Scientific Forecaster Discussion

000 fxus63 kfsd 241722 afdfsd Area forecast discussion National Weather Service Sioux Falls South Dakota 1222 PM CDT Wed Apr 24 2019 Short term...(today and tonight) issued at 425 am CDT Wed Apr 24 2019 Short term weather will be forced by a split system in the nearly zonal flow over the northern tier for the next 24 hours. One wave is moving along the international border of ND/Minnesota this morning, and will be responsible for first increasing the southerly gradient today, and then driving a frontal boundary across The Heart of the area by early this evening. Winds for the most part have been down overnight, but some increase in the gradient is beginning to keep winds at elevation in southwest Minnesota and southeast South Dakota a bit stronger over the last couple of hours. Perhaps a 20-30kt low level jet will surface briefly across areas of east central South Dakota into southwest Minnesota by later in the night, but no where near the 40-45kt gusts seen in the Sisseton hills. For today, will be generally ahead of a frontal boundary pushing into area west of the James River by early afternoon, and reaching a kmml-kfsd-kykn line by 00z. This will place the area squarely in the prefrontal warm push, with 925 hpa temps from +19 to +22. There is not a great deal of cooling with the boundary, so the shift to more west to northwest winds behind will likely continue the mixing. There appears as if there will be some mid to high clouds to deal with much of the day, particularly earlier and later in day, which could impact somewhat reaching the potential for warming. With lack of foliage, should be able to again exceed expectations a bit, so still looking for a lot of mid 70s to near 80 readings. There will be some concerns with fire weather today as humidity falls to 25 to 35 percent this afternoon. However, there again lacks the really strong winds aloft to push conditions toward critical. While have trended relative humidity down a bit from the earlier forecast based largely on values above the surface layer in model profiles, it likely won't get appreciably lower than this if mixing ends up more significant than expected. Other issue in the short term is the convective potential with the front by late day. Inverted-v profiles look to set up with perhaps 300-600 j/kg MLCAPE. Convergence on the boundary remains fairly weak as well, so likely would be isolated to scattered coverage at best, with a strong tie to the diurnal cycle, peaking by early evening. Storms that develop could contain gusty winds, but not looking for any severe threat at this time. A better precipitation chance will develop overnight as the second part of the wave, this a trailing jet punching in from the Pacific NW, spreads precipitation which develops across western South Dakota in the early evening toward the east/southeast, mainly near and south of I- 90 overnight. This area has a fairly decent mid-level lift support with strong div-q/frontogenesis. Instability is lesser, and while there could be a little thunder toward the southern area of the precip band, should be mainly showers/rain. Did increase pops and quantitative precipitation forecast with this band, which is a very consistent signal amongst the various operational solutions. Also consistent is the drier air which will be undercutting the band, limiting any threat for areas north of I-90. Long term...(thursday through tuesday) issued at 425 am CDT Wed Apr 24 2019 Rainfall will linger early in the day across northeast NE/northwest Iowa as the wave pulls away and the much drier air surges southward. The increased mixing of northwesterly gradient and very dry air will again present some fire weather concerns, as are looking as many areas down to around 25 percent or lower for a few hours in the afternoon along with winds to 15 to 25 mph. Another frontal boundary will sink into the area late day, and again with cooling temps aloft, would not be impossible to see an isolated high-based shower/storm somewhere along the boundary in southeast South Dakota after 5 PM. Pattern quickly becomes quite messy heading into Friday/Saturday period as another wave moves through the quick flow and begins to amplify Friday night. Could see some spotty warm advective showers on Friday, but will have quite a bit of dry air at low levels to overcome, especially pushing away from the Missouri River valley. Focus of the 40-50 kt low level jet suggest that precipitation will break out fairly quickly during the evening across western/central South Dakota and NE which will lift east/northeast through the night. Enough instability that thunder can be expect at least through the Missouri River, but a decent amount of elevated instability could end up further north (a typical bias to not expand far enough northward). Problem comes in that the very dry air in place at lower levels will have a tremendous potential to wet bulb, so eventually ice bearing profiles develop as the band lifts toward the kbkx/kmml/kmwm areas by later night and early Saturday morning. For the time, have not hit the snow potential too hard as don't see a lot of thermal consistency between solutions, with some spotty inch or so amounts mainly near terrain in northeast Brookings/western Lincoln counties. A degree or two either way will make a big difference in snowfall potential, so have mainly locked in on a consensus view (away from slightly colder/south NAM and warmer/north ec). Will have to watch though, as there are signals of higher quantitative precipitation forecast amounts and some near negative epv in the warm frontal zone which hint that some perhaps more significant snow could occur late Friday night and Saturday morning toward Highway 14. Unlikely highs will get out of the 40s in SW Minnesota and adjacent locations on Saturday. Cool Ridge wedges into the area Saturday night behind the system, and if can keep clouds from getting too thick, may have to watch for some frost for those attempting any early planting as lows reach the lower to mid 30s. However, another quick moving wave spreads in late, with some light precip chances north of I-90 late night. The messy pattern continues into the following week with another broad warm advective setup Sunday night, and frontal passage with into Monday, and yet another wave moving through around Tuesday. Timing/location of systems begins to get very sporadic by this time between runs/models, so will keep pops somewhat more restrained than they will likely be as the time approaches. What is clear is that temps will certainly be running a good 10-15 degrees below normal after Saturday, and rain will be a periodic threat. && Aviation...(for the 18z tafs through 18z Thursday afternoon) issued at 1219 PM CDT Wed Apr 24 2019 VFR conditions will be prevalent through the period, though brief MVFR conditions may accompany isolated showers/thunderstorms along a cool front in the late afternoon/early evening, as well as a more widespread area of light rain showers moving across the region late tonight into Thursday morning. Daytime wind gusts may occasionally top 25kt, with speeds variable in direction depending on location of the boundary and any isolated afternoon showers. && Fsd watches/warnings/advisories... South Dakota...none. Minnesota...none. Iowa...none. NE...none. && $$

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