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FXUS63 KBIS 182348 AFDBIS

Area Forecast Discussion National Weather Service Bismarck ND 648 PM CDT Sat May 18 2024

KEY MESSAGES

- Low to medium rain shower chances (20 to 40 percent) return late tonight into early Sunday morning.

- There is a Marginal Risk of severe weather (level 1 out of 5)
late Sunday afternoon into the evening hours.

- High temperatures remain cool through the next work week, mainly in the 60s. Expect daily low to medium (20 to 40 percent) chances for showers and thunderstorms through the week.

UPDATE
Issued at 643 PM CDT Sat May 18 2024

The diurnal cumulus continue to slowly die across most of the area and will be clear skies be sunset. Winds are also relaxing and will go calm after sunset as well. The Wind Advisory will be allowed to expire as winds are calming now.

DISCUSSION
Issued at 231 PM CDT Sat May 18 2024

An upper low continues to spin off to the east just north of the Canadian border over eastern Manitoba. Associated light rain showers continue over Bottineau and Rolette counties, but should move out in the next hour or two.

Western and central North Dakota generally sits under the influence of near zonal flow aloft but we will see some subtle ridging move across the state through the rest of the day. While visible satellite imagery shows a decent amount of fair weather cumulus, this progressive ridging should lead to some subsidence across the west, leading to some clearing.

The strongest cold air advection and pressure rises have mainly moved off to the east, so while it will remain windy though the rest of the afternoon (winds out of the west northwest), it should very gradually diminish as we move towards sunset. Given the tight pressure gradient, we are still expecting sustained winds up to 30 mph and gusts to 45 mph for a few more hours.
Thus, the Wind Advisory still looks to be in good shape.

Late tonight and into Sunday morning, we will gradually transition to southwest flow aloft as a Pacific Northwest trough starts to approach. Warm air advection may lead to some low to medium (20 to 40 percent) chances for hit and miss showers late tonight into Sunday morning, mainly across the south central and James River Valley. This activity should move north through the morning and into the early afternoon but uncertainty remains about how long it will be able to sustain itself.

A better chance for showers and thunderstorms (40 to 70 percent)
should start to develop over the south central and James River Valley by late Sunday afternoon and into the evening hours. A surface low will start to deepen over South Dakota with an inverted surface trough extending to it's north northeast. This boundary should be the focus for thunderstorm development by the late afternoon hours. Across the far south, RAP soundings suggest the potential for up to 2000 J/kg of MUCAPE. However, looking at MLCAPE, we generally max out in the 500 to 1000 J/kg range. Still, with forecast deep layer shear will range from 45 to 55 knots over the same area. On paper, these parameters should be more than enough for a strong to severe storm or two.
However, there are concerns with moisture quality and depth. It is also worth noting that the RAP is a bit more bullish on higher instability than most of the other short term models (but the RAP still shows fairly "skinny" CAPE which may mean relatively narrow updrafts). That being said, deep layer shear vectors will mainly be orthogonal to the surface trough so storms that do form could be discrete or semi- discrete. The Storm Prediction Center has placed Emmons, Logan, McIntosh, LaMoure, and Dickey counties in a Marginal Risk (level 1 of 5)
for severe weather. After analyzing soundings and the latest CAMS, we have decided to message the marginal severe weather potential up to Kidder, Stutsman, and Foster counties as well.
For now, we will message wind gusts up to 60 mph and hail to the size of quarters. The one caveat is that if the RAP were to verify, these thresholds would possibly be too low. However, the RAP appears to be an outlier right now, maybe not mixing out dewpoints enough.

Regarding temperatures, Sunday will be the warmest day of the week with highs ranging from the lower 60s to the lower 70s. By Monday we will only see lower 70s in our far southeast with highs in the 60s elsewhere. Highs will mainly be in the 60s through midweek. The coolest days will then be possible towards the end of week (Thursday and Friday) when we could see some highs in the upper 50s to lower 60s.

The active progressive flow will also continue through the week, leading to daily low to medium (20 to 40 percent) chances of precipitation at various locations across western and central North Dakota. Thunderstorm potential will be limited given the cooler temperatures but a slight chance seems warranted towards the end of the week across the south as most models have a potent trough moving into the region (Thursday-Friday). The eventual track of this trough and associated upper low is still a bit up in the air. Most models/ensemble members have this feature but substantial uncertainty remains regarding the track and strength. It may be worth nothing that CIPs extended analogs suggest some severe potential across our south by the end of the week, but CSU Machine Learning guidance is not showing the potential at this point.

AVIATION /00Z TAFS THROUGH 00Z MONDAY/
Issued at 643 PM CDT Sat May 18 2024

VFR with a few cumulus leftover form the daytime currently.
Calming winds and clear skies overnight before VFR clouds move in early morning. Showers are possible at every terminal at some point in the late afternoon so included VCSH. Winds will increase to around 20kts Sunday from the southeast.

BIS WATCHES/WARNINGS/ADVISORIES
None.

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