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FXUS63 KBIS 031128 AFDBIS

Area Forecast Discussion National Weather Service Bismarck ND 628 AM CDT Fri May 3 2024

KEY MESSAGES

- Expect low to medium chances (20 to 60 percent) for rain showers through the day, mainly across the northern half of western and central North Dakota. Some light snow may mix in across the west this morning.

- Temperatures will warm through the weekend, with windy conditions on Sunday.

- Chances for rain return Sunday evening and continue through the work week. The wettest period will Monday through Tuesday when most of western and central North Dakota will see medium to high chances (60 to 90 percent) of rain.

UPDATE
Issued at 622 AM CDT Fri May 3 2024

The forecast looks to generally be on track this morning. We have seen a few radar echoes across the northwest and north central, potentially bringing a few rain drops to the surface as they pass overhead. Some more substantial showers have been observed on the Williston radar over the past couple of hours, extending from around Culbertson Montana southeast down to around Grassy Butte. More showers should continue to develop across the northwest and north central through the day. No major changes were needed for this update.

DISCUSSION
Issued at 333 AM CDT Fri May 3 2024

Early this morning, western and central North Dakota sits under the influence of broad quasi-stationary troughing overhead. The upper low that brought showers and thunderstorms to the region yesterday will continue to eject out into Ontario to our northeast as the next upper low rotates down into northwest North Dakota from southern Saskatchewan later today. This low will be the trigger for showers to develop through the day (20 to 60 percent chance), mainly across the northern half of the forecast area (best chances along the International Border).
Some light snow may even mix in across the west briefly this morning. RAP soundings suggest that instability will be limited this afternoon and evening with MUCAPE values mainly maxing out in the 100 to 200 J/kg range. While a stray rumble of thunder or two cannot be ruled out, confidence is not high enough to include mention of thunder in the gridded forecast at this time.

Precipitation chances will diminish to the northeast tonight as the upper level trough finally starts to move out to our east, transitioning western and central North Dakota into northwest flow aloft on Saturday. Surface high pressure will also slide in on Saturday, leading to diminishing clouds from west to east and slightly warmer temperatures. Highs will remain cool today, ranging from the mid 40s northeast to the mid 50s southeast and then by Saturday, highs will range from the lower 50s to lower 60s.

An upper level ridge axis then approaches Saturday night and crosses the state on Sunday. This will mean even warmer temperatures with forecast highs ranging from the lower 60s to lower 70s. Behind the ridge axis, a deep trough will approach from the west, transitioning western and central North Dakota into meridional flow aloft. As this trough approaches, a surface low will deepen to around 990 mb or less over eastern Montana. This will lead to strong warm air advection, pressure falls, and a rapidly tightening pressure gradient on Sunday for our area. So, while highs will be fairly mild, it will be windy with soundings suggesting the potential for 40 knots or so available to mix down from the top of the mixed layer. The potential for gusts up to 45 mph seems reasonable for now and an eventual Wind Advisory seems like a decent probability.

The story then turns to the approaching strong storm system and widespread rain event Sunday night through the week. The sub 990 mb surface low will continue to deepen as the upper level trough starts to take on a negative tilt and nudges into the northern Plains. Widespread showers and some thunderstorms will develop through the day and night, but dry slotting is likely to setup somewhere depending on the eventual track of the low.
While thunderstorm chances will be better than in previous days, instability still appears to be somewhat lacking. However we could see some MLCAPE values up to 500 J/kg with deep layer shear forecast to range from around 35 to 45 knots. So, the severe weather potential appears to be fairly low but a couple of strong storms may be on the table.

The wettest period will generally be Monday through Tuesday (widespread 60 to 90 percent chances for showers) and another potential hazard could be heavy rain, especially if heavier showers/storms can train over some of the same areas. NBM 72-hour probabilities (ending 12z Thursday) don't suggest anything too dramatic with most of the west and central in medium probabilities (40 to 70 percent) for an inch of rain or more (the one exception being the James River Valley where the consensus currently wants to place most of the dry slotting).
When we increase that threshold to two inches, the probabilities fall off mainly into the 10 to 30 percent range. However, it is worth noting that the NAEFS is showing Integrated Water Vapor Transport, PWATs, and 850 mb specific humidity in the 97.5th percentile or greater for this time of year in the Monday through Tuesday time frame. Thus, there may be an opportunity here for some heavier rainfall in a few locations.

The upper low will start to wind down by mid week but will be slow to kick out into the Great Lakes region given a stubborn east coast ridge. Low to medium chances (20 to 60 percent) for showers will continue through at least Thursday.

Highs on Monday will not be quite as warm as Sunday given widespread clouds and rain. However, we should still see highs in the 60s. With the cool wet pattern continuing, highs will mainly dip back into the lower 50s to lower 60s for most of the rest of the work week.

AVIATION /12Z TAFS THROUGH 12Z SATURDAY/
Issued at 622 AM CDT Fri May 3 2024

Scattered showers will continue to increase in coverage over the northwest and north central today, but will generally be of the hit and miss variety. With such uncertainty about eventual location and tracks of these showers, we elected to keep mention of rain out of any specific site forecast except for KXWA where confidence is a bit higher. IFR to MVFR ceilings will be likely across the west and north through much of the period, including KXWA, KDIK, and KMOT. For now, it appears that KBIS and KJMS should remain in VFR categories through the period. Winds will become a bit gusty out of the west this afternoon, when we could see some gusts up to 30 mph or so.

BIS WATCHES/WARNINGS/ADVISORIES
None.

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