Scientific Forecaster Discussion

000 fxus63 klot 180626 afdlot Area forecast discussion National Weather Service Chicago/Romeoville, Illinois 126 am CDT Tue Jun 18 2019 Update... 931 PM CDT Going forecast on track and no big changes needed this evening. Will be expanding the higher coverage (areas of) fog to the remainder of the County Warning Area. While confidence isn't terribly high in fog development, the low dewpoint depressions, light and forecast to get lighter winds, and large breaks in the stratus would seem to suggest a somewhat greater potential for more coverage of fog overnight. Certainly don't have the confidence for a headline, but potential is there for dense fog to develop tonight and will be something for the oncoming overnight shift to keep an eye on. - Izzi && Short term... 156 PM CDT Through Tuesday night... High pressure expected to be situated across the region for much of the period, with mainly dry conditions persisting. Any showers/thunderstorm developing along surface trough/front to the south this afternoon and evening will likely remain to the south. Additional showers/thunderstorms expected to develop along approaching surface trough over Iowa and southwest WI on Tuesday. Once again, this precip will likely stay just to the west/northwest of the County Warning Area. Additional development expected to remain focused on this feature, but expect this axis to begin drifting into parts of northern Illinois Tuesday night. Thunder will be possible, however, instability is not looking overly high at this time. Some potential for some clearing skies this afternoon, but think cloudy skies will continue to be the trend into tonight and Tuesday. With the cloudy skies likely lingering on Tuesday, did lower high temps. If cloud cover holds on longer, temps will need to be lowered even more. Did add fog for tonight into Tuesday morning, as the pattern doesn't really look to change too much. Similar coverage is possible, and will once again need to monitor for the possibility of dense fog. Rodriguez && Long term... 236 PM CDT Wednesday through Monday... Guidance is in fairly good agreement on shortwave trough rippling through the zonal flow and across the region Wednesday. Most models depict some amplification of this wave as it moves across the region, probably due in part to some convective enhancement. While there has been some variability in the track of this feature, the 1200 UTC guidance suite would continue to favor our southern County Warning Area for the heavier rain/better rain chances. If the more southerly track of the wave verifies, then the warm sector with the better instability and attendant severe threat would likely be south of our area as well. Should see a break in the precip Thursday and probably much of the day Friday as well, before we potentially end up in a more active weather regime over the weekend. Medium range guidance continues to show a long wave trough digging into the western Continental U.S. With ridging over the southeastern U.S. Which would place our area on the potentially active north/northwest periphery of the ridge. Several days of moisture return with flow originating from the southern Gulf of Mexico and northwest Caribbean should provide for ample fuel for storms and when combined with stronger flow aloft on the flanks of the ridge certainly looks favorable for severe weather somewhere in the Midwest and middle/upper Mississippi Valley region. Could potentially see several waves of showers and storms Friday night through the weekend, which if it were to hit same areas repeatedly would raise the specter for flooding concerns as well. Still many days out and placement of the boundary and better chances of storms will undoubtedly be dictated by convection, so confidence on the precise details is low, but chances are there for strong-severe storms as well as heavy rainfall/flooding. - Izzi && Aviation... for the 06z tafs... Primary forecast for the overnight hours will be lowering stratus and fog development. Main concern for the daytime hours will be timing of improvement of cigs/vis as well as lake breeze development and timing of a wind shift at the Chicago area terminals. Cigs have quickly trended down to IFR at Ord/mdw and LIFR at gyy. Latest satellite imagery shows the low stratus gradually spreading inland and it may reach dpa as well, though timing there is still uncertain and the latest high res guidance is not handling the status as well as the past couple nights. Based more on persistence, have trended cigs a little lower than the previous update and it is possible that cigs at Ord/mdw could drop to 300 ft before daybreak. Fog trends are also uncertain, but based on observations from lake front web cams indicating only low stratus and little visibility restriction, and the extent of high cloud over the area, it's possible that the dense fog may be confined to gyy as far as the terminals are concerned and may be more patchy elsewhere, favoring low-lying, sheltered locations. With a very weak pressure gradient over the region, expect that winds should be mainly light and variable overnight and well into the daytime hours. This weak gradient should help set up a lake breeze and expect it to push inland through Ord/mdw by late afternoon, though with a synoptically northeast wind direction, the lake breeze may be more of a surge to 8-9 kt and a slight direction shift from nely to enely-Ely. && Lot watches/warnings/advisories... Illinois...none. In...none. Lm...dense fog advisory...nearshore waters until 10 am Tuesday. && $$ Visit US at http://weather.Gov/Chicago (all lowercase)

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