Day 1 Severe Weather Outlook

Valid 141300Z – 151200Z


Severe thunderstorms are not forecast across the contiguous U.S.
today and tonight.

In mid/upper levels, a more-progressive northern-stream pattern will
unfold as a strong shortwave trough digs east-southeastward from the
northern Rockies across the northern/central Plains. By 12Z, an
embedded 500-mb low may exist near FAR, with troughing southwestward
across NE and western KS. As that occurs, the long-lived, occluded
cyclone now over northern ON will eject east-northeastward over the
James Bay region to adjoining portions of QC, and gradually weaken.

Meanwhile, in the southern stream, a positively tilted shortwave
trough — apparent in moisture-channel imagery over western NV and
east-central/southwestern CA — will move east-southeastward over
portions of AZ and northern NM. This will lead to piecewise
ejection of vorticity maxima from the southern Baja/northwest MX
region across TX, the lower Mississippi Valley and MS/AL/GA, along
the southern rim of a strengthening 400-250-mb subtropical jet.

At the surface, 11Z analysis showed a wavy, stationary to very
slow-moving frontal zone from a low off the Jersey Shore to eastern
NC, south-central GA, coastal LA, and south-central TX, with the
TX/LA segment drawn as a warm front. As the northern- and
southern-stream troughs move eastward, the western segment of the
front will shift further inland and become more diffuse, amidst a
broad low-level warm-advection regime. Meanwhile, the Atlantic
Coast segment will accelerate eastward and offshore as a cold front.
A secondary zone of cold frontogenesis will occur over the northern
Plains today as the northern-stream shortwave trough approaches.
The resulting surface cyclone should be centered over western ND by
00Z, then in stronger form over southeastern MN by 12Z. At the end
of the period, a cold front should arch from the low across IA,
eastern KS, and the OK Panhandle. Marginal, elevated, low/middle-
level returning moisture will support isolated/overnight thunder
potential ahead of the front over parts of the mid/upper Mississippi

Episodes of isolated to scattered thunderstorms are expected through
the period in a corridor from far west TX to the southern Atlantic
Coast. Sufficient low-level and/or deep shear may exist today over
portions of the southeast TX/southwestern LA region, and late
afternoon into evening in the Big Bend area of far southwest TX, to
support brief/localized storm-scale rotation. However, mid/upper-
level lapse rates will be limited amidst a plume of moisture aloft
— streaming off a persistent and large area of convection over the
southern Gulf of CA and vicinity. With weak instability in the Big
Bend and messy storm modes expected farther east, the severe
potential appears too isolated, uncertain and disorganized for an
unconditional risk area.

..Edwards.. 10/14/2019


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