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FXUS63 KFSD 030325 AFDFSD

AREA FORECAST DISCUSSION National Weather Service Sioux Falls SD 1025 PM CDT Mon Oct 2 2023

SHORT TERM
(This Evening through Tuesday Night)
Issued at 259 PM CDT Mon Oct 2 2023

KEY MESSAGES:

1. Seasonally hot temperatures wind down after today, remaining warmer than normal on Tuesday.

2. Scattered-numerous showers/storms Tuesday, with potential for isolated severe storms (mainly wind) and locally heavy rain during the afternoon/evening.

3. Pattern transitions mid-week to bring cooler than normal temperatures with first chances for frost Friday night-early Saturday.

....................................................................

Under the anomalously strong ridge, temperatures have again soared up into the mid 80s to lower 90s as of 2 PM. Unsurprisingly, our warm temperatures are accompanied by breezy southerly winds, causing wind gusts into the mid 20s to mid 30s. With relative humidity values dipping down into the 30s and upper 20s, high fire danger is also resulting due to warm and dry conditions. Ensemble situational awareness tables (ESATs) continue to show temperatures throughout the air parcel AOA the 90th percentile of climatology, which will keep temperatures within grasp of record high daily temperatures, or create the new record. The following are the record highs for today.

Record High Temperatures for October 2: KFSD: 92/1953 KSUX: 94/1997 KHON: 94/1910 and 1997 KMHE: 93/1910 and 1953

After sunset, winds turn slightly more south-southeasterly, and after decoupling from the boundary layer gustiness will decrease though sustained winds will remain on the breezy side overnight.
Given the warm air already in place and breezy sustained winds continuing, warm overnight temperatures in the mid to upper 60s are again expected. For those keeping track, these temperatures are indeed near our daily high temperatures...and will thus likely be breaking our daily record high minimum temperatures, which are as follows:

Record Warm Low Temperatures for October 2: KFSD: 68/1938 KSUX: 66/1900 KHON: 67/1884 KMHE: 68/1953

Otherwise, a shortwave from the upper level trough axis to our west spawns a surface low pressure over western Nebraska/South Dakota along the baroclinic zone overnight. This will keep the surface pressure gradient (SPG) strengthened as the accompanying cold front is dragged eastwards, resulting in another day with breezy southerly winds. After sunrise, winds will quickly become breezy with gusts into the lower to mid 30s, aside from areas along and west of the James who will remain in the 20s due to their closer proximity to the cold front.

While temperatures on Tuesday will remain above average, there is some uncertainty given chances for showers and thunderstorms across the region throughout the day. The convection allowing models (CAMs)
that produce rainfall throughout the morning hours limit our heating potential, keeping afternoon highs in the lower to mid 70s, and the CAMs that keep us dry bring portions of the area back up into the mid 80s. While not every model shows this, some models show weak positive vorticity advection out ahead of the main wave thus keeping the possibility for scattered morning showers and thunderstorms.
However, given the weak forcing aloft and general weak instability, am inclined to think any showers/storms will be more isolated.

By the late morning and into the afternoon hours, the aforementioned surface low will be well off to our north, with a dry line possibly draped down between the I-29 corridor and the James River Valley.
Models continue to show that instability will be marginal throughout the event, coming in ever so slightly weaker than their 00Z counterparts, with most keeping CAPE values constrained in the 1000- 1500 J/kg range. Soundings from across the area show marginal/decent backing shear in the lower levels, with speed shear dominating the rest of the sounding profile along with some veering winds in there as well. The HRRR shows the mean surface to 500 mb shear tops out at around 50 knots, largely due to the speed component, and given the relatively weak instability the shear may actually limit how strong the storms are able to become. As mentioned by the previous discussion, the ESATs continue to show strong 850mb winds exceeding the 90th percentile of climatology across areas generally along and east of I-29, with the RAP also picking up on 850mb winds exceeding 40 knots.

The main wave approaching from the east-southeast will be the one to watch, as that is what the CAMs are showing will spark off the thunderstorms by the early/mid afternoon hours across central Nebraska. Given the strong 850 mb winds, the storms are expected to race northeastwards as the wave comes in from the east, with the primary threat being strong damaging winds to 65 mph. The HRRR neural network shows we could see brief hail threat towards the beginning of the event for areas west of I-29, with the CSU GEFS machine learning probabilities showing similar probabilities for areas along and west of I-29. While a tornado can't be entirely ruled out, chances look to remain on the lower side but will see best chances for areas along the dry line (again west of I-29). With locally heavy rainfall possible and numerous chances for rain given the various boundaries and forcing, can't rule out a very isolated flooding threat, but given the quickly moving storms any location would need to see multiple rounds of thunderstorms within a short time period. Showers and thunderstorms end overnight from west to east as the cold front plows eastward.

LONG TERM
(Wednesday through Monday)
Issued at 259 PM CDT Mon Oct 2 2023

By Wednesday morning, the cold front will have finished sweeping through, with early morning temperatures in the lower to mid 50s. As a weak high pressure develops at the surface just south of the region, we'll see periods of breezy north-northwesterly winds before the surface high slides eastwards which will bringing calmer winds back to the region. While Wednesday will be much closer to normal temperatures than we have been, we will have shifted into temperatures below normal with afternoon highs in the mid to upper 60s whereas normal highs would be in the upper 60s. This will be a sharp contrast to the temperatures we have been experiencing, but will remind all that fall is indeed here.

Overnight into Thursday, the jet max reinforces the trough aloft, strengthening the winds back up again as a new surface low pressure moves into the northern great plains from Saskatchewan/Manitoba, Canada. This will bring our next cold front into the region with afternoon highs a bit lower into the lower to mid 60s, and breezy northwest winds.

A surface high pressure develops off to the west as ridging develops off to the west. This will likely bring another shot of cooler air into the region on Friday, cooling us off into the lower to mid 50s for afternoon highs. Chances for frost return to the area Friday night into Saturday, though that will depend on how breezy we are and when the surface ridge axis slides overhead. Temperatures start to warm up again to become near normal by Sunday/Monday.

AVIATION
(For the 06Z TAFS through 06Z Tuesday Night)
Issued at 1022 PM CDT Mon Oct 2 2023

The main concerns will be thunderstorm chances, especially east of the James River late tomorrow afternoon and night. A few of these storms will contain heavy rain as well as bring the potential for severe weather. Isolatedf to scattered activity will be possible during the morning into the early afternoon but is not expected to have as much impact and is more difficult to forecast timing.
Otherwise some LLWS will be possible overnight during periods of lighter winds at the surface.

FSD WATCHES/WARNINGS/ADVISORIES
SD...None.
MN...None.
IA...None.
NE...None.

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