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Area Forecast Discussion
Issued by NWS Bismarck, ND
Issued by NWS Bismarck, ND
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087 FXUS63 KBIS 141606 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 1106 AM CDT Wed Aug 14 2024 .KEY MESSAGES... - Thunderstorms producing torrential rainfall are likely across central North Dakota today. The highest risk for flash flooding is along the Interstate 94 corridor from Glen Ullin to Jamestown, and the Highway 200 corridor from Beulah to Carrington. - Isolated severe storms are possible across all of western and central North Dakota today. In addition to heavy rain, the main expected hazards are hail up to quarter size and winds up to 60 mph. - Quieter weather is expected Friday into early next week, with a slight warming trend through the weekend. && .UPDATE... Issued at 1106 AM CDT Wed Aug 14 2024 A more organized cluster of thunderstorms is entering the Bismarck/Mandan area. While thunderstorms are not as intense as yesterday (Tuesday the 13th), they are still producing around an inch to an inch and a quarter of rainfall per hour, and are expected to last a couple hours. This is enough to where street flooding is expected. Therefore, a Flood Advisory has been issued for the Bismarck/Mandan area until 5 pm CDT. UPDATE Issued at 918 AM CDT Wed Aug 14 2024 Areal coverage of showers and thunderstorms continues to expand in the southwest, while thundershowers have entered the southern James River Valley of ND. There are also isolated thunderstorms occurring in the north. Fortunately, thunderstorms aren`t training as of now. However, flood concerns persist as areal coverage of showers and thunderstorms should continue to expand through the day. In addition, portions of the central, especially where the Flood Watch remains in effect, are likely to be impacted by multiple rounds of thunderstorms this afternoon. UPDATE Issued at 651 AM CDT Wed Aug 14 2024 645 AM CDT radar imagery shows convection increasing along a line from near Bowman to Rugby, with storm motion vectors roughly parallel to this boundary. Another batch of showers with embedded storms is pushing north through the James River Valley in northeast South Dakota and will soon be crossing the state line. CAMs continue to struggle with observed conditions, and there is still notable run- to-run inconsistency on timing, coverage, and intensity of storms. But all CAMs do show the increasing trend that is now starting to emerge on radar. Forecast expectations for today remain on track. && .DISCUSSION... Issued at 431 AM CDT Wed Aug 14 2024 A mid level shortwave trough will drift eastward across the region today, with a closed circulation forecast to develop around 700 mb. At the surface, lee troughing is forecast to shift into the Central Plains, with an inverted trough extending north through the Dakotas. Strong moisture transport on the downwind side of the trough axes will aid in providing lift and moisture for widespread thunderstorms across western and central North Dakota throughout the day. The highest chances for storms per CAM guidance shift from western North Dakota this morning to central North Dakota this afternoon, but CAMs do maintain a large degree of spread in finer details of both timing and coverage. The main concern with thunderstorms today is the potential for excessive rainfall. Most, if not all thermodynamic ingredients are supportive of this hazard. Precipitable water in the 1.5 to 2 inch range approaches climatological maximum (the BIS RAOB from yesterday evening sampled 1.81 in). Warm cloud depths are forecast in the 3000 to 4000 m range, with unusually high deep layer RH for this part of the country. CAMs have been consistently simulating areas of 2 to 4 inches of QPF, with pockets of +5". The 00Z HREF probability-matched mean QPF shows the highest potential for excessive rain from the Missouri River valley between Lakes Sakakawea and Oahe to the Devils Lake Basin and northern James River Valley, which is a slight northward shift from previous guidance. The one potential factor that could limit the number of flash flooding incidents today is the synoptic setup and its relation to storm motion. Exclusive examination of CAMs` reflectivity output implies a quick enough motion that would limit long durations of intense rainfall rates. This may partially be derived from the eastward progression of the low pressure system as a whole. In addition, the lack of a well-defined frontal boundary orthogonal to moisture transport does not favor training convection. However, forecast Corfidi vectors are 1) very short, especially in closer proximity to the forecast position of the closed mid level low over central North Dakota, and 2) neither orthogonal to the inverted surface trough nor parallel to the motion of the mid level trough. It is also curious that CAMs are showing a quicker progression of the system as a whole when most guidance is closing a mid level circulation, which theoretically slows a shortwave`s progression. Yet another consideration on storm motion is that cold pool driven storm propagation should be limited today. This is all to say that while there are some merits to it, we don`t fully buy into the quicker storm motions in recent CAMs. This analysis of the forecast combined with observed excessive rainfall along the I-94 corridor yesterday evening has prompted us to issue a Flood Watch for a large portion of central North Dakota, in effect from 9 AM to 10 PM CDT today. The northward shift in recent guidance led us to exclude counties south of the Interstate, though we cannot rule out localized flash flooding there. We also kept the northward extent of the watch south of Highway 2 in acknowledgement that the northward shift in guidance may be slightly over done and is a more recent development in model guidance. In addition to the excessive rain threat, a few storms may become strong to severe today. This potential seems greatly limited by deep layer shear only around 20 kts, but several CAMs have been simulating UH tracks that are indicative of at least strong storms. Southwest North Dakota may be a better target area for more of an isolated strong to severe storm this afternoon, where buoyancy and instability should be stronger. The low pressure system will slowly drift off to our east tonight into Thursday, but there is uncertainty on how quick the exit will be. Some recent guidance has trended more towards a strong deepening and stalling of the surface low over the Red River Valley late tonight into Thursday morning. Should this occur, central North Dakota would see a cloudier, rainier, breezier, and much cooler day on Thursday than what is currently being portrayed in the forecast. The favored synoptic pattern for the end of this week into next week can still be characterized by omega blocking, but there are stark contrasts among ensemble clusters that emerge after this weekend. Prior to next week, there is lower ensemble spread in the placement of the ridge axis from the Central Rockies to the Northern High Plains, with the thermal ridge inching closer to the state each day through Sunday, which has therefore emerged as the warmest day with forecast highs in the 80s statewide and closer to 90 along the Montana border. There is also a signal for a shortwave passage through the top of the ridge on Saturday, which the NBM carries 20 to 40 percent PoPs for in western North Dakota. The greater ensemble uncertainty for next week is related to the strength of longwave troughing from Hudson Bay to the Great Lakes, with both cooler and warmer solutions for the Northern Plains equally favored at this time. && .AVIATION /12Z TAFS THROUGH 12Z THURSDAY/... Issued at 651 AM CDT Wed Aug 14 2024 Showers and thunderstorms are expected to become widespread across western and central North Dakota this morning and afternoon, with chances decreasing from west to east this evening through tonight. Any storm will be capable of producing torrential rainfall with IFR to LIFR visibility restrictions. Gusty winds cannot be ruled out with storms, but winds should otherwise be light and variable. MVFR to LIFR ceilings will prevail across most of western and central North Dakota this morning, with gradual improvement to VFR from west to east this afternoon, except at KJMS where low ceilings are more likely to prevail through the forecast period. && .BIS WATCHES/WARNINGS/ADVISORIES... Flood Watch until 10 PM CDT /9 PM MDT/ this evening for NDZ019>023-025-034>037. && $$ UPDATE...Telken DISCUSSION...Hollan AVIATION...Hollan