Today's NAM Model valid for Tuesday evening shows more widespread showers and thunderstorms developing.

Afternoon Roundup: Unsettled weather arrives

Today turned out to be another lovely day, with temperatures in the low to mid 80s, mostly sunny skies, and low humidity. However, the increase of thin, high clouds can often portend unsettled weather, as high and thin clouds tend to streak out ahead of storm systems.

High pressure which dominated this weekend is slowly moving offshore, and when combined with the rigorous disturbance that is producing severe weather in the Midwest, our winds will be turning towards the southeast — an onshore component — if they have not already. As this onshore flow continues to become more established, clouds and moisture will be on the increase tonight. Any showers, however, should hold off tonight or be very light and and isolated due to the best forcing to trigger lift and precipitation remaining to our west.

This disturbance finally will approach our area as the day goes on Tuesday. This will lead to mostly cloudy skies, more humid conditions, and the chance of an afternoon shower or thunderstorm. These storms will remain well below severe level, due to the greatest energy being far removed from our area, as well as the onshore flow limiting our instability.

General weather highlights include:

  • Increasing clouds tonight with overnight temperatures generally remaining in the upper 60s to low 70s. Any precipitation will most likely hold off.
  • Mostly cloudy skies tomorrow, but the morning and early-afternoon should remain dry. As the trough and associated disturbance finally moves closer to our area, the chance for rain and thunderstorms increases during the late-afternoon and particularly the evening hours. High temperatures will be in the low 80s, but it will feel noticeably more humid than it was today.
  • A warm front will be approaching late Tuesday night and early Wednesday morning, from the south. Along this warm front, a secondary area of low pressure from the initial system in the Midwest will be developing just to our south. This will cause more lift in the atmosphere, and when combined with the already moist airmass, could lead to showers and thunderstorms with locally heavy downpours and some loud thunder. The atmosphere will not be supporting severe thunderstorms with strong wind gusts, as the mid-level flow looks quite weak.
  • Wednesday looks quite unsettled, as this wave of low pressure will be causing showers and thunderstorms throughout the day.
  • Much more pleasant weather with low humidity returns on Thursday and Friday

Something relatively rare in the meteorology community today: Surface-based CAPE values look to exceed 6,000 J/KG just southwest of Chicago. This is an extreme amount of energy and instability, which will be supportive of thunderstorms producing very large hail — perhaps some 2-3″ in diameter, 70mph wind gusts, and a few tornadoes.

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