Analysis: Thursday Storm Threat |
| Posted: 1/21/2007 03:08:00 PM |
The projected storm is now within 120 hours of crossing over its closest distance to the area. In some cases, with timing differences, it is under 108 hours. This storm has been on my radar screen along with other pro and amateur meteorologists for over two weeks now. The model guidence gave us a boost of hope on Friday when the GFS, Euro, UKMET, JMA, etc. indicated a storm worthy of a MECS lable crashing into New England. The storm is still shown by almost all the guidence, however the track forecast has shifted considerably eastward, and in some cases, the storm no longer effects SNE whatsoever.
Regardless of the model solutions, the set up is there for a significant storm to develop.
Teleconnections overview:
The AO is following the guidence forecast very well. The current drop is very favorable for cold air intrusions, and the every one of the current ensemble forecasts takes the AO sub zero SD later this week.
The NAO has also begun its plunge into the negatives, and in fact reached sub zero SD yesterday. The forecast is an increased fall to occur through this week with a good consensus of diving below -1 SD. This will allow solid blocking to develop as indicated in the short term forecast:
The PNA, while remaining relatively neutral and not making any drastic changes, is currently following the ensembles well and slowly rising. There is a good consensus on reaching +1 SD this week.
Back to the models: Individual analysis: Short term outlooks: The NAM is currently updating for the 18z run. The first images out show a weaker northern stream which is not good for a track shift west. What we want is a strong northern stream that will be able to pull the southern stream storm northward. In addition, it shows the ridge building strong in the west; a characteristic that has developed on many of the guidence solutions. This is good for cold but not good for a southern stream connection up here. Long term outlooks: The Euro seems to have been leading the pack in regards to forecast changes. In the past shift (which brought the storm back westward on Friday), the Euro initiated the shift 12 hours before any of the others did. The Euro also shifted east earlier. Right now, the Euro is the model of choice for monitoring this developing senario. The GFS was the model that everyone has been paying attention to after it showed a bomb slamming into the region giving us a solid 36 hour period of snow and indicating wide spread accumulations in excess of 20 inches. The GFS latest runs still shows an incredibly intense low, however a distance of around 300 miles further east. As said in the BOX AFD, the GFS has in the past lost storms like this in the mid term only to bring them back later. The 18z GFS is coming out as I type right now, however I'm not putting too much weight on this solution. For more confidence on this storm however, I'd like to see a westward shift starting by 12z tomorrow. If that doesnt occur, then I think we can say goodbye to the slightest chance of 6+ inches outside of the I95 corridor. Here is the latest solution from the GFS:
And as you can see, it essentially develops a sequel to the Great White Hurricane. In fact, the pressure that it brings the low down to (946mb) is bordering on the equivilent of a category 4 hurricane (<944mb). This is a bit extreme in my opinion, but it is enough to emphasize that this storm has huge potential.
The Euro's solution: Also showing a significant low:
One thing that I have yet to cover are the SST anomalies in the Atlantic. The 2006 hurricane season, as you can probably remember, was very inactive; especially for the eastern seaboard. in addition, you can probably remember as well, the heat wave that occurred in August and the above average temperatures that lasted through January 14th. The has allowed the ocean waters off the east coast to warm quite a bit and have not been forced to cool at all this winter.
With our first true cold intrusion of the winter this week and lasting into next week, there is a significant amount of energy available as well as instability present for a storm to take hold of and become a true meteorological bomb. This will likely occur with this upcoming storm, regardless of the nature of the track.
Here is the long term from the latest BOX AFD:
.LONG TERM /TUESDAY THROUGH SUNDAY/...
POPS CONTINUED TUE MORNING OVER SE ZONES AS SYSTEM EXITS COASTAL
WATERS S OF NEW ENGLAND. MODEL CROSS SECTIONS INDICATE MOISTURE WILL
BE SLOW TO DEPART TUE...SO WE DELAYED SIGNIFICANT CLEARING UNTIL
LATER TUE AFTERNOON OR TUE NIGHT.
WEAK HIGH PRESSURE WILL THEN BUILD OVER NEW ENGLAND WED. EXPECT A
GRADUAL INCREASE IN HIGHER CLOUDS...BEFORE CLOUDS THICKEN UP WED
EVENING IN ADVANCE OF NEXT SYSTEM.
OUR MAIN CONCERN REMAINS POTENTIAL FOR COASTAL LOW DEVELOPMENT IN
LATE WED INTO FRI TIME FRAME. ON LARGER SCALE WE HAVE A PATTERN
FAVORABLE FOR SIGNIFICANT E COASTAL CYCLOGENESIS PER RESEARCH DONE
BY SUNY ALBANY...WITH ABRUPT CHANGES FORECAST BY GFS ENSEMBLES IN
AO/NAO AND TO A LESSER EXTENT PNA REGIME LATER THIS WEEK. SO WHILE
WE ARE BECOMING MORE CONFIDENT IN A SCENARIO FAVORING RAPID
DEEPENING... WE ARE NOT YET CONFIDENT IN EXACTLY HOW FAR OFFSHORE
THIS WILL TAKE PLACE.
MODELS CONTINUE TO SHOW FAIRLY SIGNIFICANT DIFFERENCES IN THEIR
HANDLING OF BOTH NORTHERN STREAM ENERGY AS WELL AS SOUTHERN STREAM
CUTOFF OVER NM. WHILE THERE HAS BEEN A TREND TOWARD LEAVING THIS
SOUTHERN STREAM ENERGY BEHIND IN PAST FEW RUNS AND NOT ALLOWING DEEP
PHASING...IT IS NOW BECOMING MORE EVIDENT THAT NORTHERN STREAM WILL
BE KEY PLAYER IN DETERMINING WHERE SUCH RAPID DEEPENING OF COASTAL
LOW WILL OCCUR. 00Z MODEL RUNS FAVORED A MORE OFFSHORE SOLUTION...
AS UPPER FLOW WAS NOT VERY CONDUCIVE TO PULLING SYSTEM AS CLOSE TO
COAST AS SEEN IN YESTERDAYS MODEL RUNS...ESPECIALLY GFS. YESTERDAYS
ECMWF OPERATIONAL RUNS WERE CERTAINLY OUTLIERS AND ACTUALLY HAD
LITTLE SUPPORT FROM ITS OWN ENSEMBLE MEMBERS...BUT NOW GFS AND A
MAJORITY OF ITS ENSEMBLE MEMBERS HAVE TRENDED IN THAT DIRECTION.
12Z GFS HAS TRENDED A BIT CLOSER TO COAST...THOUGH IT IS STILL WELL
OUTSIDE BENCHMARK AND SYSTEM WILL BE WELL OUT TO SEA BEFORE TRUE
RAPID DEEPENING GETS UNDERWAY. TAKING MODEL AT FACE VALUE WOULD BRING
A GLANCING BLOW TO SOUTHERN NEW ENGLAND. MOST OF PRECIPITATION WOULD
BE FOCUSED FROM CAPE COD AND ISLANDS TO SURROUNDING COASTAL WATERS
WHERE WE WOULD HAVE ADDED OCEAN ENHANCEMENT FROM NORTHERLY FLOW AND
PLENTY OF COLD AIR ALOFT...DOWN TO AROUND -10C AT 850 MB. ONLY OUTER
WATERS E OF NANTUCKET WOULD STAND A CHANCE AT CHANGING TO RAIN DUE TO
WARMING BOUNDARY LAYER. LOCATIONS FARTHER INLAND WOULD SEE SOME
FRINGE EFFECTS...GENERALLY AS A RESULT OF MID LEVEL LIFT AS OPPOSED
TO DIRECT EFFECTS OF SURFACE LOW ITSELF.
GFS WOULD SUGGEST PERHAPS AN ADVISORY LEVEL EVENT FOR CAPE COD AND
ISLANDS...ESPECIALLY GIVEN PROBABLE WIND EFFECTS IN ADDITION TO
SNOWFALL...BUT IF RAPID DEEPENING WERE TO TAKE PLACE CLOSER TO NEW
ENGLAND WE WOULD POTENTIALLY HAVE A SERIOUS SNOW/HIGH WIND ISSUE
THERE.
THIS SHIFT TO A MORE NORTHWARD TRACK MATCHES VERY WELL WITH ITS 12Z
ENSEMBLE MEMBERS...AND IF ANYTHING IS A BIT ON NORTHERN ENVELOPE OF
POSSIBLE TRACKS. THIS APPEARS TO BE RESULT OF A MORE SHARPER 500 MB
TROUGH IN RESPONSE TO 2 DIFFERENT FEATURES...ONE IS MORE BLOCKING
DEVELOPING DOWNSTREAM NEAR GREENLAND AND OTHER IS INFUSION OF JET
ENERGY COMING OUT OF NORTHERN ROCKIES. WHETHER OR NOT WE SEE LATTER
ENTER PICTURE REMAINS DEBATABLE AT THIS TIME...BUT WE DO THINK
HEIGHTS WILL BE ON RISE DOWNSTREAM GIVEN EXPECTED SHIFT IN NAO
PATTERN.
WE AGREE WITH MORNING HPC ASSESSMENT OF LEANING FORECAST TOWARD MORE
CONSISTENT 00Z ECMWF BUT WEIGHTING WITH SOME OF GFS /60 TO 40/ TO
ALLOW FOR UNCERTAINTY. IN PAST WE HAVE SEEN GFS APPEAR TO LOSE
SYSTEMS OR SHOW RADICAL CHANGES IN 96 TO 120 HR TIME FRAME...ONLY TO
RECAPTURE THEM WITHIN 72 HRS. WE ARE NOT SAYING THAT IS CASE HERE...
BUT SINCE WE ARE STILL 4 DAYS OUT WE NEED TO LEAVE SEVERAL OPTIONS
ON TABLE. THEY ESSENTIALLY RUN FROM A MISS OUT TO SEA WITH POSSIBLE
OCEAN EFFECT ON CAPE COD AND MARINE IMPACTS...TO MORE OF A DIRECT
HIT WITH SYSTEM TRACKING A LITTLE OUTSIDE 40/70. ODDS FOR LATTER ARE
ABOUT 20-30 PERCENT RIGHT NOW.
12Z ECMWF SHOWS A SIMILAR EVOLUTION...BUT IS A LITTLE FARTHER
OFFSHORE THAN GFS. IT HAS MUCH LESS RIDGING OVER NORTHERN PLAINS AND
CENTRAL CANADA...AND A BIT LESS OVER GREENLAND...LEADING TO A
SOMEWHAT MORE PROGRESSIVE FLOW WHICH TAKES LOW MORE OUT TO SEA.
AS A RESULT OF ALL OF THIS...WE WILL CONTINUE WITH CHANCE POPS FROM
LATE WED NIGHT INTO THU...BUT CARRY INTO EARLY FRI ON CAPE COD DUE
TO PROBABLE OCEAN ENHANCEMENT. POP VALUES HAVE BEEN LOWERED SLIGHTLY
GIVEN A MORE OFFSHORE TRACK...BUT WE DO NOT FEEL COMFORTABLE WRITING
THIS ONE OFF JUST YET.
ONE THING THAT IS BECOMING MORE CERTAIN IS A VERY COLD AIRMASS IS
LIKELY TO FOLLOW IN ITS WAKE AND WE WILL LOWER TEMPERATURES ACROSS
BOARD FOR FRI AND SAT. OCEAN EFFECT WILL BECOME LESS OF A PLAYER AS
FLOW SHIFTS TO NW...WITH TOO MUCH OF A W COMPONENT TO AFFECT LAND
AREAS. SE COASTAL WATERS STAND A DECENT CHANCE AT SNOW SQUALLS
HOWEVER.
NEXT SYSTEM LIFTING THROUGH GREAT LAKES WILL PUSH A WARM FRONT
THROUGH NEW ENGLAND SAT. ALTHOUGH AIRMASS SHOULD BE FAIRLY DRY...SW
FLOW MAY BE ENOUGH TO PRODUCE OCEAN EFFECT ALONG S COAST AS WE HAVE
CONTINUED STEEP LAPSE RATES AT LOW LEVELS...SO WE PLACED SLIGHT
CHANCE POPS THERE. COLD FRONT COMING THROUGH SUN LIKEWISE SHOULD BE
DRY.
Also, a very well written AFD from yesterday is currently posted in the winter weather outlook section.
I would like to add, since it was mentioned in the AFD, that a very cold air mass appears more likely for this weekend behind the storm. A taste of what's to come: strong blocking remains in the GFS outlook through next week:
For the latest on the short term forecast period (which I assure you is no where near from dull) see the winter weather outlook section.
|posted by Sam Lillo @ 1/21/2007 03:08:00 PM 
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