Storm Prediction Center Update: 7/12/17

The Storm Prediction Center (SPC) Outlook shows some severe threats this week.

Day 1: Slight Risk

There is a Slight Risk over the lower Great Lakes Region:

Associated with the Slight Risk is a 5% Tornado Threat Ring, focused over southern Wisconsin:

Ongoing storms are dropping outflow boundaries near a surface cyclone in Michigan.  New storms are expected to fire along some of these outflow boundaries, where there is still deep moisture and ample instability in the warm sector of this cyclone.  Storms that form have a strong chance of becoming supercells considering the favorable shear environment.  The SPC notes that “all threats are possible” with these storms.


Day 2: Slight Risk

Day 2 shows a Slight Risk over the lower Great Lakes region and Ohio River Valley:

There will be a similar setup on Day 2 as Day 1, though the frontal boundary associated with the surface cyclone will become more important.  Storms will fire along the outflow boundaries from the Day 1 storms, still in an environment of deep moisture and relatively high instability.   In addition to these ingredients, a few weak impulses in the jetstream will amplify an already unstable environment.  The frontal boundary will perhaps produce too much lift, creating rapid stabilization in its wake, and multicellular clusters ahead of the boundary.  Severe hail and wind will be the primary threats.

There is also a Marginal Threat in the southwest associated with a monsoonal pattern.  Strong heating, weak shear, and inverted-v soundings mean that storms that persist will have gusty downdraft winds, though the tornado threat is very small.


Day 3:  Slight Risk

There is a Slight Risk over the Mid-Atlantic.

The shortwave trough and surface low that existed in Days 1 and 2 will continue east.  The Mid-Atlantic region will be in the warm sector of this cyclone.  The warm sector will be in an area of high dewpoints (lower 70’s F), and some destabilization will occur through the day.

Lapse rates are forecasted to be lower than a typical severe threat day, and storms may be widespread and cluster quickly.  However, there is adequate deep-layer shear to keep the stronger cells well ventilated, and rotating storms are possible.  Therefore, there is a threat for large hail, gusty winds and torrential downpours.


Day 4-8:  15% Probability to Predictability Too Low

The models are beginning to converge on an area of potential severe threat over Michigan and Wisconsin.  This threat is due to the overlap of moist air advection from the south, adequate deep-layer shear, and the approach of a jetstreak in the winds aloft.  At this time, the SPC has issued a 15% Probability of severe weather for Day 4.

After that, the models begin to diverge, and some of that is related to how events play out on the Day 4 threat.  While a more zonal flow pattern is expected over this region, small amplitude perturbations will dip into areas of deeper moisture, which will increase the severe threat.  However, the models diverge at this time.

Thank you for reading this post.

All data and images are from the Storm Prediction Center Website.


About highplainschasing

This blog is about my tales in storm chasing. My name is Seth Price and I am an instrumentation instructor at New Mexico Tech. My amateur radio call sign is N3MRA.
This entry was posted in Models, Practicing Concepts, Predictions, Severe Weather, SPC 1-7 day and tagged , , . Bookmark the permalink.

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