037 FXUS63 KFSD 232340 AFDFSD AREA FORECAST DISCUSSION National Weather Service Sioux Falls SD 540 PM CST Mon Dec 23 2019 .SHORT TERM...(This Evening through Tuesday) Issued at 258 PM CST Mon Dec 23 2019 Despite a large blanket of cirrus spread across the region this afternoon, temperatures have again risen above normal in most locations. Concerns in the short term portions of the forecast remain limited. Tonight: There will be a continuation of cirrus across the entire northern United States tonight thanks to broad southwesterly flow aloft. This cirrus should again blanket the CWA enough to hold temperatures upwards and have increased lows slightly. The question will be then if we can cool sufficient to develop stratus and/or fog as some of the colder high resolution models indicate. Currently anticipate there will be some light fog development given light winds near ill-defined frontal boundary, but not as pessimistic as many of the high resolution models. Tuesday: With a smaller drop overnight, we should start Tuesday with rather favorable temperatures. Winds will again be light, but absent any significant stratus, could see highs again climb well through the 30s in most areas. .LONG TERM...(Tuesday Night through Monday) Issued at 258 PM CST Mon Dec 23 2019 Tuesday Night: Colder air should finally begin to filter southeast, eventually allowing stratus to develop overnight behind a frontal boundary over Nebraska and Iowa. Models indicating a weak shortwave moving northeast after midnight which will increase weak lift through the night. Christmas: Reasonable agreement that the aforementioned shortwave begins to move through the local area early Christmas morning. Favorable upper divergence in the left exit region of the jet should help promote light precipitation development. The issue will be with a stubborn layer of low-lvl dry air AOA 800 MB that will hold into the morning. Given the expected light nature of the precipitation, could see a scenario where you see a light mixture of ice pellets, snow, and even rain at times given the warm nose can't completely saturate and cool. Regardless, any amounts will be very light, which shouldn't produce any significant impacts. Christmas Night: Models suggesting a weak 850 mb circulation develops as the wave passes through Wednesday night. This would enhance the lift slightly over northeastern South Dakota and western Minnesota into the evening (generally north of Highway 14). The loss of ice crystals further south could spell a brief period of freezing drizzle into Thursday morning focused mainly east of I-29 into Minnesota. Thursday: Somewhat colder and drier air moves in for Thursday, but would not discount a few flurries in the daytime hours. Friday: Mid-lvl flow transitions more westerly on Friday, which should promote warming temperatures as a sfc warm front lifts towards the area. Have bumped up highs slightly. Friday Night-Sunday: Still considerable variability in medium range models regarding the eventual evolution of two systems. One of these is a progressive wave moving through the Northern Rockies Friday afternoon, and a second is a larger upper low in the Southwestern United States. 12Z GFS remains a large outlier in the cluster of deterministic models phasing these systems together and bringing them through during the weekend. 12Z EC and EC ensembles support a slightly more favored solution, which would be the enhancement of mid-lvl forcing ahead of a slightly deeper yet progressive wave late Friday night into Saturday across eastern Nebraska, Iowa and southern Minnesota, then shifting this area east on Saturday. The larger upper low would then track far enough to the south to not influence us locally. That said, tHere have been consistent shifts in the GEFS solutions towards this potential as well with increasing probabilities locally focused Friday night into the first half of Saturday in the far southeastern zones. That said, many of the GEFS members still hold onto the phasing that the deterministic GFS suggests which is why it can't be completely discounted as a possibility. For now, will hold off any any major forecast changes in the extended given the uncertainties. Temperatures look to remain near to slightly above normal into early next week. && .AVIATION...(For the 00Z TAFS through 00Z Tuesday evening) Issued at 540 PM CST Mon Dec 23 2019 With high pressure to the north and low pressure to the south, look for weak easterly flow through the period, although winds are likely to be variable at times, especially this evening and overnight. Main concern for aviation operations will be the potential for fog tonight. Given weak winds and moistening boundary layer overnight, some fog is possible, but visibilities aren't expected to tank given persistent BKN-OVC cloud cover. For now, will maintain MVFR reductions in visibility from roughly 24.09Z to 24.16Z. Will monitor current conditions for possible updates. Otherwise, expect VFR conditions at KHON/KFSD/KSUX through 00Z Wednesday. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...NONE. MN...NONE. IA...NONE. NE...NONE. && $$ SHORT TERM...Dux LONG TERM...Dux AVIATION...Rogers