Area Forecast Discussion
Issued by NWS Bismarck, ND
Issued by NWS Bismarck, ND
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777 FXUS63 KBIS 051431 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 931 AM CDT Sun May 5 2024 .KEY MESSAGES... - Strong southeast winds across western and south central North Dakota late this morning through tonight, with gusts as high as 50 mph in southwest parts of the state. - Widespread showers and isolated thunderstorms are expected late tonight through Tuesday, especially in far western North Dakota. - There is a conditional risk of strong thunderstorms Monday afternoon, mainly across southern parts of the state. - Windy conditions continue on Monday (central and eastern North Dakota) and Tuesday (southwest North Dakota). && .UPDATE... Issued at 929 AM CDT Sun May 5 2024 Patches of fog have dissipated, and skies remain mostly clear. Southeast winds are now on the uptick over the area. Going forecast remains in good shape, so only change was some minor adjustments to cloud cover. UPDATE Issued at 647 AM CDT Sun May 5 2024 Patchy fog from south central into southeast North Dakota appears to have mostly dissipated as of 645 AM CDT. However, denser patchy fog has now settled over parts of the Devils Lake Basin and Turtle Mountains area. This newer fog should burn off by mid morning. Otherwise, the forecast for today remains on track. && .DISCUSSION... Issued at 412 AM CDT Sun May 5 2024 Patchy fog has developed across south central parts of the state this morning, but does not appear to be dense as of 4 AM CDT. The fog should lift by mid morning. A strong upper low over the western CONUS will push upper level ridging over the Northern Plains today, setting up a strong southerly return flow with a surface pressure gradient tightening between low pressure over the Northern Rockies and high pressure over the Upper Mississippi River Valley. Southeast winds are forecast to increase to 25 to 35 mph across western and south central North Dakota this afternoon, with gusts as high as 50 mph. The Wind Advisory has been expanded east of the Missouri River to include McLean, Burleigh, Kidder, Emmons, Logan, and McIntosh Counties, though the strongest winds are still expected in southwest parts of the state. Rising heights will allow for a warming of the boundary layer, with forecast highs returning to slightly above normal readings in the mid 60s to lower 70s. However, the southeast flow, which is notorious for poor BL mixing, along with a mid level capping inversion will likely prevent high temperatures from reaching the warmer 50th percentile of NBM guidance (closer to mid 70s across the western half of the state). A nocturnal low level jet will intensify over the Northern Plains later this evening through tonight. There is some uncertainty on how much of this will be reflected at the surface, especially given the well-document NBM high bias for winds in nocturnal southerly flow regimes. It is possible that sustained speeds could stay near, if not above advisory criteria through the night as the core of the LLJ translates through. For this reason, the Wind Advisory for all counties has been extended in time until 4 AM CDT Monday morning. The stronger overnight winds combined with increasing clouds will keep low temperatures much warmer than recently, with forecast values in the upper 40s to mid 50s. The aforementioned western CONUS low is expected to pivot into a negatively-tilted trough while maintaining a closed circulation over the Northern Plains on Monday, initiating strong surface cyclogenesis over western North Dakota. Chances for showers, along with a few elevated thunderstorms given HREF mean MUCAPE approaching 500 J/kg, will begin increasing over western North Dakota later tonight. High-resolution guidance then favors shower and isolated thunderstorm activity becoming more widespread across western and central North Dakota through the morning hours on Monday. There is a conditional risk for a few thunderstorms to become strong, possibly even severe, Monday afternoon, but it would require several ingredients to line up properly, the probability of which remains low at this time. Forcing and shear should be more than ample for stronger convection, with effective bulk shear likely exceeding 40 kts. The main question is whether enough low level moisture will be advected into the warm sector to create sufficient buoyancy for surface-based deep moist convection. Both the HREF and NBM are showing lower to mid 50s surface dewpoints with dewpoint depressions less than 10 degrees Fahrenheit. The resultant buoyancy forecast carries more uncertainty though, ranging from effectively none to as high as 1000 J/kg. Because this is expected to be such a strong dynamical system, it does seem reasonable that the low level moisture advection could be underestimated by model guidance. Should all these ingredients line up both spatially and temporally, low- topped supercells could develop along the northward advancing occluded front. Impressive low level kinematics and thermodynamics, including 0-500m SRH over 100 m2/s2, very low LCL heights, and long- curved hodographs with strong streamwise vorticity available for right-movers to ingest, would support a tornado threat. The background wind profile alone could support a convective damaging wind threat. Large hail would be the least expected hazard given the likelihood of low CAPE, a low equilibrium level, and stronger low level shear. It is important to note that should this threat materialize, which is a low-probability but reasonable worst-case scenario, it would likely be very limited in both space and time, most likely migrating through portions of southwest and/or south central North Dakota during the mid to late afternoon hours. On the other end of the forecast spectrum, a reasonable best-case outcome is for the shear and buoyancy to not line up, resulting in no strong convection at all. The exact evolution of the surface low Monday evening through Tuesday carries some uncertainty, with some guidance suggesting a double-barrel low system from southern Saskatchewan to northwest South Dakota. All guidance does seem to eventually congeal on a sub- 985 mb MSLP low on Tuesday, but with location uncertainty still ranging from southern Saskatchewan into western North Dakota. Cluster analysis of global ensemble systems remains evenly split on a northwest vs. southeast outcome for the placement of the surface low on Tuesday. In spite of this, there has actually been a consolidation of mean QPF among these clusters. Far western North Dakota maintains medium to high chances for exceeding 1 inch of rainfall through Tuesday, as most ensemble solutions now bring the wrap-around deformation band back into the state. Elsewhere, we are still expecting a dry slot to significantly limit rainfall amounts for at least portions of central North Dakota over the Monday night through Tuesday time frame, but there is still some uncertainty in its exact placement and timing. Near to below normal temperatures are favored through at least the middle of the week, with highs mostly in the 50s and 60s, coolest in the southwest on Tuesday and Wednesday where higher chances for rain and greater forecast cloud cover exist. Ensemble guidance now seems to favor a quicker deamplification of the low on Wednesday, but chances for lighter rain showers will remain in the forecast into Thursday, decreasing over time. A chaotic synoptic pattern is then hinted at by some deterministic guidance later in the week, but with general troughing to the east and ridging to the west. A slow transition to warmer weather in time for next weekend is favored by ensemble 850 mb temperature anomalies and NBM surface temperature distribution trends. && .AVIATION /12Z TAFS THROUGH 12Z MONDAY/... Issued at 647 AM CDT Sun May 5 2024 Patchy fog will remain possible at KJMS early this morning, reducing visibility as low as IFR levels at times. Otherwise, VFR conditions are expected through the forecast period. Southeast winds will become strong across the western half of the state this afternoon through tonight, with sustained speeds around 20-30 kts and gusts as high as 30-40 kts. If overnight wind speeds end up lower than what is currently forecast, low level wind shear could become a concern. Later in the night, chances for showers and possibly a few thunderstorms will begin to increase in western North Dakota. && .BIS WATCHES/WARNINGS/ADVISORIES... Wind Advisory until 4 AM CDT /3 AM MDT/ Monday for NDZ001-009- 010-017>020-031>034-040>045. Wind Advisory from 2 PM this afternoon to 4 AM CDT Monday for NDZ021-035-036-046-047-050. && $$ UPDATE...JJS DISCUSSION...Hollan AVIATION...Hollan