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Area Forecast Discussion
Issued by NWS Bismarck, ND
Issued by NWS Bismarck, ND
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309 FXUS63 KBIS 121936 AFDBIS Area Forecast Discussion National Weather Service Bismarck ND 236 PM CDT Mon Aug 12 2024 .KEY MESSAGES... - There are medium to high chances (40-80%) for showers and thunderstorms late Tuesday afternoon through Wednesday. A few storms could be strong to severe in western North Dakota Tuesday evening and into the night, and across much of western and central North Dakota on Wednesday. - Very heavy rainfall and slow moving storms are also possible on Wednesday, leading to the potential for localized flooding. - Temperatures will remain rather steady from Thursday into next week, with low chances for showers and thunderstorms each day. && .DISCUSSION... Issued at 228 PM CDT Mon Aug 12 2024 At the moment, surface high pressure is positioned across much of North Dakota. Aloft, a weak ridge is sitting across much of the central CONUS, with generally zonal flow across the Dakotas. Throughout the daytime hours, cumulus has formed across essentially the entire area, with the expectation that this cumulus will dissipate heading into the evening hours. Troughing to the west has allowed for very subtle moisture return, and given the ample daytime heating under the clear sky this morning, the diurnal cumulus has overperformed so far. We are not expecting any showers to form with these, however a few prominent towers in the north central have formed, with very weak radar returns present in one of them, near Rugby. As the troughing begins nudging in from the west, the surface pressure gradient has begun tightening up a bit, and we`ve seen a slight increase in winds through the day, with a few areas seeing some gusts up to 15 to 20 mph. High temperatures today are forecast to be in the 70s, with a few areas seeing the low 80s in the far northwest. Winds and cloud coverage should decrease overnight, with overnight lows forecast to be in the mid to upper 50s. As the trough to the west of us deepens and begins transiting the High Plains, midlevel height falls ahead of the trough will help provide enough forcing for the development of showers and thunderstorms Tuesday night and into Wednesday. The SPC has outlined Marginal Risks (level 1 of 5) for far western North Dakota on Tuesday, and for much of central North Dakota on Wednesday. The severe threat on Tuesday will remain mostly in the western parts of the state, as the main axis of forcing will be situated across Montana and Wyoming through most of the day. As that trough slowly progresses eastward, a narrow corridor of instability is forecast to develop across western North Dakota, helping aid in the development of thunderstorms. The main limiting factor at this point appears to be deep layer shear, with low values of 20 to 30 kts of 0-6km bulk shear values forecasted across the area. Shower and thunderstorm activity is likely, but the severe threat remains marginal at best. Our current thinking for expected threats remains hail up to quarter sized and winds up to 60 mph. Otherwise, Tuesday is forecast to be the warmest day of the forecast period, as the low level thermal ridge will be at its strongest ahead of the trough, with highs in the lower to upper 80s. Given the continued approach of the trough, breezy winds will also be possible across much of the western and central portions of the state. Wednesday will see the continuation of these showers and thunderstorms, as the slow moving wave will continue making its way across the Dakotas. This can be seen in the SPC`s Marginal Risk shifting east, covering much of the central part of the state. Given that its the same feature as before, the overall setup of this severe risk is rather similar to Tuesday`s setup. Continued midlevel height falls ahead of the trough will allow for the development of showers and thunderstorms, however these showers and thunderstorms will mostly form during the morning and afternoon hours. The chances for severe weather decrease overnight as we cool off into the 50s, but as daytime heating ramps back up again, the ample instability will return. Once again, forecasted deep layer shear values appear to be the limiting factor, lending to this being a rather marginal threat. Another area of concern, however, is the forecasted precipitable water (PWAT) values for much of our area, combined with the slow moving nature of these storms, leading to concerns for possible localized flooding. Current guidance is suggesting the potential for PWAT values to approach 1.25 to 1.75 inches, which is coming close to the climatological maximum for KBIS at this time of year. Deterministic sounding forecasts also support the potential for slow moving, training storms. As a result, the WPC has outlined a Marginal Excessive Rainfall across much of central North Dakota, in line with where chances for showers and thunderstorms are maximized. Even though the severe threat is marginal, we still do see rather high chances for rain during the day Wednesday (ranging from 60 to 80%), which brings about this risk for excessive rainfall. The main threats associated with the severe weather on Wednesday will be hail up to quarter size, winds up to 60 mph, and locally heavy rainfall. Otherwise, highs on Wednesday will be in the mid 70s to mid 80s. Heading into the weekend, current long range models suggest the development of a ridge across the Rocky Mountains, slowly expanding east heading into Friday. However, this expansion is expected to halt, as the upper level flow is forecast to settle into an omega blocking pattern, with troughing across the Great Lakes region and Pacific Northwest, keeping this ridge across the Rockies in place. As a result, from Friday onwards into the extended period, similar conditions are being forecasted each day. Temperatures are forecast to rise into the upper 70s to mid 80s, with the warmest temperatures in the west, with overnight lows consistently in the 50s. While we`ll be under the influence of a ridge aloft, the actual ridge axis will be displaced to the west, allowing for a somewhat active northwesterly flow aloft to be draped across the state. While details are a bit fuzzy this far out, we can expect to see a few shortwaves passing through this flow, allowing for low chances (around 20 to 40%) for showers each day, the spatial extent of which is uncertain at this time. The chances for severe weather remain low, as this setup will not be very conducive for adequate moisture return and midlevel forcing. The CSU machine learning guidance currently keeps all severe chances out of the area from Friday onwards. && .AVIATION /18Z TAFS THROUGH 18Z TUESDAY/... Issued at 1228 PM CDT Mon Aug 12 2024 VFR conditions are expected to prevail through the TAF period. Fair weather cumulus has developed across much of the area, resulting in a scattered deck around 4 kft, which is expected to continue until the evening hours. Winds are initially out of the southeast around 5 to 10 kts, beginning to increase in strength around 16z Tuesday. && .BIS WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Besson AVIATION...Besson