Saturday, December 26, 2015

December 26-29 Potentially Catastrophic Flooding

A potentially catastrophic flooding situation is unfolding for parts of the South Plains in the next couple of days.

The Weather Prediction Center has outlooked portions of eastern Oklahoma and extreme northwest Arkansas in a High risk of rainfall exceeding flash flood guidance to the right of a line. In essence, this is the risk of very heavy rainfall in a given area. There is a Moderate risk extending from northern Texas, portions of eastern Oklahoma, northwest Arkansas, central and southern Missouri, and extreme west-central Illinois. Those in the Moderate risk should make preparations for very heavy rainfall, while those in the High risk area should make preparations for potentially-widespread flash flooding.

The Weather Prediction Center places an 11.87" rainfall maximum over eastern Oklahoma for this storm system over the next 72 hours. Given that Oklahoma went through a record-breaking rainfall event in May of this year, this impending rain event will only add stress to an environment that has already dealt with a very intense rainfall event this year. Amounts on the order of 4-7" are projected for northeast Texas, while central and southern Missouri into northwest Arkansas and portions of east Oklahoma are outlooked for 7-10"+ of rain. Again, this is not something to take lightly. Rainfall on this magnitude, especially in areas that received extreme rainfall several months ago, is expected to cause flash flooding, possibly on a severe scale.

To Summarize:

- Heavy rain is expected to cause flash flooding over the next few days.
- This rain could be as high as 12", with amounts over 6" possible in portions of eastern Oklahoma, southern Missouri, and western Arkansas.
- Preparations should begin now for potentially severe flash flooding, especially in the areas mentioned above.


December 26-29 Significant Ice Storm and Snowstorm

It appears that a significant snowstorm, as well as a significant ice storm, will unfold in the December 26-29 period.

VALID: 12/27 at 4 AM CST
All further images with this layout are also from the CMC
The event begins as the storm system exits the Southwest. We will see a swath of moderate to heavy rainfall extending from the Great Lakes into Texas, but the real story will be the wintry precipitation. In this image, the forecasted precipitation type valid for December 27th at 4 AM CST, moderate snow will be falling in northern Texas into the Oklahoma Panhandle, as well as the western portion of Texas into New Mexico. Heavy mixed precipitation will be falling in southwest Oklahoma into northern Texas, draped across the Red River. Freezing rain is forecasted very close to Oklahoma City, while moderate rain falls across the remainder of the state. Mixing will also be a concern in northwest Missouri.

VALID: 12/27 at 5 PM CST
By the evening of December 27th, model guidance foresees snow continuing to fall in northern and western Texas, with continued moderate to heavy mixing in portions of Oklahoma. Oklahoma City now appears at risk for a freezing rain event in this timeframe, with heavy rain located in Arkansas and Missouri. Our storm system is also observed moving across the Southern Plains at this time.

VALID: 12/28 at 1 AM CST
In the early morning hours of December 28th, the system will begin to spread its influence to the north. Moderate snow will be ongoing on the back end of the system in Oklahoma, and now also falling in southern and eastern Kansas into far northwest Missouri. Mixed precipitation will be falling at this time in portions of Oklahoma and Texas, and falling at a heavier rate in northern Missouri into extreme western Illinois. Heavy rain will continue falling in Missouri and Arkansas, setting up a pronounced flooding risk which we will analyze in a follow-up post, which will be published shortly.

VALID: 12/28 at 7 AM CST
As the sun begins to rise, the situation becomes more dicey for those in the Midwest and Great Lakes. Model guidance now foresees moderate snow falling on the back end of the system in central Oklahoma, with a tight gradient from snow to mix to rain in eastern Oklahoma. Moderate snow will be falling in much of Kansas into southeast Nebraska, with heavy snow impacting southern Iowa and northwest Missouri. A large swath of mixed precipitation will be falling from central and northern Missouri into much of northern Illinois and Indiana, including much of metropolitan Chicago, IL. A narrow, but moderate band of freezing rain looks to hit central Illinois into northern Indiana at this time period, setting up a potentially catastrophic rush hour situation for urban areas.

VALID: 12/28 at 2 PM CST
By the early afternoon hours of December 28th, the storm system is now mature, with a defined dry slot shooting into southern Illinois, and a frontal band producing moderate to heavy rain from Alabama and Georgia to Tennessee and Kentucky. Light to moderate snow is projected to be falling from east Kansas into east Nebraska, as well as much of Iowa and northwest Missouri. Heavy snow is outlooked in southern Wisconsin and central Iowa. A heavy band of mixed precipitation is forecasted in central Missouri, eastern Iowa, northern Illinois, extreme southern Wisconsin, southern Michigan, and even portions of Pennsylvania. A narrow but intense band of freezing rain looks to set up immediately south of downtown Chicago, extending east into the heart of Gary, IN, and just making its way into southern Michigan. This would likely set up a treacherous commute home for any workers able to make it in during the morning mess.

Now that we've analyzed the hour-by-hour, let's check out the total accumulations.

Meteocentre - Snow accumulations
Accumulations for this image and the next two accumulation images are in MILLIMETERS.
The Canadian model projects a general swath of 20mm to 40mm of liquid-equivalent snow in the Texas/New Mexico/Oklahoma region. Putting this into a conversion chart tells us this is roughly 0.8" to 1.6" of liquid-equivalent snow. The average snow-to-liquid ratio is 10:1, meaning one inch of liquid would produce 10" of snow in that environment. However, given the lack of a preceding cold air mass, it would not surprise me to see totals lower than that, as the ratio goes down and snow becomes heavier. Thus, using a general ratio of 8:1, we could expect about 6-12" of snow for this region.

A band from northern Missouri to southern Wisconsin shows about the same liquid equivalent values, and again, a lower snow ratio is expected, so 6-12" of snow could be expected if this forecast were to verify as is. These heavier accumulations would just miss downtown Chicago, but would greatly impact Madison, WI and Milwaukee, WI.

Meteocentre - Ice Pellet accumulations
For the projected ice pellet accumulations, a swath of 10-25 millimeters exists in portions of Oklahoma, which is converted to about 0.4" to 1.0" of liquid-equivalent ice pellets. The amounts only increase, with a higher band clocking Chicago and into Detroit. Amounts on the order of 25mm to 40mm of ice pellets, converted to 1.0" to 1.6" of liquid-equivalent ice pellets, would be expected in this band. Additional, albeit lesser amounts are then outlooked for the Northeast.

Meteocentre - Freezing Rain accumulations
Lastly, we examine the projected freezing rain accumulations from this system. The main band appears to be placed immediately south of downtown Chicago, if not slightly inside the main metropolitan area. The swath contains values on the order of 7.5 millimeters to about 15 millimeters, with the highest amounts in Illinois. Using our handy conversions, we can identify that this storm drops 0.3" to 0.6" of freezing rain, a potentially catastrophic scenario with the projected wind fields.

To Summarize:

- A significant snowstorm and ice storm looks to unfold across the country over the December 26-29 period.
- Freezing rain on the order of up to 0.75" could impact portions of Illinois, Indiana, Missouri, and Michigan.
- Snowfall on the order of 6-12" could impact portions of Texas, Oklahoma, Missouri, Iowa, Wisconsin, and Michigan.
- Preparations should begin for those outlooked in snow, mix, ice, and heavy rain areas.

Additional posts on this storm are forthcoming.