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The calm after the storm or the storm after the calm?

Surface weather map (temperature) for late afternoon, May 7, 2014 across the Southern Plains.
Surface weather map (temperature) for late afternoon, May 7, 2014 across the Southern Plains.
Surface weather map (temperature) for late afternoon, May 7, 2014

Last week saw some pretty incredible storminess. First, an outbreak of tornadoes and severe wind and hail storms moved across parts of the Midwest and the Deep South. About three dozen people lost their lives as a result, mostly across Arkansas and Mississippi.

That was followed by record-breaking rainfall that flooded parts of southern Alabama and northwest Florida. There was apparently only one fatality associated with the flooding. However, flooding damage was extensive.

According to the National Weather Service (NWS) office in Mobile/Pensacola, the official NWS reporting sites at Mobile Regional Airport (MOB), AL and Pensacola Regional Airport (PNS), FL received some record rainfall amounts on Apr. 29, 2014. PNS recorded an estimated 15.55". Data were lost due to a power outage between 10:00 and 11:00 p.m. C.D.T., so amounts were estimated using Dual Polarization Doppler radar precipitation estimates. This was PNS’ greatest calendar day total on record (record keeping began in 1879). Of special interest in the PNS data was the 5.68" that fell in 1 hour between 9:00 p.m. and 10:00 p.m. C.D.T. A quick look at the NOAA Hydrometeorological Design Studies Center’s Precipitation Frequency maps shows this was a 1 in 200 year to 1 in 500 year one-hourly amount. The 24-hour amount lies between a 1 in 50 and 1 in 100 year event.

MOB saw 11.24" during the calendar day. This was their third greatest calendar day total on record (record keeping began in 1871). The MOB 24-hour total was about a 1 in 25 year event.

Of significance, the two-day estimated total for PNS of 20.47" lies between a 1 in 100 to 1 in 200 year event.

Note that these statistical measures do not mean that such events WILL occur once within the specified period. They are just another way to express the probability of the event occurring.

PNS saw its wettest April and wettest month on record (29.53 inches of rain, 25.21 inches above average); the previous wettest April and month was April 2005 (24.46 inches). MOB recorded it’s eighth wettest month 18.09 inches, 13.30 inches above average); the previous wettest April was in 1955 (17.69 inches).

The storm system also brought heavy rainfall to the Mid-Atlantic. Amounts of six inches or more fell in the Baltimore – Washington metropolitan corridor in 24 hours. This led to a retaining wall collapse in Baltimore, MD. The video showing the slow motion sinking of a row of parked cars adjacent to the wall, followed by a speedy collapse, was pretty incredible.

The system’s last gasp involved some record-breaking rainfall and localized flooding in the Tampa-St. Petersburg area late last week. Parts of Hillsborough County (just to the northeast of downtown Tampa) received seven to more than nine inches of rainfall.

That system is now history and most of the U.S. enjoyed calm weather conditions early this week. By Mon., May 5, 2014, some well-above average temperatures lingered in the southwest U.S., warm dry southwesterly winds started to raise the fire danger across the southern Plains states and some localized flooding still continued along the East and Gulf Coasts. Overall, however, the start of the workweek could be considered the calm after the storm…and the calm before the next storm.

The weather pattern is already setting up for more stormy weather.

Initially, a broad southwesterly wind flow set up across the southern Plains the past few days. Over western parts of Texas and Oklahoma, westward into New Mexico, hot and very dry air will continue to dominate the weather scene. Look for highs in the upper 80’s to lower 90’s across this region with late afternoon relative humidity readings well down into single digits (Fig. 1, Fig. 2 and Fig. 3). Hence, there is an elevated wildfire risk with widespread postings of red flag warnings.

On Monday, a controlled burn in central Oklahoma got out of control. The ensuing blaze killed one person and destroyed numerous structures.

A vigorous upper level trough will move eastward from the Rockies by later today and Thursday, allowing a surface low-pressure system to eject into the north-central Plains (Fig. 4). As a warm, moist Gulf of Mexico air mass heads northward in advance of the low, severe storms are possible from Minnesota and Iowa southward to northeast Oklahoma starting on today and continuing through Friday. The severe storms will initially ignite along the “dry line,” the boundary between the very dry air to the west and the more humid Gulf of Mexico air to the east. In fact, by later Thursday, the dew point gradient across Texas will span dew points from the mid-teens to slightly above 70 degrees (Fig. 2). Relative humidity readings in west Texas will be in the single digits again.

The trailing cold front is expected to become stationary and align itself mostly east-west (parallel to the upper level winds) across the Middle Mississippi River Valley by this coming Friday (Fig. 5). This will help focus a potential period of heavy rainfall there. Heavy thunderstorms could also develop within the warm and humid air mass. Computer models are indicating a maximum five-day rainfall approaching 5 inches across parts of southern Louisiana (Fig. 6).

Dry weather will continue to dominate the weather scene from California across much of Texas into the southeast. Clouds, showers and seasonally cool weather will again rule across the northern tier.

Based on long-range computer models, the next two weeks won’t be the stormiest for a spring season. However, the weather will be unsettled in many places, along with some bouts of severe weather and/or heavy rainfall, in others. The Deep South will escape most, if not all, of the severe weather activity, a welcome relief to many, as residents clean up from last week’s storms.

© 2014 H. Michael Mogil

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