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Revised in 13 passages since the 6:22 AM discussion. See every revision.
Crisp, fall-like conditions tonight. Some fog is possible after midnight.
Comfortable & pleasant afternoon weather conditions today through Tuesday & Thursday-Friday.
Windows for showers & thunderstorms Tuesday night through Wednesday & next weekend.
Synopsis/Mesoanalysis: Southern Wisconsin is centered between low pressure along the Ontario-Quebec border & high pressure along the Minnesota-Iowa border late this morning, resulting in breezy northwest surface winds. With the center of the high & affiliated low level subsidence well west of the area, surface heating is allowing for fair weather cumulus development, with cumulus coverage expected to increase further moving deeper into the afternoon. The aforementioned surface high will nudge into southern Wisconsin tonight, leading to clear skies and light surface winds. The clear skies & light winds will allow for efficient radiational cooling & fall-like low temperatures in the low 50s. Some spots could dip into the upper 40s. Areas of fog development are possible after midnight, particularly in low- lying locations. Fog will quickly dissipate after sunrise Monday, giving way to mostly sunny skies and pleasant weather conditions Monday afternoon. High pressure will begin to shift east of the region Monday night, allowing south-southwest winds to re-establish across the area.
Tonight: Will be monitoring for potential fog development near/after midnight as the boundary layer attempts to decouple in the presence of clear skies and light surface winds. Forecast soundings show extremely shallow near-surface moist layers susceptible to mix out and/or dew depletion, so fog coverage remains uncertain. Suspect that the best overall fog potential will remain confined to river valleys & other low-lying spots, but have included broader mentions areawide in the afternoon update. Will be monitoring visibility trends into the overnight. Slow down, use low beam headlights, and allow for extra following distance if encountering areas of fog on the roads.
Synopsis: High pressure will continue to shift east of the region on Tuesday, with southerly return flow allowing for gradual moisture returns and slight increases in humidity. The next upper disturbance and surface front will drift into the western Great Lakes from the Canadian Prairies Tuesday evening through Wednesday, bringing the next chances for showers and thunderstorms to southern Wisconsin. A new area of high pressure will move into the western Great Lakes during the second half of the week, leading to pleasant and mostly dry weather conditions Thursday into Friday. The high will shift east by Saturday, with returning southerly flow leading to steady increases in humidity through the remainder of next weekend. Concurrent increases in zonal upper level flow, in addition to embedded shortwaves, will support the next shower and storm chances within the returning moisture. Saturday night into Sunday is the currently favored window for storms in southern Wisconsin, with more precise timing clarifying through this week.
Tuesday Evening through Wednesday: Still anticipating periods of showers and storms across southern Wisconsin as an upper wave & attendant surface cold front move through the western Great Lakes. General idea of initial (likely elevated) storms within an increasing warm advection regime Tuesday evening/night, followed by a window of surface-based storm potential along the passing cold front Wednesday afternoon, remains on track in this forecast. Latest six hourly precipitation probs reflect these expectations. Whether or not any of this activity reaches severe levels remains less certain, however, largely due to the currently progged timing of the best synoptic-scale ascent. Global & longer-range mesoscale guidance is relatively insistent on a Tuesday overnight/early day Wednesday passage of the upper wave & surface front, which would be misaligned with peak surface heating & the best instability. Nevertheless, prevailing deep layer shear will be supportive of more organized storm development through this time frame, with any combination of factors promoting increased instability leading to higher potential for a few severe storms with hail and gusty winds. Will be monitoring setup and timing details closely over the next 24-36 hours.
Saturday Night through Sunday: A second window for showers and storms opens as low level moisture deepens & the mid-upper pattern becomes more energetic. More precise details will clarify through this week, though it does appear that periods of storms will be around the region as the nose of the low level jet approaches and ultimately folds over southern Wisconsin. Global ensemble guidance is in relatively sound agreement regarding low level jet evolution & placement, providing further confidence for periods of storms during this time frame. A broad overlap of sufficient shear and instability makes this a period to monitor for organized/stronger thunderstorms, though whether these parameters come together to realize any potential will depend on smaller-scale features & timing not yet captured by available guidance. Will be monitoring trends & providing additional info as this portion of the period draws closer.
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