wpostServer: http://css.washingtonpost.com/wpost

The Post Most: Local

Posted at 10:59 AM ET, 01/29/2013

Wild Wednesday: Cold front brings potential for strong storms and heavy rain


The 12z (7 a.m.) North American Model radar simulation for 10 p.m. January 30. (Twisterdata.com)
A strong storm system, expected to produce a severe weather outbreak across parts of the Mississippi Valley and broader southern United States today, will increasingly impact our weather through late tomorrow.

A line of showers should push into the area during the afternoon or evening into the overnight tomorrow, advancing east by or before midnight. Recent models have slowed it a bit, favoring the late evening and early overnight. Within the larger batch of showers, some more intense activity is a good bet.

Severe weather is not normal here in January. While this event is somewhat unusual in its strength, it won’t be like a spring or summer event. However, it could still pack a punch via brief heavy rains and even isolated strong to perhaps damaging winds.

What’s causing this?

A very powerful upper-level disturbance is set to create an equally potent surface low over the next 24 hours. That surface low rockets into southeast Canada during the day tomorrow. At the same time, a strong cold front plows eastward, with temperatures surging through the 60s to near 70 out ahead of it locally. If this was April or later, we’d probably be staring down a fairly significant severe weather episode locally.


This graph represents an average of thunderstorm days per month based on the current climatological period. Not all thunderstorm days included rain, as some storms passed close — instead of over — the location. Meteorological winter is the low point, with January thunder expected about every three to four years on average. (Ian Livingston)
While there is significant veering of winds with height that might be supportive of tornadoes in the warm season, the primary risk with this activity will be a strongly forced line of convection near the front, probably within a broader area of light to moderate rain.


Bulk shear in the surface to 850mb region at 10 p.m. tomorrow night. This shows the change in wind speeds over the lowest 5,000 feet of air. Plenty of wind just off the surface. (Twisterdata.com)
Very low instability, which favors the convection focusing near the front and not being too upwardly vigorous, may also mean the activity does not contain any lightning and thunder. Still, the intense forcing along the front, mixed with high winds not too far off the surface, presents us our “storm” risk.

Those lower-level winds are forecast to be quite speedy, with winds about 3,000-5,000 feet off the surface running about 50 to 60 kts+ or ~58-69 mph+. These winds would be at least moderately reduced in transport to the surface during heavy rain, but could retain at least 70-80% of their strength which could promote severe wind gusts.

There is some indication that there could be an inversion — or warmer layer of air off the surface. If this is the case, it will be harder to get winds to mix down from elevated storms atop this buffer of air. If there is little or no inversion, winds will tend to reach the surface easier during the strongest rains along the front.

Recent model runs have cleared out the inversion more than previous runs. But even so, the potential is still rather borderline for severe winds, and elevated areas to the west may be favored, especially if the main line holds off till late evening. Generally, maximum wind gusts in the 35-45 mph range seem most likely, with higher gusts a lesser risk.


Reports from 15 analog systems are shown from two runs (left and right) of the GFS for the setup on January 28, 2013. Several severe weather reports were reported from these similar setups in the region, but in lower numbers than further south. Other runs showed either a few more reports in the area or none. Remember, these composities aggregate reports from 15 systems! Blue are wind reports, red tornado and green hail. (Cooperative Institute for Precipitation Systems)
In addition to raw model outputs, model analogs over recent days have shown the potential for severe wind reports with this system. However, those analogs also favor the most consistent threat staying to our south, where slightly better storm ingredients like CAPE are likely to be found.

While specifics about the threat (inversion, no inversion/max temps/etc.) will probably take until tomorrow to work out, one thing is quite certain: Atypical moisture levels will be transported into the area ahead of the frontal passage.


Precipitable water values of over 1”-1.5” in three hours are shown on the 0z NAM 1/29/13. (weatherbell.com adapted by CWG)
Models have been quite insistent on pushing dew points to near or past 60 degrees in the broader region. Even hedging low based on winter climatology, dew points near or past 50 degrees would be pretty substantial for January.

Rainfall amounts between 0.50”-1” may occur, with some places possibly even picking up more. Much of that in 6 hours or less.

There is some possibility the line will begin to weaken heading into the area, especially if it comes in late evening or later, which might temper rainfall a bit.

The short story is that the broader swath of rain will have the potential for a quick soaking in addition to any embedded squall line. It has been rather dry, so flooding won’t be widespread. But with intense short-term rain rates, we can possibly expect ponding of water in spots, if not some localized flash flooding.

By  |  10:59 AM ET, 01/29/2013

Categories:  Latest, Thunderstorms

 
Read what others are saying
     

    © 2011 The Washington Post Company