Friday, June 27, 2014

Severe Weather Discussion for Monday, June 30

This is a discussion for potentially significant weather on Monday, June 30.

Model guidance indicates a shortwave trough will extend southward across the Plains and Midwest from a parent upper level low centered in south-central Canada. This shortwave trough is expected to be the firing mechanism for the anticipated severe weather, as it shifts eastward amid strong mid-level winds and a steadfast zonal flow. This chart shows the aforementioned mid-level winds at speeds of over 50 knots in a wide swath of land across the north Plains, supporting the zonal flow regime due to a slightly suppressed ridge along the Gulf Coast. It is this zonal flow that makes me concerned for a severe damaging wind event, possibly to the extent of a derecho.

A zoomed-in view of upper level winds over the Plains and Midwest shows a band of intense winds rounding the base of the upper level low in Canada, and these winds look to push down as far south as southern Minnesota and Wisconsin, per the NAM model. I am using the NAM model primarily due to observed convective feedback issues in the GFS model immediately prior to Monday's event. These convective feedback issues have been slowly eroded today, but due to their continued presence, I feel more comfortable sticking with the NAM. The presence of these strong upper level winds tells me that we are likely looking at a continued supportive environment for a severe wind event.

Let's now analyze the typical characteristics of a damaging wind environment.

This image is from an academic journal, and shows a screenshot of a tornado environment setup on the top panel, with a "typical" damaging wind environment displayed on the bottom. The area where storms are favored to form is outlined in the hatched region. For the bottom panel, we see that the average damaging wind environment has the polar jet (the primary jet stream for all intents and purposes) shooting northeast, mainly over the northern portion of where storms are anticipated to form, eastward of the cold front. We also see the lower level jet stream shooting more north-northeast into the southern portion of the storm formation region, eventually coming together later on down the road.

If we compare the aforementioned factors with the ones we've already gone over, we can see the jet stream winds slanting northeast across the Midwest, north of where storms are expected to form on Monday (they are expected to form in Iowa/Illinois/Wisconsin). A glance at the frontal boundaries on Monday shows a cold front in a similar stance to the west of IA/WI/IL (shown below) as the bottom panel above.

Forecasted frontal boundaries on Monday morning
Let's now move on to an analysis of the projected instability values for Monday evening to try and identify any other boundaries that may ignite storms.

The NAM's forecasted convective available potential energy (CAPE) is quite impressive for this event. We see the band of elevated CAPE ahead of the cold front in the north Plains, but the real story is the swath of 3000+ j/kg of instability from Oklahoma to Ohio, and it is this area of instability we will focus on. When taking into account that storms are expected to form in Iowa, Wisconsin and Illinois, we look that way and find what looks to be a boundary of sorts over northern Illinois. This appears to be defined by a steep drop-off in CAPE values from the tip of Lake Michigan to the WI/IL border. This makes me wonder if storms could fire along that potential boundary there, though the long range format of this forecast tells me to wait and watch for this particular feature. Nonetheless, it is apparent that ample instability will be present for this event.

The final item to look over is the idea of analog forecasting. This method involves analyzing past conditions, and choosing dates with conditions that best match up with forecasted conditions. In this case, the SLU CIPS program was utilized to see what dates had the most similar conditions as those forecasted to be present on Monday. The top analog that came up was June 18, 2010, and the storm reports for that day are shown above. It is quite apparent that this top analog greatly favors a damaging wind environment, as we've already evaluated for ourselves with the two-panel graphic earlier in this post. In this top analog, the damaging wind swath extended from Illinois and Wisconsin, as well as Iowa, into Indiana and Michigan. This is actually quite similar to what I think may happen if current forecasted predictions are to verify, and this must be watched closely.

To summarize, a potentially significant severe weather event is projected for Monday, with damaging winds being a primary threat. The areas affected look to be Iowa, Illinois and Wisconsin. More information will be delivered as it becomes available.


Severe Weather Discussion for Sunday, June 29

Sunday is being monitored for potentially significant severe weather.

The Storm Prediction Center has highlighted portions of the Plains and Midwest for severe weather on Sunday. The Slight Risk covers eastern Nebraska, northern Kansas, northwestern Missouri, Iowa, southeast South Dakota, much of Missouri, Wisconsin, and the Upper Peninsula of Michigan. The most elevated risk inside this overall outlook area extends from eastern Nebraska to west-central Wisconsin.

Instant Weather Maps
Projected instability values off of the NAM model show figures in excess of 4000 j/kg situated from Kansas to Iowa, which should be more than enough to fire off these storms as a disturbance propagates eastward across the northern Plains. Shear from the surface to the 500mb layer looks to grow to upwards of 60 knots across the northern Plains and Upper Midwest, with spots of 70 knots also being found. It does appear that the initial storms that fire look to have some substantial tornado potential, with the aforementioned instability and shear present. As the night wears on, the expectation is for these cells to coagulate into a mesoscale convective system, which will then likely shift off to the east overnight. A strengthening nocturnal lower level jet stream should enable this eastward progression to continue.