Forecast Discussions mentioning any of "ACARS" "MDCRS" "AIRCRAFT" "TAMDAR" "AMDAR" "WVSS" received at GSD on 06/02/19

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
647 PM CDT Sat Jun 1 2019 .MESOSCALE DISCUSSION... 608 PM CDT Since 5 pm, storms have remained similar in intensity with isolated strong/near-severe. There may be just enough "messy" convective coverage in this environment favoring outflows, that it very well could be hampering widespread robust, sustained storms. In addition, the strongest effective deep layer shear of near 40 knots remains a little north of where the current highest convective coverage is (south Chicago metro to Streator). That all said, the scattered severe threat continues, and we continue to some semi-discrete convective, at times resembling some supercellular structure. Recently on the immediate south side of Chicago there have been a flurry of hail reports with the storms there. With the mid-level winds steady or even slightly increasing in tandem with still uncapped and non-turned over air to the south, the scattered severe weather threat will continue across mainly the south Chicago metro and far northwest Indiana through 7-8 p.m. Primary threats remain hail, especially 1-1.5 inch hail, and isolated gusty winds. The threat of a more congealed wind threat is lowering with each hour. Some locations have received multiple rounds of storms in the Chicago metro and are at a little higher risk to have localized flooding, maybe even flash flooding of a favorable location given temporary heavy rainfall rates. The overall flood threat though remains somewhat low. MTF && .SHORT TERM... 155 PM CDT Through Tonight... An outflow/lake breeze composite boundary has pushed inland to near a Woodstock to Naperville to Wheeler line based on latest surface observations. The airmass to the east and north of this boundary has been stabilized, with dewpoints in the upper 50s and temperatures in the upper 60s. This stabilization is also demarcated by stable wave clouds on GOES-16 visible satellite imagery. Latest guidance does indicate the breezier synoptic southwesterly flow may force this boundary back east a bit through the afternoon hours, and how this modified airmass impacts the severe potential into portions of Lake (IL) and Cook counties remains a bit uncertain at this time. Recent ACARS soundings out of Midway show this stable layer is pretty robust, with a notable temperature inversion present near the surface. With additional diabatic heating, would expect at least some airmass recovery on the cool side of this boundary with baroclinicity starting to wash out a bit over the next 1 to 3 hours. If this boundary remains robust, however, it could locally mute the severe threat into Cook and Lake counties later this evening by limiting the potential for strong downdrafts and associated winds to reach the surface. The other question stems around a notable pocket of dry air now being analyzed in the 850-700 mb layer with 850 mb dewpoints under 0 C now being analyzed across portions of northwestern Illinois and into our far western counties. This could tend to work against updraft intensity--at least initially--although an axis of much higher moisture is analyzed near I-90. With surface temperatures having warmed into the 80s, dewpoints in the lower 60s, and steep mid-level lapse rates in place overhead, MLCAPE values are, nonetheless, inching past 2000 J/kg early this afternoon, especially near and west of the I-90 corridor. Mid-level winds are also increasing ahead of a small but sharp shortwave, and effective deep layer shear values are pushing north of 30-35 kts across the region (lower south of I-80, highest north). 500 mb height falls on the order of 50-60 m/12 hr have been noted spreading towards southern Wisconsin. Associated increases in forcing for ascent will be responsible for expanding showers and thunderstorms along and incoming cold front through the evening hours. All of this taken together spells a continued threat for strong to severe storms across our region this afternoon. Hi-res guidance continues to struggle with a rather wide disparity in solutions still noted this afternoon. Storms are beginning to develop across southwestern Wisconsin at this hour, near a developing weak surface low and at the leading edge of stronger ascent from the incoming shortwave. Latest thinking is that storms will continue to develop south and westward over the next hour or two, eventually nearing Winnebago, Ogle, and Lee counties towards 4 PM or so. Additional convection is also attempting to develop along the aforementioned outflow/lake breeze across southern Cook and into Lake (IN) and Porter counties. The airmass immediately ahead of this boundary is unstable and only weakly capped, so we will need to keep an eye on this area for potential storm development as well. Initial discrete/isolated activity will carry a risk for large hail (potentially up to ping pong-golf ball sized), but storms should quickly begin to congeal into line segments as robust forcing overspreads the area. As a result, the severe threat will begin to transition to mainly wind damage, with a potential for 60-70 mph wind gusts. At this time, think the main severe corridor may end up developing near and north of I-80, and particularly focusing along I-88 and I-90 where instability will be maximized. The tornado threat is low from a synoptic standpoint with minimal low-level hodograph curvature, but we will be keeping an eye on any stronger updrafts which manage to develop near this feature later this afternoon. A brief tornado spin-up or two can`t be ruled out in the vicinity of this boundary, but the main severe risk still appears to be from hail and damaging winds. Looks like a surging line of storms will move out of our south and eastern counties by mid-evening with a continued threat for strong to locally damaging wind gusts. Will be holding onto some low PoPs behind this line of storms with lingering moisture and some instability to work with, but the main severe window will come to a close by about 9-10 pm area-wide. Carlaw && .LONG TERM... 155 PM CDT Sunday through Saturday... Amazingly, it does indeed look like we will enter a briefly quieter weather pattern on Sunday and into early next week. Unfortunately, it does appear as if we`ll have to contend with some lingering low precipitation potential on Sunday as low-level moisture will not be entirely scoured out by today`s shortwave and cold front. Forecast soundings reveal a sliver of lingering instability, trapped between a building subsidence inversion and a strongly mixed boundary layer. Certainly doesn`t look like things would be able to cool enough for charge separation so will not be indicating any thunder in the grids, but have added some very low (10%) PoPs to the region to account for showers bubbling with daytime heating. Weak northwesterly gradient flow should allow a lake breeze to push inland Sunday afternoon, keeping things generally in the 60s lakeside while upper 60s and lower 70s become more prevalent inland. A more subsident regime will begin to build in overhead on Monday as high pressure starts to really nose into the region. Will be showing precipitation-free conditions on Monday with highs ranging from the lower 60s near the lake to the lower 70s inland. The dry weather looks to continue through most of Tuesday, although low to mid-level moisture will begin to increase during the afternoon hours as southerly return flow re-establishes itself with our transient surface high shifting eastward into the Mid Atlantic. This will allow instability to diurnally build which could support some low storm chances mainly across our western locales during the afternoon and evening. The next cold front will be approaching the area Tuesday night but there are plenty of questions on convective evolution into Wednesday. Forcing Tuesday night may be somewhat disjointed over the region and possibly focused more south. Then on Wednesday, the front may stall or wash out over the area. In west-northwest flow aloft, bulk shear up to 30 kt and steep lapse rate plume could enable some storm organization for aforementioned lower end strong/severe risk sometime during Tuesday eve- Wednesday. Also, PWAT of 1.5"+ could support localized flooding risk. Finally, looking ahead to late next week, plenty of uncertainty on synoptic evolution. Question on Thursday looks to be whether front sags south of the area or stalls overhead/lifts slowly north, which will drive convective chances. Then for Friday into next weekend, it looks like a closed low emerging from the southwest will completely cut off from the mid-upper jet as large scale ridging over central and eastern NOAM builds into Canada. Models struggle with handling of upper lows, but in general the pattern, should this possible closed low affect our region, would spell cloudy, humid and unsettled conditions, with slow moving showers and thunderstorms at times. This could be a period to watch for more widespread significant rainfall totals, as PWAT values may approach 2", or near/around 200% of normal. Carlaw/Castro && .AVIATION... For the 00Z TAFs... Multiple outflow boundaries have lead to variable and occasionally strong/gusty winds at the Chicago area terminals this afternoon. Latest radar imagery indicates that convective activity is becoming more scattered as the cold front pushes southeastward across the region. As the front pushes south of the terminals early this evening, the potential for showers and thunderstorms will diminish while winds weaken and back from prevailing nely to more nwly. While there may be the potential for some isold showers later into the evening, potential will quickly diminish with sunset. The remainder of the period from late this evening through tomorrow should be relatively quiet. && .LOT WATCHES/WARNINGS/ADVISORIES... IL...None. IN...None. 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