What the forecast actually says — NWS Area Forecast Discussions, decoded into plain English.
Area Forecast Discussion · National Weather Service Sioux Falls (FSD) · issued Saturday, October 24 at 6:34 AM CDT.
Revised in 3 passages since the 2:54 AM discussion. See every revision.
Messages
BREEZY WITH FALLING TEMPERATURES TODAY: A push of stronger winds arrives in south-central South Dakota near/just after sunrise. Gusts with this will increase to 35-40 mph for a brief period.
COOLER WITH SCATTERED WEEKEND RAIN: A slow-moving front will bring increased clouds and occasional rain this weekend. Temperatures will be notably cooler (40s/50s) by Sunday.
SIGNIFICANT RAINFALL LIKELY MONDAY-TUESDAY: A large multi-day system will bring widespread rain to start the new work week. Confidence in amounts over 1" through Tuesday morning remain high.
ACTIVE WEATHER CONTINUES: A second fast moving system may bring rain chances back by Tuesday Night and Wednesday, with temperatures near seasonal normals.
Update
Scattered showers and very isolated thunderstorms are expected today as mid level moisture streams northward. This is in response to a strong north/south oriented jet that will increase through the day. Weak (300-500 J/kg storm energy) instability will bring the threat for a little lightning and very brief downpours. The better chances for this will be near and west of I-29. With the freezing level around 11 kft, even small hail is not expected. While most areas will only see lighter accumulations, some spotty locations could see 0.10" to 0.25".
While there will likely be a bit of a lull in showery activity from about mid afternoon into the early overnight hours, the next wave looks to move into central SD later tonight into Sunday. Low and mid level winds respond to this wave, increasing warm air advection and deepening moisture. Once again, the better chances appear to remain mostly west of I-29.
The area begins to lose saturation in the dgz on Sunday as this wave moves north through central SD. Saturation remains most persistent west of highway 81, with northwest IA seeing the largest struggle to saturate aloft. Some very weak elevated instability in northwest IA later Sunday afternoon may allow for a spotty shower or two.
Monday may end up cooler than forecast with low level cool air likely remaining locked in with rainfall occurring. Interesting to note that the Nam, while an outlier, would potentially bring a little mixed precipitation or snow to the Huron and Chamberlain areas Monday afternoon/evening. Most of the other data hangs on to slightly warmer temperatures, but not by a lot. Slightly warmer low level temperatures would also be supported by the exiting of the right entrance region of the northern jet and the increase of the southern jet during this time.
To put all of this into perspective, the latest Nam and RRFS total qpf through Tuesday morning places about 1-2.5" over south central into east central SD and while the EC ensemble, GEFS and Canadian ensemble are a bit more broad with their output, the location is very similar, if not a touch farther west. A fairly tight gradient from higher amounts to lower amounts will likely set up from about Vermillion to Windom. All in all this tracks fairly well with the mid level waves (700-500mb) over the next few days.
Discussion
THIS AFTERNOON: Winds continue to increase early this afternoon with observed gusts between 30 and 45 mph. Temperatures continue to warm into the 70s, and potential for some highs in the 80s remain possible along and west of the James River valley. However, a persistent mid-upr cloud deck may limit the climb this afternoon. Out in western South Dakota, we're beginning to see the first signs of a frontal boundary forming that will gradually move eastward overnight. The fire danger remains very high this afternoon, especially for agricultural areas, please see separate discussion.
TONIGHT: An elongated upper trough stretching across the Rockies and into southern California will begin to stretch and split in two overnight. Northern energy tracking eastward across southern Canada will be strong enough to nudge the aforementioned surface boundary into the I-29 corridor by mid-mornign Saturday. Ahead of this boundary, convergence on a 40 knot LLJ and the arrival of a mid-lvl vorticity maxima should be enough to allow isolated to scattered showers to develop and and ahead of this boundary. MUCAPE may be as high as 300 J/KG after 3am, so have increased PotThunder to at least suggest isolated lightning strikes into mid-morning.
SATURDAY-SUNDAY: While northern stream energy moves eastward across Canada, a large upper trough will remain over the Four Corners region through the weekend. Overhead and closer to home, the aforementioned surface boundary will linger across the Tri-State area this weekend. This front, stacked on top by a low-lvl frontal zone, as well as somewhat persistent divergence from the right entrance region of an upper jet, will keep scattered rain chances and quite a bit of cloud cover through the area during the weekend. Now the degree of rain coverage is still up in the air, given somewhat diffuse and transient lift at times, but also some sub cloud dry air that could linger. So will continue to advertise a narrow stripe of low to mid-range chance of rain that will fluctuate north and south with the movement of the low-lvl baroclinic zone.
Temperatures will be quite tricky on Saturday as the front bisecting the forecast area could lead to a large temperature discrepancy from Huron to Storm Lake. 12Z guidance has trended downwards with highs slightly, so blended in short term guidance to account for trends the the blend-of-models forecast may not have caught onto yet. There seems to be much more consistency in guidance that cooler air may sink further south behind the front for Sunday. With a persistent and cool northeast surface flow in the forecast area, highs may struggle to break the 50s in portions of the forecast area with the warmest highs near Highway 20.
MONDAY-TUESDAY: Slight discrepancies in medium range guidance on Thursday have resolved somewhat this morning. The large upper trough in the Four Corner begins to eject northeast late Sunday into Monday, and will begin to enhanced warm advection northward and increase the intensity of the low-lvl frontal zone through the forecast area by Monday morning. We'll have to watch for any potential instability arrival, but fairly widespread moderate rain may begin to form Monday morning and gradually retreat northward as the upper trough enters the Plains.
Latest the American global model guidance has shifted the track of the upper low slightly further northwest from the Nebraska Panhandle into central South Dakota by Monday night, closely resembling the latest the European model runs. AI based models are also very similar in their track of this upper wave (perhaps a bit further south than deterministic runs), suggesting widespread rain will continue into Tuesday morning as the synoptic trough passes.
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