Love the one you're with--- snow totals increase for Thursday, chances decrease for the weekend event.

Over Running events are a fun challenge-- with the extreme blocking in place, the amount of moisture, possible mix or change to sleet and freezing rain all have to be factored in. Over running is a term used when there is cold air in place (I'm assuming you noticed the cold today) and some warm air attempts to move it out of the way. Cold air is heave and dense, so it runs over the top of the cold base and makes it precipitate.

The data has taking a strong trend towards two things
1. Cold air holds longer--
2. More moisture.

The most extreme version of this is depicted in the NAM model that gives most of our region about a half inch of liquid, mostly falling as snow (5 inches or so--- and 3 hours of rather heavy snow.) This model tends to over due the precipitation but, I do give it some credence because its a regional model--(covers the US) not a global model that does the entire earth.

Let's start the bidding at 2-4 inches for the night and go from there-- and it still will end as a little freezing drizzle but 90% of the moisture falls as snow. I would not be shocked at all if this ends up as a low level winter storm warning with a 3-6 range region wide.

Snow starts around sunrise Thursday, heavy at times in the morning and over and done by 2 PM.

This map shows the extreme solution with a band of 5 inches running along highway 460.


Event two is less promising if your a snow level--- with the lack of a sub tropical jet, many factors have to come together perfectly to get a good coastal low. There has been a decreasing amount of support from the computer models today-- so I'd downgrade this to highly unlikely. Keep in mind, we don't downgrade because a model says so--but because the pattern as depicted doesn't support a storm.

Cold air is being ushered in--with one, maybe TWO shots at snow in the next 7 days.

December is currently running -10 degrees below normal. WOW---
If you recall last winter, we had historic blocking in the AO and NAO which lead to our many snows. It was this writers belief that we would not see that type of blocking again. And, I guessed wrong.

Some of my "people" in the field hit home on the fact that the NAO runs in cycles and we are in a longer term phase where the NAO will run negative. Last winter, and so far this winter has certainly supported this belief.

The set up--

COLD COLD air is heading our way-- once we dip below freezing over night, its going to be a while before it goes above freezing.

Event 1-

Over running--

What is that? Cold air in place-- flow turns from the SW aloft (think warmer air) and it "precipitates". Some data suggest that this remains to our south while some nudges it into our region. The afore mentioned block COULD keep this thing out of our area--or make it just a minor event.

One model looks a little more ominous with strong lift (lift = precipitation) Here is a snap shot of that left just to our south and west. This is 7 am Thursday morning. This does represent the most extreme NORTH of the model data.

I'd place the odds at 50/50 we see any type of accumulating snow out of this event. Would start early Thursday.


Event 2--

El Nino= Wamer water= More energy in the jet stream..LAST winter.

La Nina= COOLER water, less energy in the STJ This winter.

Despite the cold, we've not had many snow changes because of the lack of energy in the STJ (Sub tropical Jet)

The pattern MAY allow a little more sub tropical jet energy to be introduced to the pattern which COULD spawn a storm that rides the coast. Time is Saturday into Sunday as of now but keep in mind this is attempting to time energy aloft that is still over Sibera.

As a teaser--this is a precipitation output (Disclaimer- the precip forecast are the WORST thing computers model--just using this as an EXAMPLE. Most of VA has between a half inch and inch and a quarter of liquid. If its snow-- would be a decent event.

Now, this is 7 days out. There are no promises, just a tease. And, would be on the exact day of our first big snow from last year, December 18th.

Cold, Rain and then back to cold **Edit to say record cold**

The cold will continue through the rest of the week, "warming" a little late week and a system will form in the planes and track to our north and west, leaving us in the warm sector and we get rain. This system will dump more cold air in its wake and leave us cold and dry next week.

Simply put, the pattern right now doesn't look to support snow over the next 10 days. Cold--yes, but this pattern would most likely be cold, warm up, rain, then cold. Despite a record setting - NAO (North Atlantic Oscillation) The Pacific Pattern isn't helping at all--and this "should" hold serve the next 10 days.

** Updated to say this could be record cold. Its been an unusually strong -NAO block and as this rebuilds it will dump bitter cold air over the north east and mid atlantic a few days. Be ready**

The little Clipper that could..maybe?? 2 models come back with a tad more snow.

** Data now snows 1-2 inches likely region wide**

Clippers are fun when they make it to our region--- hard to forecast, but in if your wrong-- an inch or two isn't going to shut the town down for a few days. Someone in my weather community compared this event to the Dec 24th 1999 clipper--was forecast to be flurries and ended up dumping almost 3 inches in Lynchburg, on Christmas Eve.

Computer modeling has come a long way in the past few years, but still has some basic flaws. All models often have a "south bias"--so as a storm gets closer it moves north. Think of the snowfall last winter on Jan 30th. For days they thought the best snows would fall over NC and southside, only to charge north late and drop a foot here and Danville mixed with sleet.

This clipper--again is having those same issues. It trended SOUTH, and south and south to where last night it was going to snow in upstate SC (if cold enough). Now, its slowly tracking north via model data--

I'm attaching precipitation forecasts from the 18z (Late afternoon) and 0z (late evening) of the NAM. While these maps are the LEAST reliable (that's another blog)--it shows what I mean--you can see the nudge north in the heaviest band PLUS its a little stronger. Sometimes clippers develop whats called a strong low level jet--which causes convergence (two air masses running into each other forcing the air up. This causes heavier precipitation.

Top image is the latest run--and the bottom image is late afternoon. Note there is more precip east of the Mts and its nudged north.

Last image-- Lift at 7pm tomorrow. Nice band right up and down the 29 corridor.

Lift = snow. 

I'm expecting 2-3 inches in Blacksburg, 1-2 in Roanoke and Danville and a Coating to an inch in Lynchburg. Jog this thing another 30 miles north and those totals could double.

Some light snow likely Saturday, cold and dry after for a while--

A clipper type system should impact our area Saturday with a period of light snow and flurries now likely late afternoon and evening. I'm only expecting a dusting to an inch accumulation. I've attached a precipitation map from the early morning run of a model called the GFS--it shows about a tenth of an inch liquid, which matches my thoughts.

From there--cold is the word. Some lows in the TEENS and highs struggling to get past the mid 30's a few days.
Some early indicators show potential 10 days out for a storm. 10 days out means very little other then look for future updates.

Last year, it was an El Nino winter and we had plenty of action in the subtropical jet, this year--la nina, (Colder water near the equator relative to normal) sets up a different type pattern and we tend to not have as much energy in the sub tropical jet. Basically, I would not be shocked if we remain cold and dry.

First Trackable event...

In my winter outlook-- I targeted December as the coldest month and the most likely to be colder than normal AND have some type of snow and cold.

Attached are the 9 and 10 day maps from the ECMWF--the most reliable longer range model.

This is a classic look for a good snowstorm here. If THIS were to play out as shown, it would bust my 5-10 inch winter total forecast.


Of course--this is 9-10 days out. In a La Nina winter, its harder to hold the block in place--if you note there is a storm in place to the NE of Maine. That feature most hold to suppress this storm and hold the cold air in place. If that feature is not there--this event will be MUCH different.

** In a La Nina pattern, the pacific jet is stronger (faster) making blocking harder to hold**

Winter Outlook 2010-2011

I'm running a day later than I had promised--but the crisp cool day today inspired me to put my thoughts and forecast on paper. I've had my general ideas for several weeks, was just waiting for a little more data to trickle in to double check things.


Winter 10-11

Quite the shift from last winter where we had 200% normal of snow

Season Temp. Forecast +2 above normal

Seasonal Snowfall 5-10 inches. (Same forecast for Roanoke, 2-5 inches for the Danville region) 10-18 inches for the NRV, The favored high elevations of the region get more due to upslope snows. (AKA, places were I doubt anyone will read this blog)
Higher risk for ice storms. 

Monthly breakdown temp bread down

December-

-2 below normal

January

Starts cooler but ends warm

+2

February

+4
Still has some colder outbreaks, just warmer overall.

As the long wave pattern adjusts to spring, March will feature rapid swings (Normal to some extent in March) with a higher then normal risk of wintry weather.

Technical talk--

This may bore you if you don't enjoy weather. (I need to post a glossary of terms for those who may care--but if you google these terms you can find them)


I derive my outlook by many means-- First and foremost I read as many detailed outlooks from REAL meteorologists as possible. From there, I study the global patterns based from the El Nino/La Nina regions and other terms such as GLAAM, Mountain Tourque, PDO,QBO, MJO, AO  the past season's hurricane pattern and even snow cover in Eurasia this time a year. From this information I can get a set of years that had similar pre season patterns and check the climo records for those seasons. Its not an exact science, nor do I claim to be my one and only source. With more qualified people creating these "usable comparable years" I see little sense in recreating the wheel as a less qualified forecaster.


Last year, the west based moderate el Nino made it somewhat of an easy call and I'd rate my call an A--- didn't foresee the big snow in December and missed the extreme cold in early January--but other then that it was excellent. The big factors all lined up and I went big and scored big.

This winter's outlook has half the confidence as last years--

Why?

In favor of warmth--
Strong La Nina, The Indian Dipole, Positive QBO

In favor of colder--
Long term pattern of the NAO being negative
Snowcover in Eurasia being above normal.
Some research shows that re curving hurricanes from the east coast may be an harbinger of the -NAO being negative in the winter.


We don't have many close analogs because the El Nino went rapid to La Nina and is currently strong. This COULD disrupt the forecast because the norm is a colder December transitioning to warmer as the winter progresses. (If the La Nina has peaked sooner--does the December climo norms not matter is the thought here)

Piecing this together--

Strong La Nina's don't usually bring big snows to our region. Hence, I'm starting cold and moving warmer and expecting below normal snow. However, the snowcover in Eurasia, Re-curving hurricanes and longer term trend of the NAO I expect us to end up in the battle zone between colder air that spends much of the winter bottled up well to our north.

Monthly break down--

December-- Cold pattern develops late November and is strongest until middle of the month. The pattern begins to adjust to a warmer and by early Jan we are in a warmer pattern.


January--

Early month cold gives way big and its a good month- Late month holds chances for a few severe cold outbreaks, but warm wins the month.

February-
Warm pattern holds on with still the occasional risk of a couple cold days with trailing storms as cold fronts clear (See Feb 89 as example)


Storm pattern---

Last season the mean storm track for our region was to our south and east, hence most events were mostly snow and a ton of it. This season, I expect the mean storm track to go to our west placing us in the warm sector. However, the Appalachian Mountains play a role here where the cold air gets trapped against the mountains and we end up with an ice event. Strong La Nina = south east ridge= Warmer temps on the east coast, especially south of 40 latitude. Cold air damming plays BIG role in our ice storms.

These events can be subtle--For example.

Monday is a great day- High 61, low 32
Tuesday feels a little cooler--52 high but the relative humidity drops and the dew point is 8.
Clouds come in Tuesday night and sleet develops. Temp falls down to 25 with the sleet. By the next morning it is 29 with freezing rain. On the other side of the blue ridge towards Tazwell its 44, Charleston WV is 48 and to our east Richmond is 36 and south Danville is 38. Freezing rain lingers all day. The next day the temp is back into the upper 40's. 2 days later temps are back in the 60's.

These type storms come out of the SW--- and can head up into Ohio then reform well to our northeast. That low level cold air being trapped along the Appalachian mountain (There are three types of this "cold air damming" that causes aides in these ice and snow storms) places us in a unique spot for these ice events. If we tap into the colder air that build up over the Arctic (And didn't build up last year because of the pattern) we could see a nasty ice event or two.

For the record, LYH has one of the highest "per hour" freezing rains in the nation--close to 50 hours a year of freezign rain. I use to have a map of the hours. This is a unique climate trait to our region due to the proximity of the Blue Ridge which creates the inability to remove cold air at the bottom 2k or often less feet of the atmosphere.

Much different than last winter--warmer, less snow and more ice. More rapid shifts and in some sense laster winter was cold with shorter bursts of warmth. This year will on some levels be the opposite. Warm, with short bursts of cold.

My biggest concern is 1-2 nasty ice events, not the nuisance ice events I described-- Also, La Nina's breed drought as the pacific jet dominates and everything is progressive with little interaction from the subtropical jet. This is something to monitor as we head into the spring growing season.
Will update as needed.

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