116 FXUS63 KFSD 130935 AFDFSD Area Forecast Discussion National Weather Service Sioux Falls SD 335 AM CST Mon Nov 13 2017 .SHORT TERM...(Today and Tonight) Issued at 334 AM CST Mon Nov 13 2017 As expected, fog has mostly stayed away tonight thanks to the return of light southerly gradient winds. Will need to keep an eye on things early this morning as RH values will remain high as warm moist air advects into the forecast area. This warm moist advection will drive the main foci of the short term forecast, stratus and drizzle potential. Stratus currently located over Kansas will advect northward today, eventually impacting the CWA, in particular our northwestern IA zones, by late this afternoon. It looks like another day to soar above guidance high temperatures with sunshine and mixing west of the James River. At the same time, will hold back highs to around consensus in our southeastern zones due to the expected afternoon sky cover. After 00z Tuesday, the warm moist advection will lead to a chance for patchy drizzle in southwest Minnesota and northwest Iowa. At this time, given the marginal surface moist layer depth of 1km or less and the poor vertical placement of the most favorable ascent - sometimes above the moist surface layer - will limit mentions of drizzle for now. .LONG TERM...(Tuesday through Sunday) Issued at 334 AM CST Mon Nov 13 2017 On Tuesday, a short wave will move into the northwestern high plains by afternoon. Ahead of this wave, a southerly flow of air will continue to bringing up ample low level moisture ahead of a surface wind shift to the west. Low level RH is generally very high east of the James River valley early Tuesday, then shifts slowly eastward through the day as the surface wind shifts to the west ushering in drier air. The stratus will likely hang on through the entire day in our far southeast zones around Spencer and Storm Lake. This creates a challenge in terms of high temperatures. This area may be a couple of degrees too warm. However the moisture advection is very strong which can overcome a lack of mixing. So either way, temperatures will be above normal. In our western zones where stratus will have no impact and a westerly flow of air takes over, raised highs by mixing to about 900mb, giving a lot of upper 50s, even lower 60s in south central SD. Concerning precipitation, thought about drizzle chances Tuesday morning, especially in northwest IA. However the stratus depth is not overly thick with moisture mainly only through 900mb. There appears to be no ascent in the cloud layer and although there may be some slight cooling of cloud temps early Tuesday morning, the cloud tops begin to warm again after 15Z. Therefore left out the mention of drizzle for now but certainly cannot rule out some very light patchy drizzle in our southeast zones. Fog may be limited too due to the gradient, but this will also need to be watched. The aforementioned short wave passes across our forecast area Tuesday night giving everyone a northwest flow of air coupled with clearing skies from west to east. Lows will be pretty mild as the air mass is still of Pacific origin. Wednesday will follow with somewhat of a breezy day east of I 29, and highs will be cooler. High pressure moves off to the east Wednesday night producing a stout southerly flow of air by Thursday. The gradient and mixed layer winds are maximized west of I 29 where it will be quite windy, with 20 to 35 mph common in our SD zones. Wind speeds will be a little less in northwest IA and southwest MN but temperatures will be cooler when compared to our western zones. From Friday onward, there is quite a bit of discrepancy between the medium range models with the details. At 12Z Friday, the GFS shows a short wave passage moving into the northern plains on the south side of an upper low near Lake Winnipeg. The ECMWF is absent this feature, although both models are in good agreement over the intermountain west with the next short wave trough. Because of the lead short wave, the GFS is a full 12 hours faster with a surface wind shift bringing the winds around to the northwest on Friday when compared to the ECMWF. Precipitation chances with the various waves will therefore exist this week, but with warm temperatures aloft, tried to keep snowfall to a minimum. It also looks like a windy period, especially on Saturday, and potentially on Friday if the GFS is closer. By the weekend, highs will once again be back below normal. && .AVIATION...(For the 06Z TAFS through 06Z Monday night) Issued at 1106 PM CST Sun Nov 12 2017 VFR conditions and light winds will dominate early in this TAF period. Southerly flow will gradually increase through Monday morning, with frequent gusts in excess of 20kt likely after 15Z. The southerly flow will bring an increase in low level moisture, leading to an increased potential for MVFR-locally IFR ceilings developing east of the James River Valley, and especially along and east of MN/IA Highway 60, by mid-late afternoon Monday. Lower ceilings may become more extensive in coverage late Monday evening, but confidence in whether MVFR conditions will develop at KFSD/KHON is low at this time, so will hold with SCT low cloud deck at this time. && .FSD WATCHES/WARNINGS/ADVISORIES... SD...None. MN...None. IA...None. NE...None. && $$ SHORT TERM...Ferguson LONG TERM...MJ AVIATION...JH