Forecasting a Nor’easter

Categories: Weather

I’ve heard somewhere that weather forecasting in the Northeast is one of the most difficult areas in the world. That may be, I don’t know, but we have many influences on our local weather-a meandering jet stream, the Atlantic, the Great Lakes, our Green Mountains, even the Arctic itself. On the weekend of February 21-22, the National Weather Service in Burlington was forecasting a potential Nor’easter to hit New England the end of the next weekend, on March 1. I followed the forecasting of this storm closely all week, to see how the forecast and storm would develop through the week.

A Nor’easter is a coastal storm that starts in the warm tropical waters of the Gulf of Mexico, and travels north along the coast. The storm takes moisture from the ocean it is over, and deposits it inland, in either snow or rain. Because the storm is over the ocean, New England is in the upper left quadrant of the low pressure system, and with the storm rotating counter clockwise, the winds at the surface are from the northeast, hence the name. The Valentine’s Day Snowstorm of 2007 that we still talk about in our department was a particulary impressive nor’easter, but examples are usually found most winters.

Forecasters use computer models to predict the weather. Most have esoteric abbreviations for names, such as NAM and GFS. You can read the raw data, but it doesn’t mean much without the interpetation programs that translate it into map form. When I want to see the models behind the storms (like in a dry summer when I am hoping for rain) I usually go the Weather Underground site, which has great time-based maps of a couple of the major models.

I don’t bother with the models most of the time, though, as the real forecasting comes in the interpetation of the models. The local forecasting office (in our case at the Burlington International Airport) takes the various computer models (there are many), and blends and interepets them based on local knowledge and experience. You can read about this process, and get a great feel for the actual forecast on their Area Discussion product. This is a text based discussion/forecast that, I think, is primarily used for the weather offices to talk to each other, and to give their interpetation of upcoming events. Filled with jargon, written in all capital letters, the feel dates back to the early days of the internet, when bandwidth was precious and brevity sacred. At the weather service site, the abbrevations are linked to defintions, but I prefer the Weather Underground discussion, that takes the text and runs it through a translating program to increase readability. I still don’t understand some of the jargon, it’s quite technical, but it’s still very interesting. I have found that our local office is a little more brief and jargon prone than other offices, like Boston.  The discussions are updated every couple of hours, with major re-writes appearing around 3 AM and 3 PM.

I’ll begin the story the weekend of February 22-23, when the discussion first mentions a potential significant winter event for the upcoming weekend, with possible heavy snow. In our landscape department, we pay close attention all winter long to the forecast, as the more warning we get of a storm, the more preparation we have, and the smoother things go. This is when I decide it might be an interesting storm to track for the blog, so I start cutting and pasting into a file.

Tuesday morning, and the long term discussion states ” SURFACE HIGH PRESSURE WILL REMAIN OVER THE REGION THROUGH MONDAY…SO EXPECTING MOSTLY DRY CONDITIONS. UPPER TROUGH MOVING ACROSS THE AREA EARLY SUNDAY MAY SPARK A FEW MOUNTAIN SNOW SHOWERS SUN AFTERNOON AND EVENING…BUT LIMITED MOISTURE WITH HIGH PRESSURE SO DON`T THINK IT WILL AMOUNT TO MUCH” . Well, so much for the storm it seems…

The tuesday evening discussion is about the same, but Wednesday morning “NEXT LOW PRESSURE SYSTEM TO AFFECT THE REGION WILL TAKE SHAPE OVER THE CAROLINAS ON SATURDAY NIGHT…AND MOVE NORTHEAST ALONG THE EASTERN SEABOARD. THIS LOW WILL TRACK NORTHEAST…AND PASS JUST INSIDE THE BENCHMARK SUNDAY NIGHT.  THE LOW THEN TRACKS SLOWLY TO THE NORTH.  TWO UPPER LEVEL LOWS WILL INTERACT WITH THIS SYSTEM…AND MAY PHASE TOGETHER EARLY SUNDAY NIGHT.  EXACT INTERACTION OF THESE UPPER LOWS AND VORTS IN THE FLOW WILL CAUSE SOME INEVITABLE CHANGES IN THE CURRENT FORECAST SOMETIME BETWEEN NOW AND SUNDAY.  AT THIS TIME HAVE GONE WITH LIKELY POPS FOR SUNDAY AND SUNDAY NIGHT…WITH THE BEST CHANCES ACROSS VERMONT.  RIGHT NOW LOOKS LIKE THIS SYSTEM WILL BE MAINLY A SNOW PRODUCING SYSTEM FOR OUR FORECAST AREA…BUT TRACK OF LOW AND AMOUNT OF WARM AIR WRAPPING AROUND SYSTEM MAY CAUSE SOME MIXED PRECIP TO BE INTRODUCED.  SOME WRAP AROUND MOISTURE WILL LINGER BEHIND THE STORM…INTO TUESDAY MORNING” The benchmark they talk about is 40N 70W, just below Cape Cod. It coincidentally is a good benchmark for coastal storms-to the east of the benchmark and most storms will miss us. I find it interesting in this forecast they call for a chance of mixed precipitation-hindsight says it stayed about 15 degrees all day.

Thursday morning, and it is very interesting now. “LOTS OF UNCERTAINTY IN THE LONG TERM WITH MODEL DISCREPANCIES AND LACK OF RUN TO RUN CONTINUITY.  AT THIS TIME” They state “LOOKS LIKE AREA OF LOW PRESSURE TAKES SHAPE OFF THE COAST OF THE CAROLINAS ON SUNDAY.  THIS LOW WILL BREAK INTO TWO SEPARATE AREAS OF LOW PRESSURE.  THE NORTHERN HALF WILL TRACK NORTH EAST ALONG THE EASTERN SEABOARD AND OFF THE COAST OF CAPE COD ON SUNDAY AFTERNOON AND NIGHT.  THEN SECOND HALF OF LOW WILL FOLLOW THE SAME TRACK…AND BE OFF THE COAST OF CAPE COD BY TUESDAY.  BOTH OF THESE LOW PRESSURE AREAS WILL BRING CHANCE FOR SNOW ACROSS SOUTHERN NEW ENGLAND” The second area of low pressure never develops, it’s tuesday as I write this and outside is a wonderful sunny day.

Thursday afternoon, and another change in the models. “THE LOW PRESSURE SYSTEM WHICH WAS SUPPOSED TO MOVE UP THE COAST AND AFFECT THE NORTH COUNTRY SUN/SUN NGT AS TRENDED FURTHER OUT TO SEA BASED ON CURRENT MODEL RUNS. THIS WILL SIGNIFICANTLY CUT BACK ON PRECIP EXPECTED FOR THE COUNTY WARNING AREA. WILL CUT BACK FOR NOW TO CHANCE PRECIPITATION  FOR SNOW FOR CVLY/VT AND THEN TO SLIGHT CHANCE INTO THE ST LAW VALLEY.”

Friday morning, and we’re getting closer to the time of the storm, and the models are backing away from it even more. “STILL PLENTY OF UNCERTAINTY FOR THE EXTENDED PORTION OF THE FORECAST WITH GFS AND ECMWF HANDLING COASTAL LOW QUITE DIFFERENTLY.  THE GFS KEEPS THE LOW COMPLETELY SOUTHEAST OF OUR COUNTY WARNING AREA.  HAVE INCLUDED CHANCE FOR SNOW SHOWERS IN EASTERN VERMONT FOR SUNDAY NIGHT THROUGH TUESDAY FOR POSSIBILITY OF STORM TAKING A SLIGHTLY MORE WESTERN TRACK. VERY LITTLE CHANCE FOR PRECIPITATION DURING THAT TIME PERIOD FROM THE CHAMPLAIN VALLEY WESTWARD.” So, they have played up the GFS model, but covered themselves by saying chance preciptation (usually 20-30%) in case the ECMWF model is correct.

Saturday morning, I’ve had it with the ALL CAPS, and am cutting and pasting from the Weather Underground site now.  They state “By Sunday night into Monday…models slowly coming into better agreement on an overall solution regarding addl energy riding northeast along old offshore frontal boundary. Critical in the development process is current split flow pattern…and the extent to which  digging northern stream trough approaching from the northwest interacts with potent southern stream vorticity/upper energy across the southeastern Seaboard. Best estimate is for a low center track across or very near the 40n/-70w benchmark by Monday. This would keep highest threat of precipitation to the eastern portions of our forecast area…and will lean towards this scenario for now…which is broadly supported by this mornings Euro/Canadian/UKMET/GFS ensemble solutions. primary forecast issue during the long term  is continued evolution of upper trough and associated surface low expected to track northeastward through the Gulf of Maine during Monday night. There are potential middle-level phasing issues…as strong middle-level low over the middle-Atlantic region Monday morning subsequently interacts with a shortwave trough tracking eastward across the Great Lakes area through Monday night. Whether phasing occurs with these features will determine the ultimate strength and track of the surface low and low level forcing for ascent. That said…operational 00z GFS is about the furtherest west solution…with surface low passing near 40n/70w benchmark. The GFS ensemble mean is east of the benchmark at 00z Tuesday. The 00z European model (ecmwf) has also come in similar to the 00z GFS. Felt best option for now was to indicate a west to east gradient in probability of precipitation Monday night…Heaviest snowfall with this potential storm likely remains east of our forecast area based on latest trends…so will use “light snow” wording in the forecast for Monday night into Tuesday. Some light snow accumulation is possible across central/eastern Vermont. Will continue to monitor as stronger phasing could yield low track closer to New England with greater impact still possible across the forecast area Monday night into Tuesday…just not the most likely evolution at this time. Those with travel plans through Monday night/Tuesday should continue to check back for updates. ” Hindsight says this is the correct forecast, but we still have a ways to go yet…

Saturday evening, and they write,”complex and challenging forecast for late Sunday night through Monday night across the eastern Continental U.S./Our County Warning Area with very low forecaster confidence”. The models are all disagreeing, with “Overall…models have initialized well with current middle/upper level features…but significant disagreements occur with regards to amount of southern/northern stream phasing…and depth/strength of middle/upper level trough lifting across the eastern Continental U.S. ..NAM quickly weakens middle/upper level circulation and keeps surface low pressure well east of forecast area…with limited impacts. Furthermore…with no closed circulation as suggested by the NAM…limited moisture is advected toward our forecast area and quantitative precipitation forecast fields are near zero. Will use a compromise between the more  aggressive GFS/European model (ecmwf) and increase probability of precipitation/quantitative precipitation forecast from east to west across our County Warning Area”.

Sunday morning, and the models are still all over the place. “..with several GFS members now drifting westward over or just east of the benchmark…cannot totally discount. Better solution however would appear to be a blend of either this morning’s Euro/UKMET and/or NAM scenario…which depict a slightly slower ejection of southern energy and a subsequent track slightly further east with less quantitative precipitation forecast. With such uncertainty remaining even at this close time range and out of respect for the op GFS solution…have conservatively hoisted winter storm watches for the eastern half of the forecast area for later tonight into Monday for accumulate snows”

Sunday evening, and they are watching closely now, and state “an extremely difficult and complex forecast. A system of this magnitude is going to have some surprises…even across our forecast area”. They feel “Given…depth/strength of this circulation…feel models are too quick in weakening middle/upper level circulation…which we have seen all winter long. Furthermore…latest trends have been to delay weakening the system and for southern stream to have more interaction with northern stream energy over the Great Lakes. This combined with plenty of convection helping to enhance downstream ridge…has resulted in a track slightly further west with the surface low pressure” They start forecasting snow amounts, and call for 3-6″ in the Champlain Valley, with siginifcant heavy snow in eastern Vermont.

Monday at 3 AM, and they are watching the storm nearly on top of us now. “As has been the case all along…degree of phasing/interaction with slowly digging northern stream upper trough across SC Canada continues to be the big question…and per latest radar trends full interaction may be a bit tough. As alluded to in prior discussions…other problematic issue is areal coverage/amts of quantitative precipitation forecast across our area today into this evening. While most operational solutions show excellent upper air support and deformation traversing our area this afternoon into this evening…examination of sounding profiles would indicate less than robust Omega through the boundary layer into middle levels. As a result…am a tad skeptical on whether heavier op GFS quantitative precipitation forecast will come to fruition.” This is the first indication that maybe it won’t snow as much as first forecast.

9 Am monday now, and now “well its been well advertised as an extremely difficult forecast with the potential for a sharp precipitation gradient from west to east across our County Warning Area. Based on best lift/moisture associated with coastal low pressure passing to our east…will downgrade warnings across eastern/southern Vermont to Winter Weather Advisory at this time.” The forecasters “am a tad skeptical on whether heavier op GFS quantitative precipitation forecast will come to fruition. So with that said will keep current warning/advsy headlines as is…though lower snow totals to low end criteria all areas”

3 PM monday, and do you remember the significant snow for eastern vermont they were thinking of 24 hours ago? Well, now, “a significant middle/upper level dry slot has moved into central/eastern Vermont…which has resulted in limited amounts of snowfall. Will remove advisory for eastern Vermont”

Monday late evening now, almost 10 PM, and they show some retrospection on the storm, “THE LOW PRESSURE AREA THAT BROUGHTLIGHT SNOW TO THE REGION TODAY IS OVER WESTERN NOVA SCOTIA AT THISTIME…AND CONTINUING NORTHEAST INTO THE CANADIAN MARITIMES OVERNIGHT. THIS LOW PRESSURE AREA TRACKED FURTHER EAST OF CAPE COD TODAY THAN WHAT EARLIER MODEL GUIDANCE HAD INDICATED…WHICH SHOWED THE LOW TRACKING OVER THE WESTERN PORTION OF CAPE COD. THUS…THE NORTH COUNTRY RECEIVED LESS SNOWFALL THAN WAS PREVIOUSLY FORECASTED. SO…HAVE DECIDED TO CANCEL THE REMAINING WINTER STORM WARNINGS AND WINTER WEATHER ADVISORIES FOR THE REGION”

So, it seems that the Saturday morning models were the most accurate, but it was a crazy ride all along. We ended up with about 2″ of snow on campus, while the most snow fell in Westford, at 7″. The eastern part of the state ended up with 1-2″. Very interesting storm, and a great test example of why forecasting is so difficult in New England. I never complain about weather forecasting. I know a little bit about what they go through.

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