SPC Apr 7, 2020 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook
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Day 1 Convective Outlook NWS Storm Prediction Center Norman OK
Severe thunderstorms are possible late this afternoon through
tonight across southern portions of the Great Lakes region into the
Ohio Valley and Mid Atlantic. Strongest storms will pose a threat
for very large and damaging hail, as well as potentially damaging
wind gusts, and perhaps at least some risk for a tornado. ...Synopsis...
Little change to the large-scale pattern is forecast through this
period, with a higher latitude blocking ridge remaining prominent to
the north of Hudson Bay. As blocking also lingers across the
mid-latitude and subtropical eastern Pacific, only slow movement of
the closed mid-level low near the California coast is forecast, with
a gradual digging just west of central through southern coastal
areas. To the south of the higher latitude ridge, a deep mid-level closed
low appears likely to remain quasi-stationary near the Canadian
Maritimes, while a strong jet streak digging to the lee of the
Canadian Rockies contributes to another evolving mid-level low
near/east of the Canadian Prairies. Smaller-scale perturbations
progressing around the southern periphery of this feature will
contribute to mid-level height falls and increasingly cyclonic flow
across the northern Plains into the upper Mississippi Valley by late
tonight. One preceding short wave impulse is forecast to accelerate across
the Great Lakes region through the southern New England/northern Mid
Atlantic coast by the end of the period. This is forecast to be
accompanied by a developing surface low along an initially northward
advancing warm front. It is also expected to contribute to
suppression of broader-scale ridging, initially present across much
of the Ohio Valley into Mid Atlantic, by this evening. ...Great Lakes/Ohio Valley into Mid Atlantic...
Forcing for ascent and destabilization are already contributing to
increasing thunderstorm development along the frontal zone across
southern portions of the Great Lakes region, aided by a 30+ kt
westerly 850 mb jet. This activity appears likely to progress
around the crest of the large-scale ridging, and may spread
east-southeast of the Allegheny Mountains into portions of the Mid
Atlantic region by this afternoon. Limited low-level moisture may
only contribute to weak boundary-destabilization across this region,
but deep boundary-layer mixing along and south of the warm front may
contribute to potential for strong surface gusts across northern
Virginia, aided by modest west-northwesterly mid-level flow. Meanwhile, in the wake of this activity, warm and capping elevated
mixed-layer air is forecast to overspread much of the southern Great
Lakes and Ohio Valley region. This may suppress convective
development through much of the day, but daytime heating and
increasing low-level moisture (including surface dew point increases
through the lower 60s), may contribute to "loaded-gun" type
soundings with CAPE in excess of 1000 J/kg over a sizable area. Considerable spread exists among the various model output concerning
the potential convective evolution within this environment, which
will include strengthening deep-layer shear supportive of organized
severe storm development with the approach of the upper impulse by
this evening. However, at least scattered discrete storm
development appears possible across southern portions of the Great
Lakes region by early evening. Continued southward development of
at least scattered discrete storms appears likely across the Ohio
Valley through this evening, ahead of a southward advancing cold
front in the wake of the surface low. Given the environment, strongest storms will be capable of producing
very large hail, with at least some upscale convective growth
through the evening contributing to potential for damaging surface
gusts. Tornadic potential remains more unclear, due to the forecast
veered nature of the low-level wind fields, and resultant lack of
more pronounced clockwise turning with height in the low-level
hodographs. Eastward advection of the convectively unstable thermodynamic
profiles, east of the Appalachians into northern Mid Atlantic
coastal areas is forecast overnight. This may occur above a
residual stable layer near the surface, but it should contribute to
increasing severe weather potential, at least in the form of
thunderstorms capable of producing severe hail. ...Lower Mississippi Valley...
Models suggest that a persistent zone of low-level warm advection
will set up across the region by early today. This probably will be
accompanied by increasing thunderstorm development, which may
eventually become supported by inflow of moderately unstable
boundary-layer air by this afternoon. Although low-level wind
fields may remain modest in strength, moderate to strong mid/upper
flow around the crest of the subtropical ridging may contribute to
sufficient shear to support a risk for severe hail and strong
surface gusts. ..Kerr/Grams.. 04/07/2020 

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