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December 2011

National Weather Service, Taunton, MA, verifies 79 mph Thursday morning wind in Rhode Island

Fb1Wind this morning was impressive, to say the least - this image maps peak wind gust reports.  Those who were online last night may have seen my real-time analysis post as the storm strengthened quickly over the Mid-Atlantic, and a strong pressure fall/rise couplet appeared with the storm, in addition to rapid subsidence (sinking air) evident on water vapor imagery behind the storm.  Both of these points are referenced in the excellent explanation from the Taunton, MA, National Weather Service Weather Forecast Office regarding the maximum recorded wind gust Thursday morning of 79 mph at Point Judith, RI.

Glenn Field at NWS Taunton shares:

"You may have been wondering about the validity of the 79 mph wind report we listed from Point Judith, RI from 4 AM this morning in our PNS.  Upon reviewing this further, we have concluded that it is, in fact, likely to be a real occurrence. (There were also reports of tree and wire damage at Westerly and Scituate and a 63 mph wind gust at Burrillville.)  At first thought, you might think it was thunderstorm-related, but the thunderstorms were past there by then.  Radar imagery showed a pronounced back edge to the rapidly deepening cyclone.  There was a tremendous pressure rise-fall couplet associated with back side of the storm.  Forecast model data showed that the 1.5 PV (potential vorticity) surface, which is the best PV surface to approximate the tropopause, was lowering to approximately 550 mb at that time...quite low.  If you recall the December 9th event from several years ago, where there were 100+ mph winds gusts with thunderstorms and snow squalls along the south coast/Cape Cod/Islands...the tropopause surface had 'folded' down to as low as 700 mb in that event.  In recent years, this feature has gained some notoriety and has been dubbed a 'sting jet' in the professional literature.  There was an article in the November, 2010 edition of the Monthly Weather Review journal entitled "Sting Jets in Simulations of a Real Cyclone by Two Mesoscale Models."  The original article is from 2004 by K.A. Browning, published in the Quarterly Journal of the Royal Meteorological Society ("The Sting at the End of the Tail...").  

http://onlinelibrary.wiley.com/doi/10.1256/qj.02.143/pdf

[Thanks to Frank Nocera and Joseph Dellicarpini at BOX for providing the info and references on the sting jet.]"


Flood Warnings issued in response to heavy rain

FbAs of this late-night writing on Wednesday night, flood warnings have been issued for minor flooding along the Housatonic River below Stevenson Dam in Connecticut, and the Pawtuxet River near Cranston - both for minor flooding that will crest Thursday afternoon.  I also expect the Suncook River in New Hampshire to require a flood warning for minor flooding.  The image included here is an example of the hydrograph used to monitor river levels.  In this case, the Housatonic River gage at Stevenson is shown from 11 PM Wednesday, when the river's observed stage (blue) was just about to cross from "Action Stage" into official "Flood Stage."  Note the forecast line that peaks in Minor Flood, at a stage of 12 feet, at which the water may reach the Maples area of Shelton, CT.

To check the latest river condition and forecast from the Northeast River Forecast Center, you may want to bookmark this site:

http://www.erh.noaa.gov/er/nerfc/


Technical: Storm Prediction Center keys in on heavy snow band

A heavy snow band is progressing east across portions of New England, and the National Weather Service Storm Prediction Center is tracking the roughly one inch per hour band of heavy snow.  Here was their take as of 10:30 PM EST:

Fb

MESOSCALE DISCUSSION 2376
   NWS STORM PREDICTION CENTER NORMAN OK
   0925 PM CST WED DEC 07 2011
  
   AREAS AFFECTED...ERN PA/NY...PORTIONS OF NJ/CT/MA...MUCH OF VT/NH/ME
  
   CONCERNING...HEAVY SNOW
  
   VALID 080325Z - 080730Z
  
   A NARROW CORRIDOR OF HEAVY SNOW /1 IN AN HOUR RATES/ WILL CONTINUE
   TO SHIFT EWD AND NWD WITH TIME WHILE GRADUALLY TAPERING OFF ALONG
   THE WRN PERIPHERY.
  
   AS THE PHASING UPPER TROUGHS CONTINUE TO EJECT NEWD...ASSOCIATED
   LOWER TROPOSPHERIC WARM ADVECTION IS GRADUALLY SHIFTING OFFSHORE.
   COLD ADVECTION IS BECOMING REINFORCED ACROSS PORTIONS OF THE
   NORTHEAST IN ITS WAKE...WITH COOLING TEMPERATURES PARTIALLY
   RESULTING FROM DIURNAL COOLING. THE RAIN/SNOW LINE HAS NOW
   PROGRESSED EWD...AS OF 03Z ORIENTED ROUGHLY THROUGH NRN MAINE SWWD
   THROUGH RUTLAND VT...WEST OF ALBANY NY...AND THROUGH WILKES-BARRE
   PA. HERE...A NARROW CORRIDOR OF HEAVIER SNOWFALL RATES EXISTS
   BETWEEN GREATER MOISTURE TO THE EAST /CHARACTERIZED BY PWAT VALUES
   FROM 0.85 TO NEAR 1.25 INCH PER AREA 00Z SOUNDINGS/ AND THE DRIER
   AND COLDER AIR MASS TO THE WEST. ALTHOUGH BRIGHT-BANDING HAS
   OCCURRED ON RADAR...A SNOW BAND HAS BEEN NOTED FROM NEAR
   WILKES-BARRE PA THROUGH THE CATSKILLS...WHICH WILL GRADUALLY SHIFT
   EWD AND NWD IN CONJUNCTION WITH THE UPPER TROUGH.
   ADDITIONALLY...HEAVIER RATES WILL GRADUALLY DIMINISH TO THE WEST OF
   THE BANDING FEATURE AS IT TRAVERSES EWD AND NWD.
  
   ..HURLBUT.. 12/08/2011
  
  
   ATTN...WFO...CAR...GYX...BOX...BTV...OKX...ALY...PHI...BGM...
   CTP...


8:45 PM EST Surface Analysis: Dual low-pressure...trailing storm center to become dominant storm

SAT_RAD_ANALYSIS_ACTIVE8:30 PM EST surface analysis - two areas of low pressure.  The leading storm center will essentially be absorbed by the second storm center, which is zipping northeast and will rapidly intensify.  Note the baroclinicity between 73 degrees at the buoy east of Cape Hatteras (and 60s on the Outer Banks) and temperatures in the 30s across the Appalachian Mountains.  This temperature contrast, combined with the important factors described last hour in the real-time analysis post, is helping to fuel rapid storm development.


Real-time analysis: Rapid storm intensification underway along the Mid-Atlantic coast

We're witnessing an amazing intensification of the storm center marching up the Interstate 95 corridor.  The first evidence of this intensification came in water vapor satellite imagery, in which the colors here show purple and green for deep moisture, orange for dry air.  Note the tremendous intrusion of dry air moving north over the Carolinas, a sign of intense sinking air on the backside of the northward moving, intense, upper level disturbance.  Ahead of the energetic upper level disturbance, note the fan shape of moisture, indicative of upper level divergence - air moving away from itself.  This results in rising air from lower levels of the atmosphere to fill the void aloft, and rising air is the key to cloud growth, lowering surface barometric pressure, and storm development.

Fb1

Of course, the barometer is one of the oldest and most reliable instruments in weather observation, so monitoring the fluctuations in the air pressure can be an important indicator of what's to come.  Lowering atmospheric pressure indicates a strengthening storm, and monitoring the center of fastest falling pressure often indicates the next position of the storm.  Note the two barometric pressure charts, below - the first shows current pressure, illustrating where the storm center is, and the second is a pressure tendency chart, showing where the storm may move next.

Surface barometric pressure, showing the storm center shifting rapidly from Virginia toward Chesapeake Bay:

Fb

Pressure Tendency, showing rapid pressure fall/rise couplet, indicating the likelihood of a developing swath of strong to damaging wind:

Fb2
All of this adds up to a rapidly strengthening storm, lending credence to the forecast of heavy rain - and heavy snow - for the Northeast corridor.  At the time of this writing, snowfall amounts of 9"-10" have been reported in some locales of West Virginia and Maryland, moving northeast.  Additionally, several wind damage reports are coming in from Virginia, where the combination of pronounced drying on the water vapor imagery, and a strong pressure fall/rise couplet, support the damaging wind event.  These strong winds will sweep through Long Island overnight, and New England early Thursday morning.


With rain to snow transition imminent in Northern & Western New England, fine tuning to accumulation forecast

Warm air is well entrenched across New England and it will be a slow process for cold air to take over enough for snow.  The first areas to see the change from rain to snow will be Northern New England and the higher terrain, with Western New England next as cold air moves in from both the north and west.  Highest snow amounts should be found in Central and Southern Vermont - including Okemo, Bromley, Stratton and Killington.  Of course, while those should be the highest snowfall totals, all of New England's Ski Country looks to fare well, picking up a heavy, wet snow to serve as a terrific base for building.  In Central and Southern New England, the onset of cold air will be slow enough that most of the precipitation will fall as rain - 1.5" to 2" of it, with big puddles and some stream flooding possible - with just a possible burst of snow pre-dawn Thursday as the precipitation winds down.

Southern New England Snowfall Forecast:

ACCUMS_ACTIVE_SNE

VT/NH Snowfall Forecast:

ACCUMS_ACTIVE_VT_NH

Maine Snowfall Forecast:

ACCUMS_ACTIVE_ME


Early Snowfall Estimate shows a healthy dose for Ski Country

Though there is time for forecast modification, New England's Ski Country looks to fare well from the storm slated for Wednesday night into Thursday morning.  There will unquestionably be some elevation dependency to snowfall totals when all is said and done, especially considering the mild antecedent air that will be scoured out by an increasing but relatively light northerly wind.  Nonetheless, while cold air may only creep southward, it appears as though enough cold air will trickle into New England to change precipitation to snow in Northern New England before the bulk of it falls.  Farther south, the snowfall forecast is more uncertain, and may need to be adjusted when the precise timing of cold air is determined.  At this point, it looks like a brief burst of early-morning snow will occur in much of Southern New England, but the cold air arrives just as the moisture is ready to shut down, again meaning some elevation dependency for amounts.  If any changes need to be made in Southern New England, it likely would be to reduce forecast amounts a bit/peel the eastern edge farther west, if cold air is too sluggish to arrive.

Southern New England early snowfall estimate:

ACCUMS_ACTIVE_SNE

Vermont/New Hampshire early snowfall estimate:

ACCUMS_ACTIVE_VT_NH

Maine early snowfall estimate:

ACCUMS_ACTIVE_ME


8 to 14 day forecast: Colder pattern will not persist through next week

8_TO_14_DAY_TEMPSA shot of colder-than-normal air at the end of this week will mark the first prolonged chill of winter for New England, but this week's 8 to 14 day forecast reveals the pattern is not likely to hold for the Northeast.  A trough in the jet stream will remain in place across the Western United States, and that's where the coldest air will pool next week, with the mean storm track developing along the eastern edge of the cold dome - likely to run through the center of the country.  This puts the Eastern United States on the warm side of a frontal boundary, yet again.  That said, a fast jet stream wind aloft, steering several energetic disturbances, will allow for one moderate storm near the beginning of the week that will briefly tap tropical moisture before high pressure builds across the Southeast US later in the week.  8_TO_14_DAY_PRECIPHere in the Northeast, the active jet stream pattern will likely pull just enough cold air southward to keep near-normal temperatures in Northern Maine, but also will bring a couple of energetic disturbances through that may produce rain or snow - one Tuesday or Wednesday, and another toward the end of the week or weekend.