Area Forecast Discussion
Issued by NWS Sioux Falls, SD
Issued by NWS Sioux Falls, SD
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794 FXUS63 KFSD 171128 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 628 AM CDT Fri May 17 2024 .KEY MESSAGES... - Well above normal temperatures will end the work week. High confidence (90%) in widespread highs in the mid-upper 80s today, with low-moderate confidence (30-50%) in some areas topping 90F west of I-29. - Periods of breezy conditions return today and continue through the weekend. Occasional gusts over 30 mph this afternoon into Saturday, but probability of reaching Wind Advisory criteria is low. - Sunday afternoon into Tuesday continues to look more active with the potential for periods of locally heavy rain and severe weather possible. && .DISCUSSION... Issued at 423 AM CDT Fri May 17 2024 TODAY-SATURDAY: Broad warm advection early this morning has been producing mid-level clouds, with isolated high-based sprinkles sliding southeast. Have even seen a few lightning strikes in our southwest in an area of steeper mid-level lapse rates. Expect this activity to continue to drift southeast across the forecast area through mid-morning before waning as warm advection diminishes by 15Z. Focus for the rest of today will be breezy south winds and brief push of well above normal temperatures as a low level thermal ridge builds into central SD. 850mb climbing above 20C by this afternoon could support a few highs above 90F west of I-29, while mid-upper 80s will be more common across the east. Strongest low level winds are focused east of the thermal ridge, with soundings supporting occasional gusts to around 30 mph across portions of northwest IA/southwest MN this afternoon. 35-40kt low level jet continues overnight, with another surge of broad warm advection ahead of an approaching cold front. Moisture profile is a little drier tonight than we are seeing early this morning, but with some weak mid-level instability cannot rule out some spotty sprinkles once again, so have added a mention of this ahead of the front tonight into early Saturday. This looks to be about the only precipitation chances with this front, which should push east of the forecast area before midday Saturday. Strong push of cold advection behind the front could result in a brief period of stronger winds early Saturday across our west, with some soundings showing initial post-frontal mixing tapping into 40kt atop the mixed layer. This looks to be short-lived, though, with 30-35 mph gusts more likely in the post-frontal air mass Saturday morning, and this mainly west of I-29. As we progress through the day, a slackening pressure gradient should lead to less breezy winds through the afternoon, with light/variable winds Saturday night as surface high pressure slides across the area. Highs on Saturday are still a little uncertain, though with better consensus on frontal timing, think the potential for highs topping 80F is on the low side, even in our far southeast. SUNDAY-TUESDAY: Much more unsettled period as we head into next week. Sunday will see increasing rain chances in response to a couple of mid level waves moving across the region in southwest flow aloft. First wave moves through Sunday morning, combined with a broad area of warm advection. This wave will be fighting some lingering low-mid level dry air in the wake of departing surface high pressure, but would expect to see at least scattered showers and a few embedded thunderstorms as the deep lift works to overcome the dry air. A trailing wave is progged to slide across the area during the afternoon/evening, though some questions regarding the ability to destabilize in the wake of morning activity. If the instability materializes, seeing fair agreement among models that deep layer shear will be supportive of a few organized strong to severe storms. Cannot argue with latest SPC Day 3 Marginal Risk (Level 1 of 5) in our southern counties, generally south of the Highway 18 corridor, though still plenty of uncertainty regarding timing/location of more favored severe threat. We remain in the somewhat active southwest flow through Tuesday, though ensemble joint probabilities of sufficient instability/shear remain on the low side (<30%) for Monday through Tuesday. If the surface boundary remains to our south as projected by some model solutions, this could limit our severe potential but may increase the potential for areas of heavy rainfall as a stronger wave later Monday night-Tuesday slides northeast along the elevated boundary. All in all, uncertainty is high regarding how exactly things will evolve through the first half of next week. However, ensembles are showing moderate probabilities (50-70%) for total rainfall topping 1" through Tuesday night, with low (20-30%) probabilities for over 2", both focused across the southeast half of the forecast area where rivers have already been running higher. WPC highlights this areas with a Marginal (Level 1 of 4) Risk in the Days 3 through 5 (Sunday through Tuesday) excessive rainfall outlooks. WEDNESDAY-THURSDAY: Details for this period are low confidence as there is little agreement among the longer range models. However, moderate consensus in some cooler air settling into the northern Plains mid-week with periodic rain chances possible, depending on how quickly a deepening upper low retreats to the northeast. && .AVIATION /12Z TAFS THROUGH 12Z SATURDAY/... Issued at 621 AM CDT Fri May 17 2024 Early morning ACCAS/high-based sprinkles/showers are expected to diminish by 14Z-15Z. While sustained winds will generally be at or below 12kt to start this TAF period, these isolated showers may produce brief erratic gusts up to 20-25kt. After 14Z-15Z, low level mixing will result in more widespread and persistent gusts to around 25kt through this afternoon, especially along and east of the US Highway 81 corridor. While less persistent, occasional southerly gusts to around 25kt will remain possible overnight as a 40+kt low level jet develops over the area. This will also result in a period of low level wind shear across most of the forecast area tonight. A cold front will move into the forecast area from the west after 18/06Z. While this will shift the low level wind shear off to the southeast with time, increasing gusts are expected behind the surface boundary, locally as high as 30-35kt for areas west of US Highway 81 during the latter hours of this TAF period. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ DISCUSSION...JH AVIATION...JH