Area Forecast Discussion
Issued by NWS Bismarck, ND

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129 FXUS63 KBIS 280921 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 421 AM CDT Fri Jun 28 2024 .KEY MESSAGES...
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- Showers and thunderstorms are possible this afternoon and evening, with 60-80 percent chances in northern North Dakota and 20-30 percent chances in the south. - Strong storms with small hail and gusty winds are possible this afternoon and evening, but the severe storm risk is low. - It will be breezy to windy today, especially in southwestern North Dakota where gusts of 35 to 45 are expected, highest in Bowman and Slope Counties. - Severe thunderstorm potential (a Level 2 of 5 Risk) in the form of large hail and damaging winds returns Sunday night.
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&& .DISCUSSION...
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Issued at 421 AM CDT Fri Jun 28 2024 Early this morning a weak cold frontal boundary is slowly moving eastward across the area, and is located along the Highway 83 corridor as of 08 UTC. Last evening`s convection has moved well east of the frontal zone, into the far eastern Dakotas/Minnesota as of this hour. The air mass in the wake of that QLCS is much too convectively-processed and stabilized for any renewed storm development with the eastward-moving front early this morning. However, the moist and stable boundary layer is contributing to significant stratus over central ND, and areas of fog in north central ND where light upslope/easterly winds into the terrain of the coteau region occurred prior to the frontal passage. We expect the stratus and fog to gradually lift as the front moves eastward, but it may linger into the early part of the day in parts of central and especially north central ND. The main feature of interest today and tonight is an upper-level low initially over southwestern Canada early this morning, which is forecast to move east-southeast, reaching western and central ND by late afternoon and evening. Upstream radar imagery reveals an area of showers already associated with the cyclonic flow in the vicinity of this upper-level low, and guidance is consistent in showing showers and some thunderstorms moving into the area and/or further developing with diurnal destabilization this afternoon. Precipitation chances this afternoon and evening range from 60 to 80 percent in northwest and north central ND, where synoptic-scale ascent will be strongest near and north of the upper-level wave, to 20 to 30 percent along the ND/SD border. Boundary layer destabilization with residual low-level moisture characterized by dewpoints in the 45 to 55 F range and cooling temperatures aloft will contribute to around 500 J/kg of MLCAPE. This is sufficient for scattered thunderstorms this afternoon and evening, but the bouyancy profile is rather "skinny" owing to modest midlevel lapse rates and a lack of greater low-level moisture. This may tend to limit updraft accelerations, and marginalizes the severe-storm risk despite effective-layer shear around 40 kt. As a result of the expected thermodynamic setting, and both GEFS-based machine-learning and HREF-driven calibrated severe storm guidance having probabilities less than 5 percent in our area, we have chosen to focus our messaging on the potential for strong storms with gusty winds and small hail rather than explicitly mentioning severe storms in our outlooks. However, the reasonable worst-case scenario for this afternoon and early evening would be for a marginal severe storm with large hail to the size of quarters and/or damaging wind gusts up to 60 mph, mainly in central ND where the CAPE-shear setting will be most favorable. Confidence in the coverage of that risk wasn`t high enough to explicitly message at this time, though mesoscale trends will be monitored through the day in case a more well-defined area of concern becomes apparent. Otherwise, a stronger cold front is expected to move across western ND this afternoon and central ND this evening, in tandem with the upper-level trough. The 00 UTC guidance forecasts a few-hour period of strong low- and midlevel cold air advection with the front centered on southwestern ND in the late afternoon and early evening. This includes significant "thermal packing" at the 850 mb level with the frontal passage, which often signals a period of strong winds when that cold air advection and its associated momentum transfer occur in an already-well-mixed boundary layer after peak heating of the day. Given this trend, along with a signal in ECWMF ensembles (EFI values of 0.8), and forecast soundings from various guidance suggesting mixed-layer winds of 40-45 kt, we have issued a Wind Advisory for Bowman and Slope Counties this afternoon and evening. We gave consideration to including additional counties north and east of there, but momentum transfer is more uncertain with latitude given the midlevel ascent associated with the upper-level low. Moreover, with eastward extent the timing of the frontal passage will be late enough in the day that the duration of strong winds is more uncertain. Nonetheless, it will still be windy across all of southwestern ND this afternoon and evening, and there is a low to medium probability that the Wind Advisory could need to be expanded a bit with later updates. Highs today will range from the mid 60s in northwestern ND, where precipitation and the cold front will arrive first, to mainly the 70s elsewhere. Lows tonight will be cool, in the 40s F in most areas, as high pressure originating over Canada begins building into the area. That will set the stage for below-normal temperatures on Saturday when highs are forecast to only be in the 60s F, albeit with dry weather, a mostly sunny sky, and lighter winds as ridging dominates at the surface and aloft. Southeast return-flow will develop on Sunday as middle- and upper-level ridging shifts eastward ahead of a trough that will move into the northern Rockies. This will establish a warming trend, and low-level moisture advection as a lee-side surface low develops over MT/WY. The timing of an upstream impulse in the increasing southwesterly flow suggests convection will be favored to develop over MT in the afternoon and evening, and may then move across western and central ND Sunday night. We anticipate a severe-storm risk in the form of large hail and damaging winds will accompany this activity given the expected CAPE-shear setting, and as a strong low- level jet develops overnight. However, forecast soundings do suggest strong capping associated with a warm elevated mixed layer may be a challenge to the overall event. Machine-learning guidance and the SPC outlook that contains up to a Level 2 of 5 Risk in parts of western ND nonetheless support messaging of a severe storm risk Sunday night in western and central ND. A severe-storm risk may exist Monday as well, depending on the timing of a surface frontal zone and the effects of any overnight and early-day convection. However, strong southwest flow aloft ahead of an advancing upper-level trough and rich low-level moisture would yield potential severe storms if the frontal passage is slow enough and sufficient heating occurs. Global ensemble members in the NBM are closely clustered with the large-scale pattern through Monday, heralding a confident general forecast. Beyond Monday, approximately 70% of ensemble members suggest broad troughing will persist across the area for much of next week, which would support cooler temperatures and 20 to 40 percent chances of showers and thunderstorms. However, spread in guidance increases, especially by the 4th of July, when there`s about a 30% chance of a bit warmer and drier pattern than the ensemble mean and NBM call for.
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&& .AVIATION /06Z TAFS THROUGH 06Z FRIDAY/...
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Issued at 421 AM CDT Fri Jun 28 2024 MVFR to LIFR ceilings and patchy fog are ongoing in central and especially north central ND as of 09 UTC. They will persist until a cold front passes, but improving conditions are forecast between 12 and 14 UTC behind that front. Mainly VFR conditions are expected during the day today, but isolated to scattered showers and thunderstorms are forecast, mainly after 18 UTC and mainly in northern ND. MVFR ceilings may also return to northern ND after 00 UTC (a 40-60 percent chance).
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&& .BIS WATCHES/WARNINGS/ADVISORIES...
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Wind Advisory from 2 PM this afternoon to 9 PM MDT this evening for NDZ040-043.
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&& $$ DISCUSSION...CJS AVIATION...CJS