Area Forecast Discussion
Issued by NWS Bismarck, ND
Issued by NWS Bismarck, ND
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068 FXUS63 KBIS 240008 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 708 PM CDT Thu May 23 2024 .KEY MESSAGES... - Scattered severe thunderstorms (risk level 2 out of 5) across southern North Dakota this evening. Expected hazards are large hail up to 2 inches in diameter and damaging winds up to 70 mph, with a tornado or two possible. - Periods of scattered to widespread showers are expected this evening through Friday, and some snow could briefly mix with rain tonight through Friday morning. - Well below normal temperatures for most areas tonight through Friday night, with widespread overnight lows in the 30s. - Low to medium chances (20 to 40 percent) for showers and thunderstorms over the holiday weekend, with temperatures warming back closer to normal. && .UPDATE... Issued at 708 PM CDT Thu May 23 2024 We continue to monitor two general areas for strong to severe thunderstorms - one in the James River Valley, and one in southwest North Dakota. There is one main thunderstorm line extending across much of Logan and McIntosh Counties, with uncertainty on the upstream severe potential due to cooler surface temperatures and an overturned airmass. Upstream in the southwest is one main area of convection in Adams Counties and south into Perkins County, SD. This is the convection that WoFS had been advertising throughout the past few hours, with potential for these storms to remain strong to severe into Grant and Sioux Counties. Instability drops off quickly to the north, so we are expecting the severe potential to stay south of the Bismarck/Mandan area and I-94 corridor, although scattered thunderstorms are still possible in this area. Our general expectation is for the severe weather risk area-wide to diminish by around 9 PM CDT from both airmass overturning and diurnal destabilization with the onset of sunset. UPDATE Issued at 514 PM CDT Thu May 23 2024 We have expanded the Severe Thunderstorm Watch to include Hettinger, Adams, Grant, and Sioux Counties. Per the Mesoscale Discussion below and current radar trends, we are expecting continued development in southwest North Dakota that will move into the watch area. The most robust development at the moment is in south central North Dakota in Emmons County, with a cluster of thunderstorms ongoing in LaMoure and Dickey Counties. UPDATE Issued at 440 PM CDT Thu May 23 2024 ** MESOSCALE DISCUSSION ** Severe-storm risk will continue over the Dickey/LaMoure/McIntosh County vicinity through early evening, with an increasing strong-severe storm risk westward along and north of the ND/SD border to the Emmons, Sioux, Grant, and Adams/Hettinger County areas through the next several hours. As of 2130 UTC, a mix of supercells and increasingly clustered/ messy convective modes is ongoing near and north of the warm frontal zone in the southern James River valley. Strong deep- layer shear, sufficient low-level hodograph curvature to favor right-moving supercells (over left-movers) north of the warm front, and modest bouyancy favor a continued severe risk with this activity. The dominant threat will be large hail, though peak sizes may be limited by storm interactions as mode becomes more messy by 2230 UTC. Recent increases in low-level SRH (with 0-1-km SRH over 200 m2/s2 on the KBIS VWP, which is also north of the warm frontal zone) support low-level mesocyclones and as a result a tornado or two remain possible in far south central ND and the southern James River Valley of ND. Overall though the tendency for upscale-growing modes and modest thermodynamics may temper the threat ceiling. Extrapolation takes this activity out of our area and into southeastern ND by 6 pm CDT, but recent WoFS cycles have presented moderate chances of additional development as far west as Emmons County in the next 60 minutes, which would prolong some risk (mainly in the form of large hail) in this general area. Meanwhile, further west into southwest and far south central ND to the west of the Missouri River, an axis of somewhat higher surface dewpoints in the lower 50s F extends from Mobridge to Hettinger, north of the surface warm front. Weak to moderate bouyancy and strong deep-layer shear is present in this area, and satellite trends presenting increasing ascent approaching from the southwest and deepening convective towers suggests the probability of strong-severe storm development is increasing as far west as Adams/Hettinger Counties the next few hours. WoFS cycles also support this notion. Large hail would also be a risk with any supercell(s) that could evolve in this corridor, and would likely be the main risk given a potentially more elevated nature to these storms further north of the warm front. CJS UPDATE Issued at 329 PM CDT Thu May 23 2024 Quick update for the issuance of a Severe Thunderstorm Watch for Emmons, Logan, McIntosh, LaMoure, and Dickey Counties until 11 PM CT. Still expecting potential development in this area within the next hour or two. && .DISCUSSION... Issued at 120 PM CDT Thu May 23 2024 Latest surface analysis shows a frontal boundary draped over southern North Dakota, with surface low pressure over west central SD. North of the front east to northeast winds remain over much of western and central ND. Temperatures north of the front were generally in the mid and upper 40s north, to the mid 50s central. As you get into southern ND, generally along the I-94 corridor and south, temperatures increase into the 60s and although moisture isn`t great, as get near the aforementioned boundary, we see surface dewpoints into the middle 50s. A strong shortwave trough from southeast Montana into north central WY, with another wave situated over the High Plains of eastern WY. Subtle cloud top cooling/glaciation over west central SD and Moderate cu development over far southwest ND into northwest SD may be the indication of the forcing that will be approaching this afternoon, which if you extrapolate would put possible development over south central ND around 20-22 UTC. This current area of south central ND southward into north central SD is currently characterized with ML CAPE of 1000-1500 J/KG advecting north into southern ND, along with effective shear of 45 to 60 knots currently situated over southern ND. This puts southwest and south central ND in a favorable position for strong to severe thunderstorms from mid-late afternoon and into this evening. the latest HREF Upward helicity Neighborhood Proximity values are very similar to the 00Z run this morning in the location centered around Emmons, Logan, McIntosh and Dickey counties and if anything a little stronger and extended both west and east along the aforementioned surface boundary over southern ND. SREF sig Severe potential is also very near this position and with a bit higher probabilities than earlier runs. Compared to this morning we don`t think hazards have changed. We will still message 2 inch hail, 70 mph winds and a tornado or two possible. The tornado threat looks to be favored over the southern JRV, and back possibly to the Missouri River. The hail threat would cover most of southwest and south central ND along and south of the I-94 corridor. The wind threat also looks to be favored along and south of the SD border, but as the overnight shift mentioned, the 0-3KM shear vectors are very strong over northwest and north central SD, so any convection that would tend to bow out would have an increased wind potential if it develops over, or tracks into North Dakota. For all of these threats, the area extent is pretty limited, with the I-94 corridor a cutoff for the northern extent. As for the timing. Some of the CAMS were indicating convection developing earlier over southeast ND and far southwest ND, but in general, where we anticipate strong to severe convection this afternoon, the timing seems to be still pointing to around the 20-22 UTC timeframe, through around 02-03 UTC. Although the southwest and especially the south central are favored for severe storms this afternoon/evening, all of western and central can expect scattered to showers and thunderstorms through evening and lingering into the overnight hours. Late tonight and early Friday the focus shifts to the north and west with the potential for frost. As the system begins to exit, current forecast trends clear out far western ND after around 09 UTC. The northwest looks to be the most likely area to see some frost. Will monitor but the areal extent and duration is quite small. We have areas of frost over portions of Divide and Williams counties and gardeners may want to take preventative measures. At this time tough will hold off on a frost advisory. Depending on the amount of dynamic cooling with the system, we could still see a mix of rain/snow over north central ND as the system lifts into Southern Canada. It`s possible the higher elevations in the Turtle Mountains could see some light accumulations, otherwise expect mainly a mix or a brief changeover to light snow before ending. There will also be some strong winds on the back side of this system. It`s possible we may need some wind highlights Friday morning, or even possibly during the overnight hours in the southwest. With the severe threat today and quite a bit of uncertainty in the strength of the winds overnight will hold off on wind highlights at this time. Friday will be quite cool, especially north central into northeast ND where highs will struggle to get out of the 40s. Most areas will see highs in the 50s. Frost potential will be a concern late Friday and into Saturday morning and a frost advisory may be needed for a good portion of the forecast area. A weak pressure pattern is situated over the area bringing light winds. This would favor frost if we have little sky cover. This seems to be the case, but one potential failure mode would be stratus/fog developing. There does appear to be a signal for fog in the latest bufkit soundings. Will continue to monitor. A warming trend is then expected through the remainder of the Holiday Weekend and into early next week. There will be some hit and miss chances for showers and thunderstorms through the weekend, but no big washouts at this time. The greatest risk for showers and/or thunderstorms looks to be in the far south central and into the James River Valley on Saturday afternoon/evening. Uncertainty increases in the Sunday/Monday timeframe but there will be a progressive wave or two brining a chance of threat for showers and thunderstorms. At this time, the threat for severe storms looks minimal through the Holiday weekend and through the first half next week. && .AVIATION /00Z TAFS THROUGH 00Z SATURDAY/... Issued at 708 PM CDT Thu May 23 2024 Scattered showers with areas of MVFR-IFR ceilings remain across northwest and north central ND this evening. Thunderstorms over southern ND (mainly along and south of the I-94 corridor) could be severe with large hail and damaging winds as high as 55-60 kts. Significant visibility restrictions can be expected with any thunderstorm, and prevailing MVFR/IFR visibility is otherwise possible under showers. Ceilings will continue to lower to MVFR/IFR levels from north this evening. Improvements in cigs/vsbys are expected to move into the far northwest, including KXWA, later tonight, with most of the are remaining MVFR to IFR or lower into Friday morning. Northeast winds will remain steady at 15-20 kts through the night, with gusts to 25-30 kts. Conditions will improve late in the TAF period. && .BIS WATCHES/WARNINGS/ADVISORIES... None. && $$ UPDATE...CJS/Jones DISCUSSION...TWH AVIATION...TWH/Jones