845 FXUS63 KTOP 050448 AFDTOP Area Forecast Discussion National Weather Service Topeka KS 1148 PM CDT Thu Apr 4 2019 ...Update to aviation forecast discussion... .SHORT TERM...(This evening through Friday) Issued at 339 PM CDT Thu Apr 4 2019 An upper level shortwave continues to move east of the forecast area per the 20Z water vapor imagery and profiler data. Further west, an upper trough axis was noted off the west coast with a shortwave lifting northeast towards the Pacific Northwest. In between these features was a low amplitude ridge over the Rockies. At the surface, weak ridging had set up over the western central plains as an inverted trough moved into the Ozarks. However this ridge had already begun weakening. For tonight and Friday, the weather is expected to generally be dry with no obvious forcing as the upper level ridge moves out over the plains. The bigger concern is what to do with the stratus and possible fog overnight and Friday morning. NAM forecast soundings tend to show the stratus hanging around and limiting how dense any fog may get. But the RAP soundings do not hold onto the stratus overnight suggesting there is some potential for dense fog. Earlier mixing across central KS has not cleared out the low clouds across eastern KS. So the NAM may be onto something. For now have kept a mention for patchy to areas of fog in case the low clouds break up enough. But overall guidance keeping visibilities above 1 mile adds some uncertainty in there being widespread dense fog. Lows tonight are forecast to only fall back into the mid 40s since there is not a lot of cold air expected to move in. Models suggest better mixing of the boundary layer on Friday. As long as we see some sunshine by the afternoon, think highs could make it into the lower 70s. .LONG TERM...(Friday night through Thursday) Issued at 339 PM CDT Thu Apr 4 2019 For Friday night and the weekend, models prog upper level energy to lift across the forecast area with the best dynamics impacting northeast Kansas Saturday afternoon and Saturday evening. The GFS wants to develop some light precip late Friday night, but confidence in this is low given the NAM and ECMWF tend to keep things dry and there isn't an obvious lead shortwave lifting out ahead of the main shortwave. So I just have some slight chance POPs for Friday night. The better chances will be Saturday afternoon and evening where POPs are in the 60 to 70 percent range. There is progged to be some modest instability with the wave but mid level temps are not all that cold and as a result lapse rates are forecast to be only around 6 C/km. Additionally deep layer shear looks to be on the modest side with 0-6KM shear values of 30 to 40 KT. So the severe risk looks to be muted on Saturday. By Sunday a second piece of energy is progged to move through the plains and kick the initial wave to the east. However the dynamics with this wave look to move mainly south of the forecast area. However a frontal boundary with some frontogenesis should move through the area. Because of this have continued with some chance POPs through the day Sunday. There does not appear to be a lot of cold air advection behind the front. So highs on Saturday and Sunday are expected to be in the upper 60s and around 70. Lows should also be on the mild side with readings in the lower and mid 50s. For Sunday night through Tuesday night, upper level ridging is expected to move across the area bringing a dry period to the forecast. The GFS does hint at a weak backdoor front moving into the area on Tuesday. There is not a lot of confidence in this solution as the mean westerlies look to remain across the northern plains and the ECMWF/Canadian fail to bring any boundary this far south. So have highs in the lower and mid 70s for Monday and Tuesday. Lows are expected to fall into the upper 40s. The next upper wave is forecast to move through the central plains Wednesday and Thursday. At this time the models tend to track the surface low pressure system along the NEB and KS state line with some decent spread in the ensemble solutions. So have some chance POPs beginning Wednesday afternoon and continuing through Thursday. The current operational solutions would minimize chances for snow on the back side of the system. But this could change if later runs trend south. Something to keep an eye on in any case. For now have a modest cool down for temps on Thursday with highs in the 50s until the uncertainty diminishes. && .AVIATION...(For the 06Z TAFS through 06Z Friday night) Issued at 1141 PM CDT Thu Apr 4 2019 Fog development is the primary concern for this TAF period, especially at Top sites. Very shallow fog was trying to form near KTOP at issuance time. Persistent IFR visibilities are expected at the TOP sites by 08Z and could drop below one mile after 10Z for a few hours. CIGS could drop into the low-end IFR range as well, with a chance for LIFR conditions for a brief time either side of sunrise. KMHK does not look at be as heavily impacted with MVFR conditions expected, but will have to monitor trends. Fog should begin to lift and dissipate after 14Z. && .TOP WATCHES/WARNINGS/ADVISORIES... NONE. && $$ SHORT TERM...Wolters LONG TERM...Wolters AVIATION...Teefey