Todd Gross' Boston Weather and Southern New England Forecasts

FRIDAY OUT TO SEA?

IMAGE LEFT IS FOR FRIDAY MIDDAY, 500 mb leve, approximately 18,000 feet... lower jet stream level.

CLICK ON IT.

THE LATEST ON FRIDAY: There is a tough call for St. Patrick's Day. The ECMWF computer model still looks fairly aggressive, but we have lost the "look" on the GFS. The image to the left is very important as it shows a short wave trough, the little bump with the X (vorticity) in the Ohio valley running east and out to sea underneath New England. It just does not have enough amplitude and does not come far enough north to spawn a decent low pressure system near the ground level so the corresponding area of positive vorticity advection, and warm advection slide well south and are fairly non impressive. It is still early in the game though, and as I said there is some dissent from the European based computer model... but this could end up to be an out to sea event if things keep going in this direction.

Computer Model Discussion for Thursday

UPDATE 7am - Friday: I should have believed my own technical discussion below, follow along as the radar threw me off my original track by reading oldest to newest posts, bottom to top. Note how my forecast was on target Wednesday, but I lost confidence in my convinction on Thursday.

The important factor of course, that I was getting at, is how the warm advection field, got stopped dead in its tracks. That is on the chart at the bottom right, and is airbrushed orange. ALSO NOTE HOW WELL "THE SECRET AGENT" MODEL DID (my nickname for it) 24 hours ahead of time, unreal. Click on it so you can see the nothing for Boston and big storm for SE Mass. It isn't always that good!

UPDATE 1pm Thursday: You believe this? The radar certainly is running against the NAM and the NAM based 'secret agent model' left. The RUC in fact is agressive with snow all the way up to Rt. 2, so I am upping accumulation predictions at this time along the Pike, but the NH border STILL gets shut out. Heavy amts SE Mass.

UPDATE: 7am THU: My "secret agent" model shows we have a forecast problem. Click on it. What if the belt of precipitation were to migrate just 30 miles north, a mere 30 miles. That would throw off the forecast by leaps and bounds. Right now it NO precipitation from Boston north, and yet moderate snow within a stone's throw of Rt. 128, just minutes away.

UPDATED 6pm Wed: Correct people who posted, thank you! The left shows a good belt of positive vorticit y advection moving in , but where is the warm advection. Even if you pictured a SW wind blowing "across" the shown temperature gradient aloft on the right hand panel, you can see that ribbon of temperature by 00z (7pm) is already out to sea, leaving us little time for an accumulating  snow this far north. Ideally for good snow you need to have the PVA pass OVER the warm advection area. Neither works well by itself. Oddly, last weekend south we had the warm advection, but not the PVA and places ended up with just an inch, this time it is the other way around. 

UPDATED 11:30am WED: Map below still stands, latest NAM and GFS are simliar to the Canadian GEM map below. The left is the vorticity map, the right is the surface wi th thickness lines, isotherms, if you will.. have we figured out why it isnt going to snow a lot yet? Please post your comments my friends! .. or if you want the answer handed to you, just click on option 3, technical forecasts and search down  the list, over and over again we miss out the best storms when a key ingredient is missing. What is it? *sorry to get playful, but hey, bear with me on this, it is going to be helpful* HINT : Missing element is on the right hand map.

UPDATED 7am WED: Here is a map. Ok, students of meteorology. What element is missing tomorrow 7pm, and wh at element is quite present?

"NAO" FAR NEGATIVE.. FRIGID

The "North Atlantic Oscillation" is far negative right now, which helps account for this cold weather. Generally it means there is a Jet Stream pattern that is recurringly favorable to cold air passing into the Eastern US from Canada due to blocking in the northern Atlantic. The chart shown however predicts a "positive" NAO by mid March. That sometimes results in storm development, but also means milder weather may be on the way, and NOT just because winter will be waning. You can find links to computer models and other charts such as this NAO prediction right here on toddgross.com in Todd's World of Links.

"ZONAL FLOW" MEANS DRY WEATHER

We have a zonal flow pattern my frieds this week! That means a general west to east jet stream with just little "bumps" or "short waves" in the Jet that have to travel far enough south, and be positioned just right to give us any precipitation at all. This eventually will change and a more major trough (U shape in the Jet) will develop in the Jet Stream pattern, perhaps as early as this weekend, but until that time tranquility reigns. The chart you are looking at is for Tuesday at the 500mb, or 18,000 ft. level.

What is a "back door" front?

I have been eyeballing a possible "back door" cold front for Thursday. It may just barely make it through for a time, or it may pull the trick that sometimes happens just ahead of it.. with lighter winds, the coast, esp. from Boston up to Newburport, flips into a seabreeze even AHEAD of the front before it moves by. It is called a back door front because it drops down from the north, taking the back door in, rather than the normal side/front door from the Ohio valley. Even if it does move in for a time, it will bounce right back out Thu. night making for a warm Friday start before that cold cold finish. (with rain in between)

MORE on Positive Vorticity Advection..

Just "GIVE ME MY SNOWFALL TOTALS!" you may say. I'm sorry, can't help myself.. have to show you the thinking behind it. The chart left shows one model which REALLY goes to town with the storm in producing both warm advection (see option 3, earlier discussion) and something called Positive Vorticity Advection. The latest close up for tomorrow morning clearly shows that the heavy snow may make it back on this chart all the way to Worcester! Click on the map, and you will see how the winds at the 700mb, 10,000 ft level are blowing across from "positive' to "negative" vorticity, or "twist" in the atmosphere. When that blows across a good temperature gradient, it snows or rains. Simple as that. It also produces high upward drafts called vertical velocity, and sometimes we get thunder inside snowstorms when that happens. Indeed our NAM computer model is calling for that too during our blizzard Sunday. For more info on reading weather maps, order the book WEATHERMAPS by PETE CHASTON. JUST CLICK THIS LINK.

TWO Models - TWO Solutions

I hate going for exact amounts early on, because things can simply "change", it is best to "hone in on" the exact amounts of snow when you get closer to it. That way you aren't flip flopping every 12 hours. In TV land, they used to like exact accumulations 3 days in advance, if you do that, you will drive yourself and the viewers crazy... start broad then work in. However, I have a major problem this afternoon honing in.

What produces heavy snow here in New England is not really the storm, but the result of positive vorticity advection blowing across a warm advection field. (I know, sounds complicated). Let's see if I can show you graphically. Click upper left, and you will see the 850mb low pressure system offshore on the NAM computer model, clearly it shows the warm advection, or blowing of winds across the temperature field, offshore, teh heaviest would be on the Cape only! So that is why many forecasters will start playing things down a bit midday today. HOLD ON! Take a look at the Canadian forecast model to the left. Here I am highlighting not only the fact that this is a Sunday storm with plenty of warm air advection, but a great PVA field, or positive vorticity advection running through it, also highlighted in a separate chart (click on it). That would mean rain cape, heavy snow inland, mix... Boston!!! Not to mention a super-deep surface low hugging the coast! What a difference! I'm going to leave my forecast as is, in between for now.

To understand how to read all these public accessible internet charts, just get Weather Maps by Peter Chaston, the best book written on it. I am carrying it at this time.

If it move close enough, there is rain involved...

Alright, if you look back through Category 3, you will see my Tuesday post about a baroclinic zone "ripe" for storm development. Now that we have the storm on the charts, what about that warm ocean temperature? Will it rain on the coast? The 1000-850mb thickness is crucial when it comes to rain vs. snow, when it is above 1300 meters, it means the atmosphere has too many warm layers somewhere within the bottom part so that snow will "melt" on the trip down. The chart on the left clearly shows that SE Massachusetts may go over to rain in the middle of the storm if it does indeed hug the coast 7am on Sunday. This is the "next thing" that I'm starting to keep my eye on. However, a look at the vertical profile of the atmosphere, also known as a "sounding" (click on it) at Chatham on the Cape, way out there surrounded by water, and you can see that the only really warm layer is right near the surface, it is cold everywhere above, so the marine layer is pretty shallow. Heavy enough precipitation would mean wet snow and perhaps not rain along coastal Massachusetts based on this sound. For information on how to READ weather charts that you can get off the net, purchase WEATHERMAPS 3rd Edition, by Peter Chaston.

WARNING FLAG - Saturday PM

(note.. this was written on Tuesday before any of the computer models indicated a storm possibility for Saturday night) 

Most, not all, but most meteorologists would scoff. In fact, they might think I'm out of my head. The fact is, there is a breeding ground sitting offshore, and despite what computer models are currently calling for (little or no snow late Saturday) there happens to be the potential for sizable snow Saturdayafternoon or evening IF AND ONLY IF a "short wave trough" deepens just a tad, enough to work on the "baroclinic zone" or sharp temperature contrast field offshore Saturday afternoon. It is just starting to be picked up on the GFS computer model, but I am always "wary" of offshore baroclinic zones in a SW flow aloft. Baroclinicity is what fuels low pressure near the ground, and causes "cyclogenesis" , the development of storms. While we don't have a big storm depicted quite yet on the computer models, we are getting there. Take a look first at the small low pressure system NOW forecast on the GFS model (I actually wrote this before that came out and had to re-write to show that there is now some "real" evidence of a surface low) and take a look at the upper air 500mb map below it. That trough is NOW on this computer model ONLY sharp enough to kick off warm advection snow ahead of the developing low pressure that "should' form in time to give us shovelable snow if this trend continues. Something to watch. For more information on troughs, temperature fields, computer maps, etc, in technical terms I highly recommend the only book written on the subject for weather whizzes like you, Weather Maps by Peter Chaston, which I do carry.

KICKERS - Why it won't snow

Alright, in our technical discussion today, now it is cold, so where is the snow? The flow is just not right for it. The mean trough position is just offshore, meaning that every time a storm forms, it will likely be out at sea. In addition, the lining up of the"short wave troughs" (described in Chaston's Weather Maps book) is all wrong right now, by coincidence. Take 500 mb map (you can get this from Todd's Wx Tools) on the left, (click it) and you will see 3 short waves. Wave 1 on the east coast is being caught by wave 2, but too late, it will already by out at sea when they merge. The reason for this is wave 3 is "kicking" wave 1 and is called a "kicker" , kind of like in football. Anyway, that is why we can't get any storms to lodge right now on the Eastern seaboard in the right place.