Tuesday, December 6, 2011

Rainmaker incoming! (ending as snow??)

Despite the record setting +AO, a storm approaches and bring a TON of rain-- upwards of two inches. Here is a smoothed map by the NWS RNK. Free plug for them, they are now on Facebook. If you keep up with your news feed they do often link various maps of interest.

The fun and tricky part of this even is that is COULD end as snow, especially in the Mountain Empire, NRV, Highlands where an inch or two is possible and maybe even into Roanoke (Maybe a coating) Lynchburg may end as snow, but the last 2k feet are a tad warmer and I just can see a few gloppy flakes falling. I'm never a fan of these events as often the models are a little too slow pulling out precipitation and as that doesn't always equate to the cold air being faster with the snow. So, I like 1-2 in the higher elevations west of 81-- maybe 3-5 above 2500 feet. If your an 11 PM news watcher-- Wednesday night may have some flakes flying as far east as Lynchburg. The Storm is all but done by 2 AM- even in Lynchburg.

I've attached 2 MAPS-- one is from the 6z NAM-- it's not that aggressive on the western flank with snow and the second is the outlook from the HPC about where they think 4 inches or more is possible. It's pretty bullish, more bullish compared to my current thoughts. 

HPC MAP- Best threat along 81--
 Blue is low risk and green is medium risk of 4 inches.

NAM 6z- 1-3 inches along 81.

For Giggle-- I will show the most aggressive model with snowfall- I think it's too aggressive and 
not factoring in the low level warmth well enough. 
I think this is over done as it has an inch WELL into NC, south of Danville and the 3 inch line knocking at Lynchburg's door.

Sunday, December 4, 2011

Record setting POSITIVE Arctic Oscillation could be harbinger of coming winter--

I know my readers don't always get the technical weather talk and I strive to break it down where it's easy to understand. Back in the past 2 winters-- the Arctic Oscillation has been VERY negative in our very cold times, RECORD negative in 09-10 when we had those major snowfalls. This scale tells us in simple terms whether the cold air will be bottle up towards the north pole or be shoved away from the pole. Currently we have a NEAR RECORD POSITIVE Arctic Oscillation. (It hit OVER a +5, 2 highest reading EVER)

This doesn't mean we won't get any cold - but future events can be hedged by looking at past events. We can get cold here-- one of the years the DAY we set the record "Christmas Eve 99"-- there was a surprise snowfall in the Lynchburg area of close to 3 inches. (One of my favorite surprise events ever- I was tracking the clipper and it was expected to be dry east of the Mts-- the snow started just as a dropped off some gifts for a VERY special family-- was a special day) We can get cold air-- but long lasting cold air is harder to get. Ice events are more likely as there is nothing to FORCE the storm to our South and East.

Years with the record AO+  that took place in December,
Winter 51-52,  71-72, 75-76, 79-80,  83-84, 91-92. 92-93, 99-00, 06-07

Big snow winter-- 79-80

Average- 99-00, 71-72

The rest were BELOW-- some worse than others with 75-76,  83-84, 06-07 being among the 10 LEAST snow winters ever.

The two that are second yea La Nina's-- like this is would be 75-76 (Yikes) and 99-00.

99-00 had a really aggressive winter pattern with 4-5 events in a two week stretch and that was IT. The last even LOOKED to be a big snow event but as it trended closer became a snow to sleet to ice to sleet with 2-3 inches near LYH of ice/slush and more like 5-7 out in Roanoke. The BIG event was a surprise event on Jan 25-- that shocked us all. The synoptic pattern was unique in that there was a negative tilt troff sitting just of the SC/GA coast and a shortwave dug in like crazy and blew up. There was a ton of dry air aloft and so it was literally 10 minutes from 8 inches of snow in LYH to NONE in Bedford. My point-- that was a rare event and atypical for a La Nina winter.

With that, I'd strongly hedge that we pull back our winter expectations. Mts will still get plenty of upslope snows, I'd be shocked if anyone east of the mountains (Roanoke included) breaks 10 inches on the year. I'd increase the risk of an more significant ice storm with low level cold lurking and a bad storm track.

In the near term-- we do have a few shots of cold air incoming, but they won't last long.  I don't see much to think we get into a pattern that looks anything like winter in our region. A cold front passes Wednesday and SOME data has hinted a shot of snow as it passes, especially to our north. Those events are RARE and I don't expect it to pan out.