Area Forecast Discussion
Issued by NWS Grand Forks, ND
Issued by NWS Grand Forks, ND
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
918 FXUS63 KFGF 171957 AFDFGF Area Forecast Discussion National Weather Service Grand Forks ND 257 PM CDT Tue Sep 17 2024 .KEY MESSAGES... - There is a level 1 out of 5 risk for severe storms Wednesday afternoon and evening, including the potential for supercells. && .DISCUSSION... Issued at 202 PM CDT Tue Sep 17 2024 ...Synopsis... Large upper cyclone is readily viewable on water vapor satellite imagery this afternoon centered over the Great Basin, with the Northern Plains residing under southwest flow aloft just downstream of the cyclone`s exit region. At the surface, low pressure is deepening in the northern High Plains around eastern MT, with a stationary front extending eastward through ND into far northwest MN. South of this front, summer-like temperatures well into the 80s are occurring, with some locations still liable to touch 90 in the southern Red River Valley this afternoon. North of the front, cooler temperatures in the 70s and low stratus clouds will set the stage for another night of fog potential in northeast ND as upslope easterly flow allows the stratus cloud deck to scrape against the ground. Tonight into Wednesday morning, the upper cyclone rapidly moves into the northern High Plains in a negatively tilted fashion, with rapid deepening of the surface cyclone also anticipated. This will provide a branch of forcing and moisture within its moist conveyor wrapping into the low from the south and east into ND and western MN. This will bring around a 40% chance for showers and thunderstorms sweeping across our area tonight into Wednesday morning. Thunderstorms are not anticipated to be severe tonight due to lack of better shear and overall wind energy associated with this branch of forcing. Getting into Wednesday, as the low rapidly develops, wind energy markedly increases over ND into western MN. This coupled with sufficient instability and moisture will drive the chance for severe storms Wednesday afternoon into evening. More on this below. By Thursday, the low starts to trek northeastward bringing with it a moisture scouring cold front, gusty winds, and beginnings of an eventual cooling trend. Depending on how fast the front moves into MN, there could be a small area that holds potential of seeing stronger storms Thursday afternoon near central MN. This weekend into next week, most ensemble guidance depicts broad troughing into the Plains and Upper Midwest. This will continue chance for rain and possibly thunderstorms along with trending temperatures closer to average this time of year, which will feel more fall-like rather than the summer-like conditions we`ve been experiencing as of late. ...Severe storm potential Wednesday... Setting the stage for severe potential Wednesday will be continued poleward advection of moisture within eastern ND and western MN ahead of a dryline stretching into eastern ND. This along with daytime heating under marginally steep lapse rates aloft will allow for a sector of instability with MLCAPE values progged in the 1000-2000 J/kg. With such strong dynamics nearby, the wind field over eastern ND into MN will be increasing, contributing to 40+ kt of shear to 500 mb. More impressive regarding the wind field is in the lower levels. Most guidance depicts strengthening winds in the 0-3 km, with notable veering with height contributing to increased helicity values. This is providing high chance for 0-1 km storm relatively helicity values exceeding 100 m2/s2 by Wednesday late afternoon into evening within the relatively moist/unstable sector. Should discrete storms develop in this regime, supercells will be favored with all hazards possible, including the chance for large hail and tornadoes. The potential for all hazards is illustrated nicely by some recent ML guidance such as Nadocast and CSU severe probabilities. However, there are notable questions on the potential coverage of robust thunderstorms. This is mainly driven by the unfavorable timing and/or displacement of forcing for ascent depicted by guidance. Most of the synoptic forcing for ascent will tend to move northward into Canada. At the surface, there isn`t much evidence to suggest a surging dryline to help lift, with the cold front more likely to move through after sunset when capping increases and surface-based parcels become increasingly unlikely. At this time, the chance for widespread severe storms is very low due to this, although mesoscale forcing in a few areas coupled with broad weak ascent with the nearby upper low may be enough to help a few robust thunderstorms. && .AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/... Issued at 112 PM CDT Tue Sep 17 2024 VFR conditions are forecast at all sites this afternoon through Wednesday morning. LLWS is forecast to develop at KGFK, KFAR, KTVF, and KBJI, generally around 35-45kt at 020 kft. Winds will become gusty out of the south 1-3 hours after sunrise Wednesday, gusting between 20-30kt, and likely continuing into the day. There is a chance for scattered showers and thunderstorms between 03-15Z across the region, 15-18Z for Minnesota, as a low pressure system approaches. However, confidence wasn`t high enough in timing of potential impacts from these to include TAFs for now. Thunderstorms are not anticipated to be severe, although could still hold gusty erratic winds over 30kt in addition to lightning. KTVF still holds an AMD NOT SKED due to AWOS outage at the site. && .FGF WATCHES/WARNINGS/ADVISORIES... ND...None. MN...None. && $$ DISCUSSION...CJ AVIATION...CJ