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The tight inter-band correlation and the lag-wavelength relation amongUV/optical continua of active galactic nuclei have been firmly established.They are usually understood within the widespread reprocessing scenario,however, the implied inter-band lags are generally too small. Furthermore, itis challenged by new evidences, such as the X-ray reprocessing yields too muchhigh frequency UV/optical variations as well as it fails to reproduce theobserved timescale-dependent color variations among {\it Swift} lightcurves ofNGC 5548. In a different manner, we demonstrate that an upgraded inhomogeneousaccretion disk model, whose local {\it independent} temperature fluctuationsare subject to a speculated {\it common} large-scale temperature fluctuation,can intrinsically generate the tight inter-band correlation and lag acrossUV/optical, and be in nice agreement with several observational properties ofNGC 5548, including the timescale-dependent color variation. The emergent lagis a result of the {\it differential regression capability} of localtemperature fluctuations when responding to the large-scale fluctuation. Anaverage speed of propagations as large as $\gtrsim 15\%$ of the speed of lightmay be required by this common fluctuation. Several potential physicalmechanisms for such propagations are discussed. Our interestingphenomenological scenario may shed new light on comprehending the UV/opticalcontinuum variations of active galactic nuclei.

EUCLIA - Exploring the UV/optical continuum lag in active galactic nuclei. I. a model without light echoing