textproduct: Sioux Falls

KEY MESSAGES

- Areas of dense fog will persist along/east of US Highway 81 this morning, especially in higher elevations of the Coteau des Prairies in east central South Dakota, and in portions of northwest Iowa/northeast Nebraska. A Dense Fog Advisory remains in place for these areas through Noon CDT.

- Areas of drizzle are also expected with the widespread low clouds along and east of US Highway 81 today into tonight. Near freezing temperatures early this morning will rise into the mid-upper 30s today, so impacts from possible freezing drizzle are expected to be minimal.

- Scattered light rain showers/sprinkles are still forecast near and east of I-29 on Friday, with seasonably mild high temperatures in the 40s and little to no travel impacts on tap for this post-holiday weekend.

- Will be monitoring potential for a stronger system which could impact the region early next week. Moderate (40-70%) confidence in precipitation occurring in some form, but low (<30%) confidence with regard to precipitation type/amounts due to variations in potential storm track.

UPDATE

Issued at 535 AM CST Thu Dec 26 2024

Webcams are still supporting greater coverage of dense fog across the Coteau in east central South Dakota, along the Highway 71 corridor in northwest Iowa and in portions of northeast Nebraska, so will leave the Dense Fog Advisory in place for these areas.

However, have not yet seen anticipated drop in visibility across far southeast South Dakota, much of northwest Iowa, and southwest Minnesota as has been indicated by the models throughout the night. Thus have cancelled the advisory from Yankton to Sioux Falls, into southwest Minnesota and northwest Iowa west of Highway 71. Could still see some lower visibility develop toward/just after sunrise, but widespread dense fog is no longer expected for these areas.

DISCUSSION

Issued at 356 AM CST Thu Dec 26 2024

TODAY: Early morning challenges have focused on fog and drizzle as abundant low level moisture persists in a broad south-southeast flow across the region. Subtle low level lift within/just above the stratus layer per forecast soundings had resulted in areas of drizzle/freezing drizzle developing during the late evening hours, and it seems this also helped boost visibility for a time. Have been hesitant to drop the existing Dense Fog Advisory as high-res guidance continues to show deteriorating conditions as we move through the pre-dawn hours, especially in the higher elevations of the Coteau in east-central South Dakota and the Buffalo Ridge in southwest Minnesota. Will continue to monitor trends over the next couple of hours, but may still cancel portions of the advisory before daybreak if this deterioration does not materialize.

As far as the drizzle, have seen a marked decrease in shallow echoes on the KFSD radar since over the past hour indicating a decrease in drizzle coverage, for now at least. Soundings still show some weak lift within the stratus layer at times through the remainder of today, so will keep drizzle in the forecast along/east of Highway 81 where stratus is deepest. However, as has been the case thus far, actual amounts are not expected to measure and will stick with very low (sub-15%) pops through the day. Regarding potential for light icing, with temperatures steady to slowly rising and low potential for measurable precip, think impacts from any freezing drizzle will be limited to untreated roads/sidewalks or elevated surfaces and do not plan to issue an advisory at this time.

TONIGHT-FRIDAY: Low level moisture persists across the eastern CWA through tonight with limited mid-level moisture until late tonight Friday ahead of a mid-upper level wave. The stratus will bring a continued low chance for areas of drizzle, though by this time temperatures should reside firmly in the mid-upper 30s. As the wave approaches, the increase of mid-level moisture will provide some potential for more bonafide light rain. Trends have been shifting higher rainfall amounts east of our forecast areas, but portions of southwest Minnesota and northwest Iowa could still see rainfall up to 0.10" before any rain slides east of the area Friday night.

Also of note with this wave will be a slight shift to westerly winds, which should help scour our some of the low level moisture which has been so persistent across the region this week. This should allow for a greater chance at more prevalent sunshine west of the James River Friday afternoon.

SATURDAY-SUNDAY: Although light, westerly low level flow does become more widespread on Saturday and models are in good agreement in pushing the low level moisture east of our area, so this weekend shows greater promise of partly to mostly sunny skies. This should also support more favorable mixing into the warmer air aloft, with daytime temperatures finally warming into the 40s over most of the region for the final weekend of 2024.

SUNDAY NIGHT-MONDAY NIGHT: Focus will shift to a stronger mid-upper level trough and associated surface low ejecting out of the Rockies Sunday night, and moving across the Plains Monday-Monday night. This system looks to be fairly progressive, but uncertainty in track is high due to a wide variance in model solutions. Clusters support the slight north-south differences seen in the deterministic models, but most clusters show at least moderate (40-70%) probabilities for at least 0.10" liquid-equivalent occurring somewhere in our forecast area late Sunday night into Monday night. The bigger question with the track will be with the thermal profiles and resulting precip type, though most favor an initial rain/freezing rain potential late Sunday night into Monday, switching to snow as the colder air pours in later Monday-Monday night. Too much uncertainty to determine amounts for any specific type, but this will be a period worth watching if you have travel plans early next week.

TUESDAY-WEDNESDAY: The transition to 2025 will see temperatures settle back closer to seasonal normals. Precipitation chances are low, though cannot rule out some light snow at times as weak waves slide through the prevailing northwest flow aloft.

AVIATION /12Z TAFS THROUGH 12Z FRIDAY/

Issued at 535 AM CST Thu Dec 26 2024

IFR-LIFR ceilings will remain the rule for most areas along and east of US Highway 81 throughout this TAF period, including KFSD and KSUX airfields. Low stratus may occasionally be accompanied by MVFR-IFR visibility in fog or periods of drizzle and may have to watch for light icing in higher elevations north of I-90 where 11Z temperatures are at/just below freezing. Locations along and south of I-90 have sufficiently warmed above freezing and icing is not expected.

West of Highway 81, the James River Valley areas (KHON-KMHE) are starting off the period near the edge of IFR stratus with moderate (50-70%) confidence of clearing for KHON by midday. Any clearing would be short-lived though, as the stratus is expected to work back into the James River Valley after sunset tonight.

FSD WATCHES/WARNINGS/ADVISORIES

SD...Dense Fog Advisory until noon CST today for SDZ039-040-054>056- 061. MN...None. IA...Dense Fog Advisory until noon CST today for IAZ003-014-022-032. NE...Dense Fog Advisory until noon CST today for NEZ013-014.