December 26th Significant Severe Weather Threat

The Storm Prediction Center is forecasting another dangerous severe weather situation tomorrow, December 26th, after an already destructive day along the Gulf Coast.

We are already seeing the storm system responsible for this destruction start to ease up on the Gulf Coast and start its trek northeast. However, as it does so, another area of potentially dangerous weather will evolve along the Southeast coastline. Cities like Raleigh, Charlotte and Wilmington will be at risk for the worst of these storms.

Discussion: As the storm treks northeast, lower level winds will maintain their speed, possibly strengthening, providing a base to strong storms. Readily-available Gulf moisture and instability will flourish in the Southeast, as the warm front lifts this moisture north. A trailing cold front will kick off the event with individual cells, possibly in linear form thanks to overnight outflow boundaries and existing convection. As the day goes on, lower level winds will strengthen over the area of interest, promoting both a damaging wind and tornado threat.

-Andrew

Shifting Pattern Favors Central, East US For Snow

Early January appears to be favoring the Ohio Valley and Northeast for the best snowfall and cold. I'll explain why.

This is the GFS forecast for Hour 168, or the afternoon of January 1st, at the 200mb level. The 200 millibar level is commonly identified with the level best used to observe the jet stream. If we look at the GFS forecast above, we see that there is a deep trough in the Southwest/eastern Rockies. Such a disturbance causes high pressure to form on the West Coast, creating a positive PNA index. This positive PNA would then act to keep the storm system going to the south rather than north and into the Plains.

The ECMWF forecast for the same timeframe and the same level in the atmosphere shows a very similar set-up, with high pressure on the West Coast and a depression in the jet stream over the eastern Rockies. Again, we see a very amplified jet stream in the Mid-Atlantic. Considering both models also show high pressure prevalent over eastern Canada, it's likely that storm systems exiting the Rockies will be forced into a track that goes through the South US, and eventually up the East Coast.

In the long range, both models show a below normal height anomaly at the 500mb level, indicating low pressure will be present in the Gulf of Alaska. In response, high pressure attempts to form on the West Coast and thus tries to initiate a positive PNA situation. As exhibited by the amplified jet stream in the Mid-Atlantic, we would then see the storm track favor a track that goes along the Ohio Valley and possibly up the Northeast.

ECMWF (left) and GFS (right) long range 500mb height anomalies

Problems arise when both models show a positive North Atlantic Oscillation (NAO) evolving, with a stormy pattern over Greenland. When you get a positive NAO, storms are not favored to go up the coast. However, the jet stream is aligned in a more zonal (west to east) pattern that then goes into the Mid-Atlantic. This would explain why models amplify the jet stream in the somewhat-long range.  If we have a positive PNA in place with this positive NAO, I find it plausible for storms to push into the Southwest US. After doing so, they will eject east into the Southern Plains and get picked up by a powerful jet stream. When that storm gets picked up, the jet stream will be forced to buckle south, much like when you place a brick on a horizontal bendy-ruler. As the system's vorticity orientation tilts to the southeast, it attains a negative tilt, meaning the highest storm's vorticies are tilted to the southeast. As this happens, a strong severe weather situation is likely to set up in the Gulf Coast region, and this severe weather will be a multi-day event, likely starting in the western Gulf Coast before going to the Southeast over a period of a few days. On the wintry side, the storm system will slowly move northeast, spreading snow across the southern Midwest, southern Ohio Valley before getting off the East Coast and bringing a better snowstorm to the Northeast. A strong cold outbreak then follows.

If you didn't get that, here's what I think will happen:

-Andrew

Large Tornado Strikes Mobile, AL

A tornado has struck Mobile, AL. Here is what I know as of now:

At 6:36 PM:
-Murphy High School has been damaged.
-The Mobile Infirmary has sustained major damage.

At 5:45 PM:
-Image added, courtesy of CBS42.

At 5:43 PM:
-Some houses have been reported "down".
-Injuries are being reported.
-Damage has been observed on Williams Street and Senator Street.

At 5:40 PM:
-It was a large wedge tornado.
-It did go through the downtown area.
-Damage is being said to be "all over".
-A debris ball was spotted on radar after the tornado went through the city (a debris ball means debris is showing up on radar after being thrown up in the air).

This is a REAL-TIME post, and will be updated as I get more information.

Andrew

Christmas Day Tornado Outbreak- 3:05 PM CT Update

First and foremost, a Critical Storm Action Day (CSAD) is in effect for the entire South Coast for the rest of the day, and a Storm Action Day is in effect for the Southeast tomorrow.

Highest values favorable for tornadoes are now confined to south Mississippi, south Alabama and southeast Louisiana. The tornado threat should start to move slightly north as time goes on, in correspondence with newly-issued tornado watches.

It is advised that everyone in the following states turn full attention to the developing life-threatening weather situation:

-Louisiana
-Mississippi
-Alabama

Heed all watches and warnings as given by the National Weather Service and Storm Prediction Center.

Andrew

**PDS Tornado Watch For: LA/MS/AL**

Text from Storm Prediction Center


SEL5
 
   URGENT - IMMEDIATE BROADCAST REQUESTED
   TORNADO WATCH NUMBER 695
   NWS STORM PREDICTION CENTER NORMAN OK
   1255 PM CST TUE DEC 25 2012
 
   THE NWS STORM PREDICTION CENTER HAS ISSUED A
   TORNADO WATCH FOR PORTIONS OF
 
          SOUTHWESTERN AND WEST-CENTRAL ALABAMA
          EASTERN AND SOUTHEASTERN LOUISIANA
          CENTRAL AND SOUTHERN MISSISSIPPI
          COASTAL WATERS
 
   EFFECTIVE THIS TUESDAY AFTERNOON AND EVENING FROM 1255 PM UNTIL
   800 PM CST.
 
   ...THIS IS A PARTICULARLY DANGEROUS SITUATION...
 
   DESTRUCTIVE TORNADOES...LARGE HAIL TO 1.5 INCHES IN DIAMETER...
   THUNDERSTORM WIND GUSTS TO 75 MPH...AND DANGEROUS LIGHTNING ARE
   POSSIBLE IN THESE AREAS.
 
   THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 125 STATUTE
   MILES EAST AND WEST OF A LINE FROM 25 MILES SOUTHWEST OF
   BOOTHVILLE LOUISIANA TO 40 MILES NORTHEAST OF JACKSON
   MISSISSIPPI.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE
   ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU5).
 
   REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR
   TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH
   AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR
   THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS
   AND POSSIBLE WARNINGS.
 
   OTHER WATCH INFORMATION...THIS TORNADO WATCH REPLACES TORNADO
   WATCH NUMBER 692. WATCH NUMBER 692 WILL NOT BE IN EFFECT AFTER
   1255 PM CST. CONTINUE...WW 693...WW 694...
 
   DISCUSSION...WARM SECTOR SPREADING INLAND ACROSS WW AREA WILL BECOME
   INCREASINGLY JUXTAPOSED WITH STRENGTHENING DEEP SHEAR AND ENLARGED
   LOW-LEVEL HODOGRAPHS.  MCS MOVING EWD INTO THIS REGIME WILL
   ENCOUNTER MLCAPE 500-1500 J/KG...WITH EFFECTIVE SRH MAXIMIZED NEAR
   WARM FRONT AT 400-600 J/KG.  ANY RELATIVELY SUSTAINED/DISCRETE
   SUPERCELLS WILL OFFER RISK OF LONG-TRACKED/DAMAGING TORNADOES.
   DAMAGING GUSTS AND OCNL TORNADOES ARE ALSO LIKELY FROM QLCS/LEW/BOW
   STRUCTURES.
 
   AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL
   SURFACE AND ALOFT TO 1.5 INCHES. EXTREME TURBULENCE AND SURFACE
   WIND GUSTS TO 65 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO
   500. MEAN STORM MOTION VECTOR 24040.