Consensus

January 5, 2017 / posted in SNOW / 7 Comments

I have to make this short and sweet…

While there remain some quantitative differences among the various models as to the morphology of the precipitation shield, there is a consensus we will see a significant winter storm beginning tomorrow evening and through Saturday.

As demonstration of the unanimity across the GEFS (American) ensemble members, check out this AM’s run compared to a couple days ago. (again, thanks to WeatherBell for their great graphics)

screen-shot-2017-01-05-at-3-05-03-pm

screen-shot-2017-01-03-at-1-14-04-pmObviously, with each ensemble member producing 10″+ and an ensemble mean nearing a foot, this is the real deal.  The Euro and its ensembles are in similar agreement, albeit with ensemble mean closer to 6-7″.

The trick is figuring out the orientation of the precip axis and then factoring in any warm tongue intrusion that may change over the southeastern boundary of snow to sleet for some or a significant duration of the storm.  These are details to be ironed out over the next day.

But qualitatively, I’d expect Wake county to range from 6-10″ west to east.  Bust potential to the low side is 2-3″, bust to the high side is 12-13″.

It will be chilly on Sat night down to the mid teens, but Monday AM will be brutal in the 0 to 5F range with a solid snowpack.

Quick Midnight Update

January 5, 2017 / posted in SNOW / 1 Comment

I’m interested to see how the model runs come in this cycle and over the next two, particularly.  The upper level energy that will fuel our weekend winter event is just now being sampled on the West Coast tonight.  Preliminary indications show that our thinking is on point.

I’m going to hold off defining snow accumulation zones until early afternoon tomorrow.  However, I will say this…I’m fairly confident that 2″ in Raleigh is a lock.  4-6″ is a prudent forecast.  6-8″ is not really a stretch.  More than 8″+ is possible…not likely.  (Of course, to the alternative, you have to consider that the forecast could bust to the low side.  Best estimation to the low end is 1-2″)

For now, it’s safe to put big money on white ground cover from Durham, East and North.  We’ll sort out details tomorrow afternoon.

Plinko!

January 3, 2017 / posted in SNOW / 9 Comments

Happy New Year!

PLINKO IS THE NAME OF THE GAME

We all remember Plinko from The Price Is Right, don’t we?  And now there’s a new show, ‘The Wall’, that’s basically high-tech Plinko.  In case you have no idea what I’m talking about, here’s a gif to refresh your memory.  A contestant drops a chip from the top of the peg board and it randomly bobbles back and forth, down the board until it lands in money slot.  As you can see $0 is very close to $10K!

plinko-o

So what does this have to do with the weather?  Well, using numerical modeling to predict the weather isn’t as easy as numbers in–answer out.  There are a bunch of factors:  the basic mathematics in the billions of equations being solved, the accuracy of the initial conditions assigned, rounding and truncation errors, and the sensitivity of a particular fluid dynamic setup to chaos.  To mitigate the impact of all these potential pitfalls, modern numerical weather prediction relies heavily on ensemble forecasting.  (Here’s where the Plinko comes in…individual model runs in highly dynamic scenarios is like dropping Plinko chips and getting a different answer each time.) So… we don’t use any one run of a particular model.  Instead, we look at dozens of variations of the same model to ascertain the veracity of the operational or deterministic run of a model.  i.e. if the spread is small among all the runs, you’re confident in the outcome. If it’s all over the place, then you have to tread with caution.

This weekend’s potential snow for the Southeast feels more like a random Plinko board at this point, although some focus is beginning to come.

 

WHAT OUR PLINKO BOARD LOOKS LIKE

As with all winter weather events, it will be fun to see how this turns out, versus what things looked like from a week out.  If, in fact, NC receives a major snowfall, it must be said that the GFS American model had it first…in both it’s operational and ensembles…altho the run to run variability has been rather Plinko-like.  Over the past couple cycles, the Euro has come on board with the American and Canadian to heighten the threat.  And even as there appears a significant threat for the late Fri thru Sat, things are far from certain and the range of solutions continues to be wide.  It is conceivable for central NC to get a flurry or a dusting, and it’s also quite possible to get a foot.  As with life, the answer maybe falls in the middle…but with the climatology of NC not in our favor…perhaps the answer will fall toward the lower end.

So, here’s an example of what our Plinko board looks like today…some $10K, some $0, some $100.  This graphic shows accumulated snow (and note that these plots show all frozen precip to include sleet and freezing rain in addition to snow) from each of the control and ensemble members of the American suite.  No doubt, there are some exciting possibilities, and yet there are disappointing outcomes too.  (This graphic is courtesy of the good people of WeatherBELL.)  screen-shot-2017-01-03-at-1-14-04-pm

 

BOTTOM LINE

Central/ Eastern NC has the potential for significant snowfall this weekend.  Perhaps begin planning what you might do differently, particularly on Saturday.  Don’t clean out the grocery stores just yet, though.  Also, be aware of the potential for some sprinkles, flurries or snow showers overnight Thursday that will proceed the more significant weekend system.

 

Dark Side of the Moon

October 7, 2016 / posted in Hurricane / 1 Comment

If Matthew is listening to a playlist, right now he’s probably playing (Pink) Floyd. (insert groan)

Thankfully, the eyewall of Matthew has maintained just offshore through FL.  As it moves northward, I still like the idea that it tracks over water or just grazes the coastline all the way to Cape Fear.  As mentioned last night, this will put eastern NC in a really bad position to receive a lot of rain.  The numerical modeling continues to highlight this threat and is converging on a solution that would produce widespread inland flooding.

I make the reference to Floyd in 1999 as the last great flooding event in eastern NC.  One of the things that exacerbated the flooding with Floyd was the fact that Hurricane Dennis had dumped several inches of rain over eastern NC just 10 days earlier.  This time around, areas around the region, including Fayetteville, are still recovering from flooding last week.

screen-shot-2016-10-07-at-12-20-25-pm

Here is the observed rainfall over the past 2 weeks centered over Raleigh and Fayetteville.

 

screen-shot-2016-10-07-at-12-38-31-pm

 

Here are predicted storm totals for Matthew from today’s 12Z GFS.  That’s a lot of rain on top of already saturated ground.

 

 

For comparison, let’s look at totals from Dennis followed by Floyd 10 days later in 1999…

fig03

 

Here’s Dennis…

 

 

precip-19990915

 

 

Here’s Floyd…

 

 

 

It’s a serious situation to watch over the next 36 hrs.

 

[Untitled]

October 6, 2016 / posted in Hurricane / 2 Comments

Matthew is poised to graze Florida’s Space Coast early tomorrow morning as a category 4 hurricane.  There is still much debate at this late hour as to whether it actually penetrates image-10-6-16-at-10-01-pminland or remains slightly offshore.  It’s the trickiest of forecasts with significant implications to subsequent storm strength farther north up the coast, including in NC.

I’ll leave the nowcasting to your local media outlets as Matthew tracks north.  What I’d like to put out there are a couple of points:

The models have waffled back and forth as to the track.  I’ve always been of the opinion that Matthew will track just offshore from Melbourne, FL all the way north to Cape Fear… mimicking the shape of the coastline.  Models have insisted on a sharp right turn out to sea near Charleston, but this evening’s 0Z guidance leans more towards my camp, suggesting a farther northward motion before the push to sea takes place.  Obviously, this would mean more rain, wind and surge for NC.

No matter the exact track, the interaction of Matthew with an inverted trough along the coast and the impinging cold front from the west, will combine to funnel a ton of moisture into eastern NC.  Indications this evening are that totals could trend higher.  At least 2-3″ in Raleigh on the low end.  More likely to be 4-6″ for us.

 

Matthew…so complicated

October 4, 2016 / posted in Hurricane / 6 Comments

I suppose it’s time to weigh in on Matthew.

I’ve gotten a lot of emails asking for an update, saying that I’ve been silent.  Not really.  Twitter is a fairly effective tool for quick bursts of info.  And so, I encourage you to follow me there:  @trextrex14

There’s a lot of good information in the media on this storm, so I’ll write this post from the perspective that you’ve heard all of that…and more.  I hope to value add, to give you a couple of considerations in why the forecast could go perfectly as planned, or why it may be a total bust.

As of midday today, all the numerical guidance seemed to be converging to a tidy solution (albeit problematic in itself…for reasons I’ll talk about next).  That solution had the track of Matthew basically mirroring the concave SE coastline from FL thru GA and SC to NC then curving out to sea.  In this scenario, Matthew would track just offshore, pounding the coastline from FL to NC.  The problematic part of this is that since the shape of the coastline and the track would be nearly the same.  Any deviation of the track to the left would push the hurricane inland and significantly weaken Matthew for all locales north, i.e. less wind and surge, but still decent wind and lots of rain.  So that’s what’s hard if there was just that.

However, the Euro came in this afternoon with a literal curveball.  It depicted Matthew grazing the FL coast before doing a loop off GA back toward FL.  Then a couple of the Euro ensemble members had loops, and several had a hard right turn near GA.  Then the 18Z American ensembles came in continuing this new idea of out to sea and not up the coast.  Now, hot off the presses, the 0Z dynamical guidance echoes a similar theme…out to sea.

So, at this point I would like to be able to tell you there is confidence in a solution.  Tonight, there is more uncertainty than ever.  Hold tight, make plans for how to be prepared, but I would say there’s no need to act upon them until we see more data.  Remember we still have more than 3 days to work this out.

 

Squall Line on Our Backdoor

February 24, 2016 / posted in General / 1 Comment

Don’t be fooled by the sun and blue skies.  If anything this just adds fuel to the fire.  The main line of storms will affect Raleigh close to 5pm.  Expect strong gusty winds (even outside of the storms), could be some hail, and we’ll monitor any spin ups for the possibility of a tornado or two.  Obviously, there is that possibility as evidenced by several already observed today.

Once this line moves through your area with will clear behind and the severe threat will be gone.  Although remember that winds may still gust above 40 mph and with rain swollen ground trees can uproot more easily.

Twitter: @trextrex14

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Severe Threat Imminent

February 24, 2016 / posted in General / 1 Comment

Today’s potential for severe thunderstorms, tornadoes and strong winds has been well advertised in the local media.  Hopefully everyone will be extra attentive today to warnings when they are issued.

The greatest threat will be from Raleigh and points east, however that doesn’t mean there will be a magical line akin to NC winter weather setups where you can rest easy to the west.

This AM’s cold air is being scoured out as warm air rushes in behind a couple of warm fronts from the south. (The temp at RDU rose 9° in the last hour!).  This should rocket temps thru the 60s and into the low 70s by lunch.  Showers and thunderstorms will increase in coverage thru the morning.  The main threat of severe storms in Raleigh looks to be between 12 and 5pm.  And there could be two rounds before things head east thru dinner time.  Yes there is the potential for tornadoes, but in general a greater number of people stand to be affected by strong straight-line winds inside AND outside of the thunderstorms.  So don’t think that you got off easy if you “just get” a severe t-storm warning  and not the tornado warming.  The winds will be gusty this afternoon anyway with gust 40-50mph, and the t-storms will enhance these even more.

I expect to see a tornado watch issued shortly.  So, stay alert and heed the warnings!

Follow on Twitter for more frequent updates: @trextrex14

 

Heavy Rain for the AM Commute

February 15, 2016 / posted in SNOW / 1 Comment

Indeed the cold wedge has held firm as it took until 7pm for RDU to reach the freezing mark.  And even as the temperature slowly climbed thru the afternoon, the glazing was kept in check because of indirect solar radiation bleed thru and upwelling ground warmth.  It’s nearly impossible to achieve a significant ice storm without a continuous supply of fresh cold air to counteract the self-limiting process of freezing rain drops, which tends to warm the local environment through release of latent heat.  That’s what you saw today…despite temps in the upper 20s, there was only light glazing to 1/10th of an inch.  And in the end, it all went pretty much according to plan…but maybe a little lighter on the snow/sleet than i thought.

Looking to tomorrow AM, temps will rise over night.  And we’ll have our second messy commute in a row.  This time, we’ll just be fight heavy downpours and gusty winds.  There should be moderate to heavy off and on rain with the narrow heaviest band progged to sweep thru Raleigh close to 7:30am.  Here’s the HRRR’s (short term model) depiction of what the radar might look like at 7:15am tomorrow:

Screen Shot 2016-02-15 at 7.09.50 PM

The rest of the week should be mild.  Enjoy.

Looking ahead…Is winter over?!  Nope.

We have elements coming together for the next major East Coast winter storm next Wed or so.  Will we be cold enough?  The storm track looks favorable.  Stay tuned.

 

 

For Raleigh, Things Seem on Plan

February 14, 2016 / posted in SNOW / No Comments

The northern part of the Triangle got some bonus snow on the front end this evening with only 27 flakes in Raleigh…or was it 28?

So the idea that the steady (light to moderate) snow moves back in early morning before sunrise still holds. And at that point begins the game of nowcasting banding structure and figuring out who wins for the initial snow/sleet accumulation.  Right now indications are that the enhanced axis probably lies north of Raleigh.  But we’ll see…that’s why you play the game, right?

More tomorrow and for more frequent updates, follow me on Twitter @trextrex14

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