Finding Nemo – Blizzard Forecast to Impact New England

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For some reason I think it’s just hilarious that The Weather Channel has named our Friday blizzard Nemo! While naming storms is just a silly enterprise it looks like Nemo may really be a beast!

Earlier today we were all having fun with the 12z NAM that was showing 40-60″ of snow in parts of New England. No big deal, right? The NAM is just not designed to be used for rip and read snowfall forecasts. It’s a curiosity and is of no use. Toss it. You can toss other QPF forecasts from non-hydrostatic models out the window at this juncture as well.

The global models are in exceptional agreement that this storm is going to be huge. The GFS/Euro combination along with their respective ensemble members have honed in on an impressive solution. Here’s my latest thinking in a probabilistic way.

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While a chart like this would give TV news consultants heart palpitations it’s really the best way to express forecast uncertainty! I know my blog readers have a lot of weather geek in them… so enjoy! If you notice here the odds of more than 18″ of snow are pretty low… there’s a reason for that! The greater Hartford area has only recorded 5 snowstorms (officially) of 18″+ in the last 108 years! The Bridgeport coop observer has never recorded an 18″ snowstorm since the 50s! They’re just not all that common. We’re also still 36 hours before the storm’s onset and a lot can change.

Before I get accused of being a debbie downer. Let me now talk about how amazing this storm looks meteorologically. Here’s the 18z GFS valid at 6z Friday.

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Here’s the GFS which is in fair agreement with the Euro (though the GFS is a bit more impressive with an earlier phase/capture). Synoptically, a digging northern stream disturbance mananges to be timed and located perfectly to dig and capture a moisture laden southern stream disturbance. Beauty! Without blocking downstream there’s not much wiggle room. The timing has to be PERFECT for this to work out for Connecticut. 6 hours in either direction will make a huge difference (keep in mind the northern stream disturbance is over Montana and the southern stream is over Texas).

All of our models show the perfect phase though the Euro is a bit late and therefore a bit less impressive in Connecticut compared to places around Boston. So assuming that actually occurs and it’s not late (a late phase would still clobber the Cape and eastern Massachusetts but give us a more pedestrian storm) let’s watch the beauty unfold at 700mb from 18z Friday to 6z Saturday in 6 hour increments on the 18z GFS.

700mbEvolution

I mean if that’s not breathtaking I don’t know what is. We even manage a little loop-de-loop there as the 700mb low tightens and matures. Exceptionally powerful frontogenesis on the northwest flank of that mid level low would result in a super band the likes of which you rarely experience. All of this is taking place under an area of strong divergence thanks to a coupled jet streak (classic KU setup).

The result of this “perfect scenario” is a large swath of 1.5″ to 3.0″ of liquid and likely a snowfall on the order of 1 1/2 to 3 feet.  Wowzers.

The perfect scenario is only one such possibility, of course, and small changes in the next 24 hours with the 2 disturbances we’re watching can make a large difference down the line. In order to start picking up over 15″ of snow in this part of the country you need small-scale (mesoscale) features on your side. These are notoriously challenging to forecast even 6 hours ahead of time!

That said, as of right now this has all the makings of a classic. Odds are better than 50/50 that many inland areas see a foot of snow. Along the shoreline some sneaky mid level warmth may  bring a period of sleet and a bit of mid level drying may promote some dry slotting. Big “IFs” here though with plenty of potential for a crippling snowstorm if the shoreline is able to hold the sleet and dry slotting at bay.

Will this turn into an historic storm? It’s possible. Too early to say for sure. The amount of liquid being generated by the normally reliable models (like the GFS and Euro – ignore the NAM) are staggering. I’m excited for this one 🙂

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Ensemble Modeling – A Great Tool!

The atmosphere is always moving. It’s infinitely complex. It’s a fascinating fluid that can provide meteorologists with amazement, humility, and a whole lot of fun! To figure out how the atmosphere will evolve over a given time we use computer modesl.  In order to do this we need to take a snap shot of the atmosphere to give a that model a set of initial conditions.

Since it is impossible to sample every point (both in the horizontal and vertical) in the troposphere our initial conditions will never be perfect. Weather balloons, surface observations, and increasingly remote sensing do provide us with a good first guess! Errors in that first guess amplify with time due to the chaotic nature of the partial differential equations that the model solves. Other issues, such as the imprecise parameterization of variables, also lead to issues with forecast accuracy.

Ensemble modeling is a powerful tool that tries to alleviate the issues associated with the initializing a model.  We do this by “tweaking” the initial conditions for each ensemble member and then run the model for each different member with different sets of (reasonable) initial conditions. When all the ensemble members are reasonably close with a 48-hour or 72-hour forecast, for instance, predictability is high and there’s a low sensitivity to small changes in the initial conditions. When ensemble members are all over the place, however, we can infer there is a high degree of sensitivity to the initial conditions and the forecast uncertainty is high.

Every storm is different. Some storms are well forecast and modeled and forecast confidence is high. Other times there is a huge spread in our ensembles and the forecast confidence is low. When ensemble members agree with their respective operational run things are in great shape!

Enter in the Monday night/Tuesday storm. If you were to look at the 15z Short Range Ensemble Forecast (SREF) members you’d be ready for a big snowstorm! Here’s a plot of the probability of >4″ of snow (the plot is created by looking at how many members produce >4″ of snow at a given grid point and assigning a percentage).

15z SREF 69-hour >4" of snow (in 12 hour) Probability

15z SREF 69-hour >4″ of snow (in 12 hour) Probability

So… yeah… about that 70 percent chance! While the vast majority of SREF members are quite amped up and quite wet most of our other models are quite blah (except, of course, the NAM)! The 12z European, UKMet, GGEM, and GFS produce very little snow over Connecticut. Maybe an inch or two!

So what’s the deal? It’s important to understand the difference between the different models and the forcing behind what’s producing the precipitation. Let’s start with the synoptics.

Since the NAM is the most robust let’s look at why. First here’s pressure on the 2 PVU surface (dynamic tropopause). The orange lines are isotachs on the DT. The image below is 500mb absolute vorticity and the orange shading is upward 700-500mb vertical motion.

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So what’s going on? We have a strong PV anomaly that’s heading toward the coast. The PV anomaly is strengthening as it heads east. The PV forcing is strongest off the east coast of New England. In addition, the low static stability thanks to the moist/mild air over the Ocean favors upward motion. Using a conceptual jet streak model we can also see forcing for lift. I marked the 2 jet streaks that have developed downstream of the PV anomaly. One over Nova Scotia and another south of Nantucket. The left exit region of the latter and right entrance region of the former are right over eastern Massachusetts and the adjacent Atlantic Ocean.

Here’s the 18z NAM QPF and 1000-500mb Thickness / SLP valid 12z Tuesday. Under that area of strong synoptic forcing an inverted trough/NORLUN signal has developed with a localized maximum in QPF.

18z NAM - 66 Hour Forecast

18z NAM – 66 Hour Forecast

A combination of strong synoptic-scale forcing and mesoscale forcing along that inverted trough result in heavy QPF values. So why do the other models not have this? While all the models are reasonably close with the synoptics they have dramatically different QPF forecasts.

Some of our models are referred to as global models and others as mesoscale models. The mesoscale models tend to have a finer resolution than their global counterparts and they’re also non-hydrostatic (i.e. don’t assume a balance between vertical pressure gradient force and gravity). The NAM and its ensembles (the SREF) also have different terrain and convective parameterization schemes designed to resolve mesoscale details.

So what are we to do? This event is driven by synoptic and mesoscale phenomenon. While this is true of all events it’s particularly true of these kind of setups where mesoscale boundaries are absolutely vital. We’ve seen storms like this where only miles separate 3″ of snow from 15″!

In general a few things to note about these inverted trough/NORLUN deals:

  • I’d like to see more QPF from the global models. Stronger synoptic forcing and better inflow off the ocean would be a start.
  • These things always seem to trend north and east with time! When models have something over central New Jersey 72-hours out more often than not it ends up over the north shore of Boston by verification time!
  • Simple geography tells you that eastern New England is more vulnerable to these NORLUNs as they “stick” out to the east more. Pressures lower and the trough forms as the PV anomaly is able to interact with the low static stability over the ocean. 
  • Mesoscale models can hint at the potential for super-big totals but are notoriously poor with figuring out where exactly this will be. Global models are tend to catch on and show something.
  • Where the NORLUN does setup it can dump! Normally the airmass is quite cold and many times the strongest omega is near the top of the boundary layer up through 700-ish mb intersecting with the dendritic growth zone.

This is something to watch over the next few model runs. While the NAM and SREFs are honking a big event I want to see the Euro jump on board. Until it does there’s no reason to play up the snow threat though these things are always a pain in the butt to forecast.

Think snow!!!!

Sneaky Snow?

For people who love winter this weather pattern just plain sucks. Surprisingly, I haven’t been complaining about it. After October a break from winter has been nice – and it’s been nice for our heating bills too!

It appears that this may be one of the rare times that we are able to get a couple inches of snow to fall in a hostile early season weather pattern.

12z NAM / 48 Hours 500mb Vorticity & Height Forecast

Our computer models are showing a very powerful area of upper level energy ejecting from the southern Plains toward southern New England by Thursday. This vorticity maximum gets deamplified as it is crushed by a growing area of confluence over the Canadian Maritimes and the North Atlantic.

Occasionally this kind of setup can deliver what I like to call a drive-by snowstorm. A 6 hour window of snow in a somewhat localized swath. The storm will never become a powerful or mature cyclone as its upper level energy is zipping along and weakening. In addition in the mid levels of the atmosphere a low pressure system never really closes off and you’re left with an “open wave” effectively limiting the amount of precipitation that can fall.  See the 700mb NAM forecast to the left.

So what’s the bottom line? In a storm like this there’s little room for error. The storm has to “thread the needle” so to speak with a perfect track of the 500mb shortwave. At this point our models are in fairly good agreement with a track just to the south of Connecticut. With warm waters and a period of upper level divergence we should see a deepening surface low and a period of warm advection produce precipitation along and north of the shortwave’s path.

At this point things look cold enough for a rain/snow mix or snow as long as the storm maintains its current forecast track. Any adjustment north would back winds near the surface enough to warm the boundary layer and result in more rain especially for the valleys and shoreline.

The maximum amount of snow we can reasonably expect in a setup like this is around 6″ given an “ideal” track. Something less than ideal will result in less snow whether it’s due to mixing or due to the storm tracking too far south. Odds favor less than ideal but it’s worth watching.

By midday tomorrow we should have a better idea how this “drive-by” snow event will play out. It would be pretty incredible if such a mild meteorological autumn and beginning of winter was able to produce 2 plowable snowfalls before December 10. If you like snow don’t get too excited yet and if you don’t like snow don’t worry nothing is set in stone at this early juncture.

Freak October Snowstorm Becoming More Likely

Hello, Snow!

This evening’s snow is a rare enough event. It looks as if the greater Hartford area has picked up measurable snow in October for the first time since 1979 and for only the 4th time in the last 100 years. We will need to wait to hear from the weather observer at Bradley Airport to get the official word.

This may be just a prelude for a significant and high impact winter storm that appears to be brewing for the weekend. All day I’ve spoken to meteorologist friends who have all said the same thing, “I’ve never seen anything like this.”

Obviously things can change and a variation in the track could mean only a little bit of snow. As of right now, however, the model consensus is a major winter storm that has the potential to bring down trees, tree limbs, and power lines given the amount of foliage still on the trees.

As we’ve seen tonight even though it’s October the sun angle is quite low and given a heavy enough snow rate snow can accumulate on the grass and on the roads. The GFS and the NAM computer models along with the Euro show substantial snow totals (most certainly “plowable”) even down to the shoreline.

For entertainment purposes here’s a look at the >12″ snow probability from our SREF guidance suite. The blue shading is 25 percent chance. Obviously this isn’t what I’m forecasting but this shows just how incredibly bizarre, rare, and unusual this storm could be.

By midday Friday we should have a better handle on this storm. At this point it looks like this storm has the potential to be unlike anything we’ve seen in recent memory.

Thursday Evening Snow

Most areas in Connecticut haven’t seen a frost and most certainly haven’t seen a hard freeze! All that will change Thursday as a burst of cold will move south as a departing storm heads into the Atlantic.

18z NAM 3-Hour Precipitation Forecast 00z Friday

While the NAM computer model drops temperatures fast enough to change rain to accumulating snow just about everywhere the GFS is much milder. As is typical the NAM Is likely too cold and the truth will lie in the middle.

At this point it seems like a slushy coating to an inch of snow is possible in the northwest and northeast hills Thursday evening. For the valley locations some places may see some wet snowflakes mix in but that’s just about it. One of the differences in the model is that the stronger solutions develop stronger northerly winds at the surface which is able to tug colder air to the coast much faster. This will be something to watch.

15z SREF 1" Snow Probability

The 15z SREF snowfall probabilities are pretty similar to previous runs with about a 50% chance of >1″ of snow in the higher elevations of Litchfield County. The green shading is probability over 50% and red is over 75%. As you can see there is a low probability of >1″ of snow in some of the lower elevations but at this point that is rather unlikely.

Even a slushy accumulation could make roads slippery as temperatures drop below freezing in the hills. Watch for some black ice as well early Friday morning.

As for Saturday’s storm… stay tuned! At this point it looks like a rain/snow mix glances by Connecticut but there are some indications the storm could be closer to the coast and much more powerful.