When the frontal boundary was eventually pushed eastward by a small upper-level disturbance
, convergence ahead of the front and the higher upper-level winds created conditions for supercell storms over the Northeast U.S.
The storms and suspected tornadoes, which forecasters say were caused by an upper-level disturbance
from Mexico, hit an already-sodden swath of Texas that was still drying out from the remnants of Hurricane Patricia.
And just when you think you're getting a break in the clouds, you might find that you were the source of the upper-level disturbance
On December 14th, an upper-level disturbance moving across southern portions of California and Nevada spread moisture into a chilly air mass that was in place across the Mojave Desert.
A series of upper-level disturbances and storm systems moving through the base of the trough resulted in unusual low elevation snow in the Mojave Desert during a roughly two week period.
When an upper-level disturbance approaches the front and removes mass from the column of air, such as upper-level divergence, this causes pressures to fall at the surface.
And when this happens, the upper-level disturbances intensify and can produce stronger forces.
The jet stream winds largely retreat northward to Canada by summer, but the Midwest gets countless fronts, upper-level disturbances
, and jet stream energy that move through the region or even stall out here.
Clouds come primarily from eastward-moving upper-level disturbances
or weakening cold fronts blown in from the Great Australian Bight in the south.
But on certain days, upper-level disturbances
will approach the area and break down the cap, causing the storms to spread up and down the stronger boundaries and possibly even develop along weak boundaries.