Category Archives: Winter Weather

Education and information about Winter weather.

Multiple Severe Threats April 3rd-7th

It seems like we have been dealing with the threat of severe weather for the last three weeks . Must be spring time in the southeast . The coming week offers no break and in fact, the two upcoming systems could be the strongest we have seen over the last month. Our first system gets going tomorrow over Texas and Louisiana where a big severe weather outbreak is expected .

The threat slides east on Monday bringing a severe weather threat to Mississippi, Alabama , Georgia and into the Carolinas overnight Monday into Tuesday. The main threat at this point will be from strong straight line winds and large hail. The tornado threat looks rather low right now say from the i20 corridor northward . However there will be a greater tornado threat over southern Mississippi, Alabama, Georgia and the Florida panhandle. Here is the day two outlook

A large squall line will move through the above mentioned areas bringing the severe threats along with the potential for some very heavy rain. Some locations could recieve between 1-2 inches. Here is the 3k NAM

If that’s not enough, we are looking at yet another possible threat Wednesday into Thursday and that threat looks to be the most significant of the season so far . The obvious strong wind and hail threats apply to this threat but , there appears to be a much greater tornado threat with this system . Cape values are very high and the low-level winds appear to be more favorable for supporting tornadoes . Still plenty of time to watch this period, just know this has the potential to be a substantial outbreak. Here are the latest cape values from the 12z euro

So a busy week is shaping up weather wise. Please make sure you have a reliable source for receiving warning information. I heard someone at the store this past week talking about how they don’t go to their safe place until they hear the outdoor siren. That is a HORRIBLE idea . The outdoor siren system was created back during the Cold War to alert people of any incoming attacks it was NOT CREATED FOR TORNADO WARNINGS. Often times the sirens sound minutes after warnings are issued . Minutes can mean the difference between life and death. Please make sure to have a weather radio or another reliable source this week. We will have numerous updates in the coming days

 

Cold Is The Word

The big winter storm that brought feet of snow to the NE is lifting northwards off the coast of Maine this evening . Heavy snow will continue for the next few hours  before ending later this evening .

For us in the southeast we turn cold and stay cold for a few days . Highs on Wednesday and Thursday will be in the 40s for most of the region with overnight lows well down into the 20s. In fact , don’t be surprised if you hear that some locations set new record lows for the month of March . There might be a few snow flurries scattered throughout the region tonight and into tomorrow morning but no issues are expected from those brief flurries .

A slow warming trend begins late in the week and into the weekend . Most locations will be in the upper 50s and low 60s by Sunday .

 

SE US daily winter temperature anomaly analysis by MJO phase: inside the left side of circle coldest

I got curious about the actual SE US tendencies associated with each MJO phase after seeing the charts showing temperature anomaly patterns for each phase. I knew the left phases tended to be cold and the right phases mild. But I wanted to know how cold and how mild. When I had some time and since I love analyzing wx stats, I decided that it would be interesting to see how Atlanta, which I figured could be used as a proxy for the SE US, was for each phase. So, I started off just looking outside the circle by phase. I did just January to be more efficient. I did this for 1975-2014 since this was done before January of 2015 was completed. 1975 was the first year that MJO day by data is available from this source:

http://www.bom.gov.au/climate/mjo/

As I was going along, I noticed a strong warm bias with these ~800 days. I noticed that phases 4 & 5 were quite warm as one would expect but what confused me was that the remaining phases were near normal with no phases averaging cold! That’s when I figured out that INSIDE the circle is where many of the cold days must be hiding.

So, that’s when I decided to analyze the ~430 inside the circle days. I discovered that inside is, indeed, where many of the coldest days were hiding! I then learned that inside the circle is also within the 8 phases (think 8 pie slices). So, when you see a map showing a cold SE phase 8, for example, the cold average is largely being carried by inside the circle phase 8 though just outside the circle in phase 8 was also cold.

By the way, every phase came in 3-6 F colder when inside vs outside and even phases 4-5 inside averaged near normal. The remaining 6 phases inside averaged colder than normal with inside 7 & 8 being the coldest.

The following diagram (ignore the 1978 reference) shows the Atlanta normalized average daily temperature anomaly for each phase, both inside and outside the circle (inside the circle phase 8 has an asterisk because it is the coldest at -6 F)(you may have to enlarge it to see it well):

Based on these findings, I drew an optimal path for the best chance for lengthy cold in at least January MJOwise (see diagram below). Optimally, start around the cool phase 6 within the circle and slowly circle around counterclockwise within the circle through the cold phases 7-8 and then through the still chilly phases 1-3 within the circle. This could easily take 2-3 weeks or so (the longer the better):

 

The following are many of the closest examples I could find to the optimal path….note that all of them resulted in anywhere from a 10 to 19 day period of solid cold domination:

1. 1/10-28/1977:

http://www.bom.gov.au/climate/mjo/graphics/archive/MJO_phase/197701.phase.90days.gif

2. 1/6-15/1988:

http://www.bom.gov.au/climate/mjo/graphics/archive/MJO_phase/198801.phase.90days.gif

3. 1/8-19/1994:

http://www.bom.gov.au/climate/mjo/graphics/archive/MJO_phase/199401.phase.90days.gif

4. 1/4-14/1996:

http://www.bom.gov.au/climate/mjo/graphics/archive/MJO_phase/199601.phase.90days.gif

5. 1/20-31/2000:

http://www.bom.gov.au/climate/mjo/graphics/archive/MJO_phase/200001.phase.90days.gif

6. 1/13-27/2003:

http://www.bom.gov.au/climate/mjo/graphics/archive/MJO_phase/200301.phase.90days.gif

7. 1/1-10/2014:

http://www.bom.gov.au/climate/mjo/graphics/archive/MJO_phase/201401.phase.90days.gif

Neutral negative ENSO by far gives ATL best shot at major ZR

I thought this would be of extra interest for especially the SE US major CAD regions due to the current ENSO being neutral negative (NN). I counted a total of 36 Atlanta major ZR and/or IP on record. It is very impressive that half of these (18) were during NN ENSO, alone, when one considers that NN have occurred only about 1 every 5 years. I counted 28 NN’s. Of these 28, 16 (57%) had at least one major ZR and/or IP. In contrast regarding the other 109 winters, only 15 (14%) had at least one major ZR and/or IP. So, the % of NN winters with a major ZR and/or IP at Atlanta is a whopping four times as high as those for non-NN winters!

 

Dates of 18 ATL major ZR and/or IP during NN ENSO:

12/11-3

12/14-5

12/24-5

12/25

1/13-4

1/17-8

1/18-9

1/21

1/23

1/30

1/30-1

2/6-7

2/9-11

2/12

2/14-5

2/17-8

3/2

3/9

Miller A vs Miller B

Miller A:

Miller A storms are typically a single low pressure that develops in the gulf and heads toward the NE. The 93 Super storm is an example of a Miller A low pressure. These storms tend to have a narrow ptype transition zone and generally are rain or snow versus having freezing rain/sleet.

Miller B:

Miller B low pressures are the more complex of the 2. They feature a primary low that moves into the Tn Valley and begins to occlude over this area. This can also happen as a clipper systems moves into this area. As the primary low occludes the secondary low starts to take shape along the coastal front on the Carolina coast. This surface low becomes the primary low as it heads northeast. Miller B systems often lead toward messy precipitation transition zones as warm noses occur aloft.

Overrunning

Overrunning is characterized by a warmer air mass being lifted over a colder/more dense air mass. The most common way to see this produce winter weather in the southeast is an arctic front is pushed toward the gulf coast while the subtropical branch of the jet stream becomes active. This sub tropical jet leads to warm air moving over the cold dome and helps generate precipitation. Embedded within the southwest flow aloft we will see a few weak impulses and surface lows that will help generate areas of enhanced precipitation. 

 

Wedging/ Cold Air Damming

Wedging and cold air damming are a common occurrence across parts of the Carolinas and Georgia during the cool seasons. This wedge of cold air can lead to cloudy cool weather in the fall and spring and freezing/frozen precipitation in the winter. So what causes this?

The first step is for a strong surface high pressure to move across the northern US or southern Canada and into the Northeast. This surface high then builds southward on the east side of the Appalachian mountains. You can often see the progression of the wedge on current surface maps by the isobars, dew points and current temps.  The wedge air mass will be characterized by much lower dew point air and cooler temperatures than areas south, southeast, and southwest of the wedge front. The wedge air mass is also generally quite shallow and confined to the lowest few thousand feet of the atmosphere.

As the low level cold builds southward we often see warmer moist air forced over top of the wedge and we see precipitation break out. Since we are only looking at a shallow layer of cold air many times in the wedge areas this precipitation is in the form of freezing rain as you head into the deepest portions of the cold air mass there may be a change to sleet or even snow.

With the colder drier air deposited in the wedge zones and precipitation falling temperatures will fall to the wet bulb temperature and depending on wedge strength can slowly continue to fall as the precipitation falls. What happens as the event continues is one of 3 scenarios:

#1 Classic Damming: Classic cold air damming is where the surface high pressure locks into the northeast United States . This continual dry, cold feed will sustain the cold air damming regime the longest and can push the wedge into Alabama.

#2 Hybrid: In a hybrid damming scenario the high pressure is generally in the classic spot to start the event however it is either A. too weak to provide a strong cold air feed or B. kicks out of the NE during the event cutting off the dry cold feed. These events usually see a change to rain for a large area but can continue to see freezing rain along the foothills.

#3 Insitu: Insitu damming is created when dry cold air has been left behind in the damming regions and its overridden by a warm moist flow. Most times in the insitu damming scenario the cold high pressure over the NE is not present. These events are self limiting as rain falls and freezes on contact latent heat is released into the atmosphere and eventually

 

Once the event is over the wedge air mass itself is quite hard to break as low level moisture remains. Many times computer models will forecast the wedge breaking and temperatures warming while in reality the wedge stays in place and temperatures are significantly cooler.