THE EVENING/TONIGHT: Taking a look at satellite imagery, areas of lingering stratus continue to lift northeastwards across our eastern-most column of counties with a few pockets of light showers. Expect this lingering activity to continue to progress northeastwards before exiting our area around 18z. Looking aloft, a vertically stacked ULL situated over Eastern Montana and the far western Dakotas continues to rotate northeastwards keeping our overall flow locked into a northeasterly regime through tonight. Shifting gears to our precipitation chances, isolated shower and thunderstorm could develop by as early as 21z as a subtle shortwave rotates around the ULL. However, with rather weak forcing and a modestly stable airmass in play due to this morning`s rain/lingering cloud cover; not too confident in our overall severe weather chances this afternoon and evening. Nonetheless, with up to 1500 J/kg of instability, 30-35 kts of speed shear, and a dry sub-cloud layer; can`t completely rule out a stronger storm or two mostly between the James River Valley and I-29 corridors with the main threats being damaging winds up to 60 mph and quarter sized hail. Otherwise, expect mostly quiet conditions to return overnight as breezy southerly winds gradually decrease with the approaching surface high. Lastly, lingering cloud cover and southerly surface winds will keep temperatures elevated overnight as lows gradually decrease into the low to upper 60s for the night. THURSDAY: By Thursday morning, could potentially see a few isolated pockets of light to moderate showers develop east of I-29 as the LLJ strengthens ahead of the cold front. Expect this area of activity to gradually scoot eastwards through parts of the morning with most of the activity exiting our area by about 15z. From here, cloud cover should begin to clear out from west to east with an abundance of sunshine returning for the rest of the day. With this in mind, expect our overall temperatures to stay above normal with highs topping out in the upper 70s to upper 80s with the warmest conditions expected across portions of the Missouri River Valley and northwestern IA. THE WEEKEND ONWARDS: Heading into the weekend, mostly quiet and breezy conditions will continue as a surface high moves across the region by Friday. A tightening SPG will lead to periodic breeziness over the weekend with wind gusts up to 25 mph possible each afternoon. From here, an active pattern returns aloft with a northern stream trough progressing eastwards across Montana and an ULL ejecting out of the four-corners region. Both these features will likely influence the precipitation chances across our area from Saturday through Sunday. However, to what degree is still in question. Most long-range guidance continues to vary on the strength of both systems which has result in some uncertainty. Nonetheless, most ensemble guidance continues to show low to medium confidence (30%-50%) chances of up to half an inch of QPF with the focus being areas south of I-90 which could mean some beneficial rain is on the way for parts of our area. Either way, we`ll have to monitor these systems as they approach. Lastly, the increasing precipitation chances will lead to temperatures trending downwards heading into the new week with daily dropping from the low to mid 80s on Friday to the low to upper 60s by Monday. && .AVIATION /00Z TAFS THROUGH 00Z FRIDAY/... Issued at 636 PM CDT Wed Sep 18 2024 While there is still a 20-30 percent chance for showers and thunderstorms this evening at KFSD/KSUX, recent observational trends suggest a drier evening overall, so have removed all shower/thunder mention from the 19.00Z TAFs. Will continue to monitor observations for possible updates. One caveat with precipitation is that multiple runs of the HRRR this afternoon have tried to develop a cluster of showers and storms across southeast SD late tonight into Thursday morning. This is currently an outlier compared to other high resolution guidance, but something to watch. For the rest of the evening, expect some scattered VFR ceilings to give way to cirrus and/or clear skies overnight. VFR conditions are then expected to continue through the rest of Thursday with mostly sunny skies. Southerly breezes with some gusts up to 20 kts are likely to continue into the overnight at KHON/KFSD, whereas winds will be lighter and from the southeast at KSUX. Do expect some low-level wind shear at KSUX overnight, with winds at 2000 ft agl to increase to near 40 kts from the southwest. Surface winds to veer to the southwest and eventually the west Thursday morning, gusting at times between 20 and 25 kts by the afternoon. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ UPDATE...Rogers DISCUSSION...Gumbs AVIATION...Rogers