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Very Interesting Weather Ahead

23rd 1am
23rd 1am
There has been much discussion on the weather boards as to why the continental US has had such an early cold outbreak, despite the fact that the teleconnection indices for the most part have been unfavorable.
 
Anyone that has been watching the standard teleconnections such as AONAO, and PNA, would tell you that these indices are in an unfavorable state to bring cold weather to the US, and they would normally be correct. So what is the reason for the recent cold, and the deep cold that is forecast to reach our area around the 22nd?

Right now we're in a positive AO pattern which means there is an expansive area of low pressure (polar vortex) parked over the north pole southward to Greenland. Out west, we have a large area of positive height anomalies over the Bering Sea into Siberia. A +AO pattern would normally mean a nice toasty winter for most of us in the US as it keeps the cold air bottled up in Canada. But we have a little different setup this year compared to the last several years. The thought is that the large high pressure that's creating the -WPO is countering the the +AO and allowing cold air to penetrate the central part of the US. 

According to a meteorologist:

"If you take the 20 most negative WPO Decembers, you get 13 with a -AO, 5 with a +AO and 2 with a neutral AO. So, overall from this small sample, the combination of a very anomalous -WPO with an overall +AO is the rarer combination. The 20 most positive WPO Decembers, on the other hand, have about 10 with +AO and 8 with -AO. That group, too, had about 2 with a neutral AO.

The -WPO/neutral AO years were 1956 and 1980. Both directed the pipeline of cold into Canada but the less negative AO either forced it well north of the CONUS (1956) or only into the eastern third of CONUS (1980). The PDO state likely played a role there. 1956 was, more or less, a blowtorch December. 1980 was exceptionally warm for the western 2/3s of the CONUS.

The +AO Decembers with an anomalous -WPO (in the 20 most negative only) looked more like a hybrid El Nino-like pattern. The cold anomalies were more focused in south, even if you take away the very cold 1983 year, and the northern-tier was above normal. This rare combination consists only of 5 years out of the 20 -WPO set: 1954, 1974, 1975, 1999 and 1983."

So as you can see, we're in a very unusual pattern. 

According to WXRisk.com, "the -WPO is running 2 standard deviations above the normal and establishes cross-flow polar flow despite the +AO".

In really simple terms, it opens up a hole for the polar air to come tumbling into. Again, all of this happening despite the AO and NAO in unfavorable states. 

Here's what could happen according to WXRisk.com. "This combination of -WPO/-EPO have allowed the PNA values to rise toward positive values. With the +AO weakening, the +PNA goes "boom". Cross-polar flow would be firmly established. And IF... the -WPO moves north toward the Arctic Circle and the +AO goes negative... according to WXRisk.com, "the crap would really hit the fan".
:-) Lovely way to put it, but you get the jest.

All of these future "what if" scenarios will be determined over the next several weeks, but we may be moving into a pattern that will allow some VERY cold air to drop into the heart of the country. After this next week, the rest of the month appears to be well below normal for this time of year. I've seen model temperatures as cold as -40F invading the country with teens into Georgia, so LOTS of cold air will be available. Just to give you an idea of what may be on the horizon, here are the 2 meter temperatures from the GFS at 1pm on 12/28/13. AGAIN... we're 9-10 days away so much can change

The thing that is so cool about this weather (no pun initially intended...!) is that it goes against what most people, including some meteorologists, might expect based on the setup we currently have now. And what is even more promising, is that the models are in such good agreement at this distance, which is pretty unusual. Tracking winter weather is like putting together a gigantic double-sided puzzle for us here in the southeast, and this puzzle appears to be coming together rather nicely despite the odds! 
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North Georgia Weather December 14, 2013 at 06:23 AM
This mornings GFS isn't looking as favorable as before. We've got to loose the southeast ridge before we're going to get any real cold shot here.
North Georgia Weather December 14, 2013 at 09:44 AM
Made an update with images on Facebook: https://www.facebook.com/NorthGeorgiaWeather
North Georgia Weather December 15, 2013 at 07:53 AM
Wow, what a difference a few days makes. All the massive cold set to invade the south... retreated. South 1 North 0
Elizabeth December 17, 2013 at 09:08 AM
NGW, what does the weather look like for this Saturday afternoon in Marietta, GA?
North Georgia Weather December 17, 2013 at 09:46 AM
Elizabeth, the temperatures will be relatively mild this weekend, but there will be a pretty good chance for showers and possibly severe weather. Right now the models are all over the place in terms of timing, so trying to pinpoint an exact time for rain would only be a guess. http://www.daculaweather.com/4_afd_ffc.php
Elizabeth December 17, 2013 at 11:32 AM
Thank you, NGW.
North Georgia Weather December 17, 2013 at 02:03 PM
.LONG TERM /THURSDAY THROUGH TUESDAY/... MODELS IN GOOD AGREEMENT WITH THE FIRST PART OF THE LONG TERM...HOWEVER AFTER THURSDAY THERE ARE MAJOR DIFFERENCES IN STRENGTH AND TIMING OF THE SHORT WAVES AND PRECIP. GFS APPEARS TO BE FLIP FLOPPING ON PRECIP AMOUNTS WHILE THE ECMWF IS REMAINING MORE CONSISTENT AND MORE CONSERVATIVE. WITH THESE DIFFERENCES IN TIMING AND STRENGTH...AND THE CHANGES TAKING PLACE FROM RUN TO RUN...WILL NOT MAKE ANY ADJUSTMENTS TO THE LONG TERM.
North Georgia Weather December 17, 2013 at 08:12 PM
SHOWERS RETURN TO THE CWA FOR FRIDAY AS THE FLOW TURNS MORE SOUTHWESTERLY. THIS BEGINS THE NEXT WET PERIOD THAT LASTS INTO THE WEEKEND. STRONG SOUTHERLY FLOW AHEAD OF A COLD FRONT WILL KEEP SHOWERS AND A CHANCE OF THUNDERSTORMS OVER THE FORECAST AREA. AIR MASS BECOMES MOST UNSTABLE SATURDAY THROUGH SUNDAY...BUT POTENTIAL FOR SEVERE STILL UNCERTAIN. GFS AND ECMWF STILL HAVE TIMING DIFFERENCES WITH THE SURFACE FRONT...THE GFS PUSHES THE FRONT THROUGH BY MID DAY SUNDAY WHILE THE ECMWF LAGS UNTIL OVERNIGHT SUNDAY NIGHT. SO HAVE KEPT A CHANCE OF SHOWERS IN FOR MONDAY BUT DIMINISHED THEM FROM THE NORTHWEST.
North Georgia Weather December 18, 2013 at 06:10 AM
.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... DEEP MOISTURE INCREASING TO OUR WEST AT THE START OF THE PERIOD. STRONG SOUTHERLY FLOW GRADUALLY BRINGING BETTER MOISTURE INTO THE CWA ON FRIDAY. HAVE INTRODUCED CHANCE OF SHOWERS OVER THE NORTH MAINLY FRIDAY AFTERNOON. SHOWERS AND A FEW THUNDERSTORMS SHOULD DEVELOP OVER THE CWA AHEAD OF THE FRONTAL BOUNDARY. BEST POPS SHOULD REMAIN GENERALLY ACROSS NORTH GA. THIS FRONT REMAINS IN THE OHIO VALLEY AREA UNTIL SUNDAY...AND SHOULD CROSS THE CWA LATE SUNDAY INTO EARLY MONDAY AS A COMPROMISE BETWEEN THE GFS AND ECMWF TIMING. A FEW STRONG TO SEVERE THUNDERSTORMS ARE POSSIBLE NEAR THE FRONTAL BOUNDARY...LIFTED INDEX VALUES GO NEGATIVE SATURDAY NIGHT AND SUNDAY...AND 925 MB WINDS ARE FORECAST TO REACH 40 TO 50KT. BOTH MODELS AGREE ON DRYING FROM THE NORTHWEST CORNER OF GA ON MONDAY.
North Georgia Weather December 18, 2013 at 08:05 AM
Just upstream from us... Birmingham morning AFD------- THE SEVERE WEATHER RISK ON SATURDAY AFTERNOON AND EVENING WILL LIKELY EXTEND ACROSS A RATHER LARGE AREA TO OUR WEST WITH AN EXPANSIVE AND HIGH-QUALITY WARM SECTOR IN PLACE. WHILE A QLCS IS LIKELY TO DEVELOP CLOSER TO THE STRONGER FORCING ADJACENT TO A HYBRID DRYLINE/FRONT NEAR THE ARKLATEX AT 00Z SUNDAY...IT APPEARS THAT PREFRONTAL SEVERE THUNDERSTORMS COULD DEVELOP ACROSS LOUISIANA AND MISSISSIPPI AROUND 21Z SATURDAY AND MOVE NORTHEAST. MOST OF THIS ACTIVITY SHOULD STAY TO THE WEST OF OUR CWA THROUGH 06Z SUNDAY BUT A FEW STORMS COULD CLIP THE NORTHWEST COUNTIES. THE MAIN RISK WILL COME AFTER 06Z AS THE QLCS MERGES WITH THE PREFRONTAL ACTIVITY. DEWPOINTS RISING INTO THE UPPER 60S WILL ALLOW FOR CAPE VALUES OF 500-700 J/KG OVERNIGHT...AND VERY STRONG 0-1 KM SHEAR BE SUFFICIENT FOR DAMAGING WINDS AND TORNADOES AS THE QLCS TRAVERSES THE CWA SUNDAY MORNING. WE SHOULD BE ABLE NARROW DOWN THE DETAILS OF THE THREAT LEVEL AND TIMING OVER THE NEXT COUPLE OF DAYS...BUT I WOULD DEFINITELY TAKE THIS EVENT SERIOUSLY.
North Georgia Weather December 20, 2013 at 06:04 AM
Elizabeth, you might catch a little bit of a break with the rain on Saturday morning. It appears the heavier stuff may hold off until later.
North Georgia Weather December 20, 2013 at 07:40 AM
The 00Z Global deterministic and ensemble medium/long range guidance continue to advertIse a return to a -EPO / +PNA regime as we near the end of 2013. It will be interesting to see if the suggestions of a negative EPO of -3 prove correct with the return of a slightly positive PNA and increasing sub tropical jet activity as some of the guidance is suggesting.
Elizabeth December 23, 2013 at 09:06 AM
NGW, I am please to report that on Saturday, the weather in Powder Springs was passable. We took our umbrellas (which we needed a couple of times) and our coats (which we did not need at all). We were able to sit and enjoy seeing our grandson play in the Cobb All-Star Football Game. We have watched him play rec, then high school, sports for about 12 years. As he is a senior, the game Saturday was the last rec/public school game in which he will ever play - a bittersweet moment, as his team won.

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