766 FXUS63 KFSD 041103 AFDFSD AREA FORECAST DISCUSSION National Weather Service Sioux Falls SD 603 AM CDT Sun Apr 4 2021 .SHORT TERM...(Today and Tonight) Issued at 410 AM CDT Sun Apr 4 2021 Subtle wave and associated surface boundary will traverse the region today, with the boundary expected to stall over northwest Iowa this afternoon. Relatively weak surface gradient around the boundary should allow winds to remain light enough to hold shy of critical levels today. That said, despite a less favorable easterly flow in areas north of the boundary, have seen deeper than expected mixing of late regardless of wind direction. Thus have nudged winds upward in areas away from the boundary, and our western areas could see a few gusts approaching 20-25 mph at times this afternoon. Even with the boundary, the air mass remains extremely dry. We do have a projected increase in low level moisture along and south of the boundary through the day, and mid 40s dew points have worked their way into far southern Nebraska as of 08Z. However, with the very dry air aloft, think expected deeper mixing will overcome this boundary layer moisture advection, and have dropped afternoon dew points toward the lower end of guidance yet again. With temperatures likewise expected to mix deeper/warmer, yielding highs in the 80s most areas, expect to again see afternoon humidity levels fall below 20 percent, perhaps below 15 percent in areas north of the boundary across our SD/MN counties. Given these very low humidity levels, will have to closely watch winds north of the boundary in case they begin climbing even higher than currently forecast. Tonight, this boundary lifts back to the north in response to another approaching weak wave. Given the expected depth of dry air, have slowed the start time of our low pops across northwest Iowa, but will still carry a chance of showers or isolated storms from late evening through the overnight in response to the subtle wave and warm advection. .LONG TERM...(Monday through Saturday) Issued at 410 AM CDT Sun Apr 4 2021 The boundary lifts north Sunday night, placing our entire CWA in the warm sector ahead of yet another subtle wave brings a cool front into central South Dakota late in the day. This boundary looks a little slower in the latest model runs, allowing even warmer air to build northward into the forecast area for Monday. Winds are a much more favorable south-southwest most of the day, which should again promote deeper mixing than most models indicate, leading to a breezy day for most areas. In this case, we do see a deeper layer of low level moisture work into the forecast area from Sunday night through Monday. This should help keep humidity levels higher in areas east of the boundary on Monday, though still opted to hedge toward the drier/warmer end of guidance. This yields highs in the lower to mid 80s in our eastern areas, with afternoon humidity levels from 30-40 percent. Farther west, areas toward the approaching boundary are farther removed from the influx of deeper moisture, and also closer to a strong thermal ridge just ahead of the boundary. Low level temperatures support highs approaching 90 degrees in areas west of the James River, with the drier southwest-westerly flow allowing for humidity levels nearing 15-20 percent once again. At this time, winds appear to remain shy of critical levels, but this will have to be closely evaluated in later forecasts. Monday night onward should see a pattern change away from our warm and dry conditions, as a cutoff upper low slowly works its way east across the Plains. Could see a few storms develop along the cool front as it pushes east Monday evening, and SPC has maintained a Marginal Risk for portions of the Highway 14 corridor. Instability would support isolated hail threat, while the dry mid-levels are supportive of downburst winds, though deep layer shear is rather low in our area, more supportive of organized storms farther to the northeast. The cutoff upper low, with its associated TroWAL wrapping around the north side, will then spread increasing rain chances across the forecast area from Tuesday through Wednesday, lingering across our eastern areas into Thursday. Still some uncertainty on how far north/south the system will track, so will cap pops in the likely range for now. Though model averages point to a decent potential for total rainfall amounts in excess of an inch across much of the CWA through the midweek period. This system works east of the area by Friday, with seasonable temps and limited rain chances into next weekend. && .AVIATION...(For the 12Z TAFS through 12Z Monday morning) Issued at 603 AM CDT Sun Apr 4 2021 VFR conditions and light winds will prevail for most of the forecast period, though the afternoon/early evening hours may bring gusts of 15-20kt to some areas west of I-29, and toward KSLB. Some indication that areas from KSUX-KSLB could see a period of low level wind shear after 05/04Z as southwesterly low level jet increases aloft. However, some uncertainty regarding where the stronger winds aloft will be located, so will forgo mention of LLWS in KSUX TAF at this time. A few isolated high-based showers or storms will also be possible in northwest Iowa overnight, but risk for any one point is too low to include in TAFs. && .FIRE WEATHER... Issued at 410 AM CDT Sun Apr 4 2021 Very warm and dry air mass will remain in place through Monday. Winds will be marginal today, perhaps gusting 20 to 25 mph across portions of southeast South Dakota this afternoon. However, humidity levels will be very low this afternoon, as low as 10 to 20 percent, leading to continued elevated fire danger. Areas along and east of the I-29 corridor should see some increase in humidity levels by Monday, though southerly winds will also be a bit more breezy, so we will see one more day of elevated fire danger before a slow moving system brings widespread rain chances across the region midweek. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && $$ SHORT TERM...JH LONG TERM...JH AVIATION...JH FIRE WEATHER...JH