Area Forecast Discussion
Issued by NWS Grand Forks, ND

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091 FXUS63 KFGF 170804 AFDFGF Area Forecast Discussion National Weather Service Grand Forks ND 304 AM CDT Mon Jun 17 2024 .KEY MESSAGES...
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-Training showers and thunderstorms will bring the threat for flash flooding across the region through Monday and Tuesday. The Flood Watch has been expanded to include the majority of eastern North Dakota and all of northwest Minnesota. -Embedded severe thunderstorms are likely to arise through Tuesday with the threat for golf ball size hail and a lower risk for 60 mph winds.
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&& .DISCUSSION...
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Issued at 1237 AM CDT Mon Jun 17 2024 ...Synopsis... 00z upper air analysis indicates an 850mb warm front that has become the focus for strong isentropic ascent across South Dakota and southern Minnesota. This warm front will lift further north with its strong warm air advection through the day today. At the surface, a very strong theta-e gradient will set up across the central plains and keep temperatures today very cool in the 50s. Efficient moisture transport associated with this warm front will create widespread showers and thunderstorms this afternoon and evening, lasting through a majority of the night into Tuesday morning. Thunderstorms should exit the area by late Tuesday afternoon at the latest, with additional showers lingering overnight Tuesday. There are signals for another stout wave to bring efficient rainfall to the area, although instability with this wave is much less so there is lower confidence in potential flooding. ...FLASH FLOOD THREAT... There is a strong signal for widespread heavy rainfall beginning Monday afternoon and lasting through the overnight hours. The main question at this point is how much of the area sees flooding and where the strongest impacts will be felt. Confidence has diminished in flooding potential across the southern zones due to the transient nature of the 850 warm front. Of greater concern for flooding based on CAMs is redevelopment west of the Red River Valley. For this reason, the Flood Watch has been expanded to encompass the majority of the area. A very strong low level jet will generate continuous convection, feeding persistent instability and moisture into the zone of forcing. This is what we know 100% will occur overnight tonight. What we don`t know is the evolution of surface features that will provide for more focused ascent and create a zone of potential flash flooding. CAMs have been really struggling to resolve certain things, but one of the most concerning is a few CAMs bringing a stationary front across eastern North Dakota. This is a very low confidence scenario at this point, but one that may arise nonetheless. At this point in time there are two potential outcomes through Tuesday: SCENARIO 1: The stationary front is fairly weak or nonexistent and there is no zone of focused ascent. This scenario would have limited flash flooding potential as the main forcing zone (the elevated warm front) will be fairly transient. Heavy rainfall is still likely to occur. SCENARIO 2: The stationary front is strong enough to induce convection along a line. In this scenario, where convection fires WILL have flash flooding. The zone will be fairly tight thanks to strong instability. This is the higher impact scenario. Where this zone of ascent sets up though we do not know at this time. It could set up anywhere as far west as the Sheyenne River Valley and Devils Lake Basin to as far east as northwest Minnesota. Given the convective nature of this and the multiple moving parts, it is very likely portions of the Flood Watch will need to be removed periodically throughout the day and overnight tonight, particularly as confidence increases in the greatest area of convection. This is a very low confidence scenario with the potential for higher impacts so take the entire situation and forecast as fluid at this point. Despite a downgrade from moderate to slight in the latest Excessive Rainfall Outlook, it is important to note that this is due to uncertainty. The potential severity of flash flooding has not changed. Prepare for at least heavy rainfall with the potential for flash flooding. ...SEVERE WEATHER THREAT... Through the day today, strong differential diabatic heating will likely strengthen a surface theta-e warm front across southern South Dakota and southern Minnesota. This will keep the bulk of surface instability away from our area, meaning the bulk of convection that fires this afternoon will be elevated. Strong southwesterly flow within the 850mb layer will advect a persistent elevated mixed layer to our area, giving strong sustained convection with strong instability. This means the primary threat will be hail. The main uncertainty at this point in time is how large potential hail will be and will depend on anything becoming a supercell. Despite strong bulk shear from the surface up, it is more likely that thunderstorms will only see shear from 850 and up, which is substantially less. Effective-layer bulk shear north of the warm front is on the order of 20-30 knots, so there is a greater probability for multi-cellular clusters. If parcels can tap into a bit more shear closer to the surface, the potential for elevated supercells is there. Without supercells, the greatest hail size we will most likely see is 1.25, but with supercells it would be possible to push past 1.5. As the surface warm front pushes into our area after midnight, convection in the vicinity of this warm front may pose the risk for severe wind gusts to 60 mph, although confidence is low due to timing with a potential diurnal inversion near the surface. Redevelopment is likely Tuesday afternoon. Flow parallel to the initiating boundary will create more linear structures, creating a primarily damaging wind driven impact. Severe convection should only last about 1-3 hours before exiting into northeast Minnesota.
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&& .AVIATION /06Z TAFS THROUGH 06Z TUESDAY/... Issued at 1111 PM CDT Sun Jun 16 2024 VFR conditions are expected through tonight and tomorrow morning. There may be some brief morning showers that may reach KFAR by 14z. However, Thunderstorms aren`t expected until after 02z moving from the south to the north possibly affecting all the TAF sites by 06z Tuesday. Winds are expected to become gusty after the evening. There`s a little uncertainty with the timing of these storms especially if the associated front stalls between KFAR and KGFK. && .FGF WATCHES/WARNINGS/ADVISORIES...
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ND...None. MN...Flood Watch from this afternoon through Wednesday morning for MNZ024-028>032-040.
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&& $$ DISCUSSION...Perroux AVIATION...MM