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February 20th-21st Winter Storm OBS/Discussion


LithiaWx

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Discuss this storm here :

 
 

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE PEACHTREE CITY GA
240 PM EST TUE FEB 17 2015



EXTENDED FORECAST CONTINUES TO BE DOMINATED BY LATE WEEK WINTER
PRECIP POTENTIAL. GFS AND ECMWF ARE IN BETTER AGREEMENT WITH
TIMING OF PRECIPITATION ONSET EARLY FRIDAY AND THE PROLONGED
PERIOD OF MOISTURE OVER THE AREA THROUGH THE WEEKEND. OVERALL THIS
CONTINUES TO BE A TRICKY...LOW CONFIDENCE FORECAST. FOR
NOW...MODELS CONTINUE TO SHOW SNOW AT THE ONSET ACROSS NORTH
GEORGIA...TRANSITIONING TO A FREEZING RAIN/SNOW MIX FRIDAY
AFTERNOON AND OVERNIGHT INTO SATURDAY...AND THEN EVENTUALLY TO ALL
RAIN SATURDAY NIGHT. BUFR SOUNDINGS ALSO HIGHLIGHT THIS TRANSITION
OF P-TYPES AS FAR SOUTH AS ATLANTA. THIS TRANSITION IS HIGHLY
DEPENDENT ON THE EXTENT OF THE WEDGE SATURDAY AND HOW QUICKLY IT
ERODES...ALLOWING FOR WARMING TEMPERATURES OVER THE AREA. FOR
NOW...CONFIDENCE IS HIGH ENOUGH TO INCLUDE SOME SNOWFALL AMOUNTS
IN THE GRIDS AT THIS TIME. HAVE GENERALLY GONE WITH 0.5-1 INCH
ACROSS THE ATL METRO AREA...AND 1-3 INCHES ACROSS NORTH GEORGIA
AND 2-4 IN THE HIGHER ELEVATIONS. EXPECT THIS FORECAST TO CONTINUE
TO EVOLVE...PARTICULARLY WITH RESPECT TO TEMPERATURES AND
SNOWFALL/ICE AMOUNTS.

 

MAPS COURTESY OF WEATHERBELL - NOTE SOME OF THESE MAPS MAY CONTIAN WEDNESDAY SNOW.

 

CMC 12Z 2/17/15

 

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EPS Control from 12Z 2/17/15

 

post-2727-0-09836100-1424205141_thumb.pn

 

EPS Mean from 12Z 2/17/15

 

post-2727-0-80209900-1424205176_thumb.pn

 

 

GFS 12Z

 

post-2727-0-93187400-1424205689_thumb.pn

 

post-2727-0-43007000-1424205697_thumb.pn

 

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18z GFS soundings show decent precip with a frozen column for 9 hours Saturday morning in parts of WNC.  That would support snow before a switchover to freezing rain/rain.  Column also looks frozen or close to for GSO and those within around 50 miles.  High has trended a little weaker but seems to be in a better position to provide cold to the CAD area on the 18z GFS.

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It didn't seem like that dramatic of a change from the 12z, but I guess it could be the beginning of a trend, I'm no expert. But yes with the way the models have been, it could be all rain next run. That risk is always there.

I'll glady take my 2-3" of snow and call it a day.

If I were offered 0.5-1.0" of snow from this system on the northside of ATL, I'd gladly take it and call it a storm. I think that getting 2-3" is quite a longshot as of now as this is a clipper system with a WNW 500 mb flow and there is no closed upper low or even a clearcut sfc low. It is so weird! I have found zero instances since the late 1800's of a 3.5"+ storm in the city, itself, from a clipper. So, I think that the chance for over 3" is probably no more than 1%. I'd say maybe 5% chance at best for 2-3". So, heck yeah I'd take 2-3". That's a no brainer! I mean there could easily be no snow at all just as the 18Z GFS shows!

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Nam drops 3-4 inches from north ga into wnc. Then at 84 hours 850 temps warm above freezing with heavy moisture coming in with temps in the low 20's. 84 hour nam so....

 

The NAM tends to be overamped at 84 hours so its a bit surprising to see that its not overly more north and west of the GFS/Euro at 84 hours.

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The overall storm is going to be a cutter. But we will be getting wintry precip from overrunning out ahead of the main storm.

Exactly.  I don't think anyone is a fan of how the storm ends up, but there's no denying the opportunity for the western half of our subforum to see the early moisture being thrown into the very cold airmass.

 

check out the 850 winds at the end of the run, coming from the sw out of the gulf.

Yes...the mechanism for generating the precip looks legit....850mb warm air advection shown below (overrunning).  

 

It's just a matter of how far to the south and east the precip will extend.  The 12z Euro didn't extend it as far to the south and east, but other modeling did.

 

850mb_TA.gif
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00z NAM had it in the low 20's Saturday morning at GSO with almost 1" of snow by 7am.  If steady precip continues, it will take some time to warm the surface 10 degrees.  So, lot's of options to be worked out.  While I hope not, one of the options would be for the precip to not move in until later on Saturday once the temps warm up above freezing.

TW

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I'll preface this with that fact that I am very much an amateur, but it seems over the past few years that the modeling has consistently underestimated CAD in this area (latest storm is an example).  This weekend's event will be coming on the back of the coldest air this area has seen in 30 years and a decent snow/sleet pack over central NC.  I'd bet that CAD will be more resilient than the models suggest....

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