Heavy Rain and Snow Event This Week

 A vigorous weather system will sweep across the mid section of the nation today before pushing east on Wednesday. While BWO focuses on precipitation, I did want to point out the potential for severe weather, including tornadoes, for parts of the region, especially Arkansas and surrounding states today.

Here is the set-up for heavy rainfall over the next few days. What concerns me is the tap of both low-  and upper-level moisture. Here is a close-up of the Midwest. Note the very high Precipitable Water (PW) that has streamed north as far as Missouri and Illinois.  PW is an indication of the total moisture content of the atmosphere. Readings of 1” or more represent unusually high atmospheric moisture for January.

While a PW near 1” might not be unusual during the summer months, it represents a reading near the 90^th percentile for late January. This image shows climatology of PW for Springfield, Missouri. The red triangle shows the current value of PW near this location.

Also, an analysis of a moisture channel satellite image, you can see that Pacific moisture is feeding into the system at mid and upper levels.

I have a rule of thumb that when PW values are near the 90^th percentile and there is a mid/upper level tap of moisture—very heavy rainfall is possible–at least inisolated spots. One mitigating factor could be that this system is progressive, passing through within 24 hours. This could limit the overall amount of rainfall.

As the system moves east on Wednesday, the upper dynamics (or energy) become somewhat less focused thus rainfall amounts could be a bit less. However, amounts in excess of 2” are still possible, especially over higher elevations.

Generally, key runoff thresholds range from 2 inches or so in Missouri and Illinois to 3 inches further south. Rain from this event likely will near these threshold levels in some spots and could produce enough runoff that would result in strong within-bank river rises and scattered areas of flooding,

Runoff thresholds are lower over parts of the Mid Atlantic and Ohio Valley, closer to 1.5” where more significant runoff could occur.

Keep in mind that snow is expected within the colder air over the Upper Midwest into the Great Lakes region.