Active Weather Pattern May Bring Winter Surprises

Very interesting things going on with the weather right now.  We currently have a large 3 contour upper level low (ULL) just hanging out over southern California and the Baja of Mexico. The reason it's just hanging out is because it's been cut-off from the northern branch of the jet stream, and has no upper level steering current to push it along right now. In the winter time, this is EXACTLY what we like to see for winter precipitation. As this large ULL begins to move, it heads east, and the exact speed and track it takes, determines who does and who doesn't get winter precipitation. Go to this page and click on the menu option "Basic UA" and the "500mb Analysis". 

One thing this ULL is going to bring for us is rain... and lots of it. The Weather Prediction Center had this to say in their QPF forecast:

"A VERY DYNAMIC WEATHER PATTERN IS IN THE OFFING AS THE UPPER LOW OVER THE SOUTHWEST US BEGINS TO HEAD EASTWARD AND THEN INTERACTS WITH A SERIES OF SHORTWAVE SYSTEMS DROPPING SOUTHEASTWARD FROM THE NORTHERN PLAINS ACROSS THE GREAT LAKES. THE INTERACTION OF THESE SYSTEMS CURRENTLY IS STILL UNCLEAR AS SEPARATE AREAS OF QPF DEVELOP IN RESPONSE TO THIS COMPLEX INTERACTION.

BY THE END OF THE FORECAST PERIOD... PRECIPITATION ALSO SPREADS NORTH AND EASTWARD ACROSS THE LOWER APPALACHIANS AS THE TROUGH OVER THE LOWER MISSISSIPPI VALLEY BEGINS TO INTERACT WITH A SERIES OF UPPER TROUGHS MOVING TOWARD THE GREAT LAKES... WITH THE EXPANSION OF THE QPF NORTHEASTWARD RELATED TO THE DEVELOPMENT OF A DOWNSTREAM 250 MB UPPER LEVEL JET WITH MAXIMUM WINDS INCREASING TO GREATER THAN 150 KTS EARLY TUESDAY MORNING. THE AREA OF DEVELOPING QPF WOULD OCCUR WITHIN THE ENTRANCE REGION OF THE AMPLIFYING JET STREAK. AS THE QPF EXPANDS NORTHEASTWARD... THE MODELS SEEM TO BE CONVERGING ON A COMPLEX SOLUTION THAT KEEPS ONE DEVELOPING LOW POTENTIALLY RACING NORTHWARD WITHIN THE JET INTERACTIONS WITH AN ENHANCED BAROCLINIC ZONE ALONG THE EAST COAST WHILE THE UPPER LOW OVER THE SOUTHEAST STILL REMAINS AS A SOMEWHAT SEPARATE ENTITY THAT APPEARS NOT TO INTERACT OR PHASE INTO ONE MORE RAPIDLY DEVELOPING SYSTEM... AS IT APPEARS NOW AND AS IT GETS MORE INTO THE MEDIUM RANGE PAST DAY 3. THERE IS STILL PLENTY OF TIME FOR THIS SCENARIO TO PLAY OUT IN ANY NUMBER OF WAYS. FOR NOW... THERE IS AT LEAST AVERAGE TO ABOVE AVERAGE CONFIDENCE THAT QPF COVERAGE AND AMOUNTS WILL INCREASE AS WE GO FROM DAYS 2 TO 3 BUT CONFIDENCE PAST THAT DROPS RATHER QUICKLY. "

And amounts are already high, as evident from this 5 day QPF map. 

The key is how suppressed this low stays as it ejects eastward. The models have been going back and forth with the exact track. With a series of shortwaves diving down through the fast flow, they tend to squash or pull the northern branch of the jet a little further south. That southward tendency would then force the ULL a little further south. Here's a great view of what I'm talking about. Manually step through this progression and watch as the ULL takes a dip as one shortwave rides down ahead of the ULL. As the ULL reaches the Gulf coast, it can then tap into Gulf moisture. Then... based on the timing of the cold air, the ULL can potentially shove warm moist air over a shallow dome of cold air, and you have sleet, freezing rain, and ice. It's a very complicated pattern of stream interaction with multiple shortwaves embedded in the flow. This is really going to be interesting to see how this finally evolves. 

Remember our big snow from January 2011? Very similar setup today. Here's exactly how that happened. The only real difference today versus then, is that we won't have as much cold air to work with, remember, this is only November still. 

Looking at the latest 06Z GFS forecast this morning, the 114-120 hour time frame is showing a chance of precipitation with temps below freezing, and Robert Gamble at WXSouth.com noticed the same thing the other day.
"One thing I ‘m noticing still on GFS , is it continues to have that convergence or deformation band holding well back west to northern Alabama, GA, SC and NC even when the low is off the NC coast.   That precip could be snow, if it’s real."

The normally conservation Atlanta NWS office said this in the morning AFD:
"UPPER COLD LOW MOVING ACROSS THE CWA ON WEDNESDAY. LOWERING THICKNESSES COULD GIVE A MIX OF PRECIPITATION ESPECIALLY TO THE NORTHWEST CORNER... DEPENDING ON TIMING OF RAIN ENDING AND HOW FAST THE COLDER AIR ARRIVES. AT THIS TIME HAVE KEPT ALL RAIN FOR THE AREA BUT WILL CONTINUE TO LOOK AT LATER MODEL RUNS."

Take a look at some of the current temperatures this morning, as well as coldest morning lows, departure from averages, and 24 hour temp change.  Also note that there is already some probability for ice between now and (Tuesday) Wednesday. Watch the slideshow on that page for the ice probability map. Also, watch the latest Atlanta NWS video for the upcoming week.

Also notice that the cold and potential wintry outbreak coincides with what I talked about in my previous post, with both the NAO and AO going negative, as well as the PNA going positive. Here's the Climate Prediction Center (CPC) 6-10 Day prognostic discussion. 

We've in a very active pattern right now and some surprises may be in store, stay tuned! 

UPDATE - Sat, 11/23
The new forecast is just out, and shows a chance for sleet late Monday/Tuesday AM. http://www.daculaweather.com/4_forecast_gwinnett.php
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