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Winter weather "event" - again
hawklady wrote in atlanta
This is one of the best almost-layman's explanations I've seen yet of the factors behind this week's winter storm, and why there's so much freak-out.

It also gives an idea of just how frickin' many "moving parts" are involved in this type of event and making the forecast "challenging". It's particularly challenging with winter wx, where a couple of degrees makes all the difference as far as what happens on the ground is concerned.

PROBABILISTIC HEAVY SNOW AND ICING DISCUSSION
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
439 AM EST TUE FEB 11 2014



VALID 12Z TUE FEB 11 2014 - 12Z FRI FEB 14 2014

...SOUTHERN PLAINS/LOWER MISSISSIPPI VALLEY TO SOUTHEAST AND UP
EAST COAST...
 
A HIGH SOCIETAL IMPACT WINTER STORM IS IMMINENT ACROSS THE SRN
TIER OF THE COUNTRY AND VERY LIKELY TO PROCEED UP THE EAST COAST
ON WED AND THURS... BASICALLY EXTENDING FROM TX/LA TO NEW ENGLAND.
 
A VERY COMPLEX UPPER PATTERN WITH SPLIT FLOW REGIME ACROSS THE ERN
TWO-THIRDS OF THE COUNTRY... WHILE A MAMMOTH DOME OF SURFACE
ARCTIC HIGH PRESSURE CONSUMES THE ENTIRE REGION AND ESPECIALLY
EAST OF THE APPALACHIANS. AN IMPRESSIVE AND DYNAMIC IMPULSE OVER
THE MID MS VALLEY WILL GLIDE ALONG THE NRN STREAM AND REACH THE
NORTHEAST COAST ON TUES. MEANWHILE... THE LAST SHORT WAVE THAT
IMPACTED THE CENTRAL CA COAST WILL ADVANCE DOWNSTREAM ALONG THE
ACTIVE SRN BRANCH AND TRACK ACROSS TX TO THE LOWER MS VALLEY.
ALREADY MOISTURE IS OVERRUNNING THE QUASI-STATIONARY FRONT NEAR
THE GULF COAST FOR A WELL-ESTABLISHED PRECIP SHIELD ACROSS THE
SOUTH CONTAINING SNOW AND FREEZING RAIN. THIS SHIELD OF MOISTURE
WILL ADVECT DOWNSTREAM DURING THE DAY INTO THE SOUTHEAST OR WITH
THE PACE OF THE NRN STREAM IMPULSE. THIS WILL RESULT IN LOCALLY 1
TO 3 OR 4 INCHES OF SNOW FROM NRN AL/GA THROUGH UPSTATE SC INTO
SERN/ERN NC WITH POSSIBLE .10 TO .50 INCH OF ICING THREAT FROM
NRN/CENTRAL GA THROUGH MUCH OF SC INTO SERN NC AND THE BEGINNING
OF A PARALYZING ICE STORM. AS THE SRN STREAM FEATURE CROSSES TX TO
THE LWR MS VALLEY... SOME SLIGHT AMPLIFICATION WILL OCCUR AS PAC
DYNAMICS NEAR THE NW WILL DIG DOWN THE FRONT RANGE OF THE ROCKIES
INTO THE CENTRAL PLAINS. THUS THE GENERALLY SUPPRESSED PRECIP
SHIELD ACROSS TX/LA WILL BEGIN TO EXPAND AND LIFT ACROSS THE SOUTH
AND TOWARD THE SRN APPALACHIANS... WHILE INDUCING A WAVE OF LOW
PRESSURE NEAR THE CENTRAL GULF COAST. THIS WOULD RESULT IN SOME
LIGHT AND MINIMAL ACCUMULATIONS OF SNOW ACROSS NORTH TX BUT A
DECENT AMOUNT OF ICING FROM THE HILL COUNTRY OF TX THROUGH ERN
TX/MUCH OF LA TO NORTH CENTRAL MS. WPC STAYED VERY CLOSE TO A
COMPROMISE OF THE 00Z GFS... ECMWF AND 21Z/10 SREF MEAN.
 
THE DIGGING UPSTREAM DYNAMICS OVER THE CENTRAL PLAINS WILL RAPIDLY
CATCH UP WITH THE SRN STREAM DYNAMICS TO POTENTIALLY PHASE OVER
THE SERN PORTION OF THE COUNTRY. THIS WILL ALLOW A SIGNIFICANT
INCREASE IN MOISTURE ADVECTION FROM THE GULF OF MEXICO AND
ATLANTIC TO INTERACT WITH A DEVELOPING 
COASTAL LOW ALONG THE SOUTHEAST COAST... WHILE ONE OF THE MORE
IMPRESSIVE CONFLUENT ZONES ALOFT KEEPS THE ARCTIC AREA OF HIGH
PRESSURE ENTRENCHED OVER THE EAST. A TRUE BAROCLINIC LEAF PRECIP 
SHIELD WILL EVOLVE FEATURING MAJOR ICING FROM ATL TOWARD CAE THEN
SHIFTING NORTH TOWARD RDU AND A VERY HEALTHY DEFORMATION ZONE OF
BANDING SNOWFALL FROM NRN GA/SWRN NC/NWRN SC NEWRD TO SWRN/CENTRAL
VA BEFORE REACHING THE DC/BALT/PHIL METRO AREAS. THE ICE
ACCUMULATIONS REMAIN MIND-BOGGLING IF NOT HISTORICAL WITH
ADDITIONAL .50 TO 1 INCH AXIS FROM ATL/AHN/AGS TO CAE TO RDU AND 8
TO 10 INCHES OF SNOW FROM AVL/GSP TO ROA/LYH/CHO AND JUST SOUTH OF
IAD/DCA WITH A POSSIBLE FOOT NEAR SWRN NC/NWRN SC. WPC CONTINUED
THE COMPROMISE OF THE GFS/ECMWF/UKMET WITH SUPPORT FROM THE SREF
MEAN FOR THE BEGINNING OF A CLASSIC EAST COAST WINTER STORM.

THEN ON THURS... THE INTENSIFYING SURFACE LOW GOING THROUGH RAPID
CYCLOGENESIS WILL TRACK FROM THE OUTER BANKS OF NC TO THE NEW
ENGLAND COASTLINE. THIS WILL RESULT IN A WEALTH OF ATLANTIC
MOISTURE SURGING INTO THE NRN MID-ATL STATES AND NORTHEAST FOR A
NARROW BUT IMPRESSIVE AXIS OF HEAVY SNOWFALL. THE GUIDANCE IS ALL
UNANIMOUS IN A WELL-ESTABLISHED DEFORMATION AXIS/COMMA HEAD OF
HEAVY SNOW BUT DIFFER ON THE EXACT CORRIDOR. WPC TOOK A MEAN
APPROACH OR TAKING OUT THE SPECIFIC DETAILS AND RESOLUTION OF THE
GUIDANCE FOR A HEAVY SNOW AXIS JUST WEST OF THE I95 CORRIDOR FROM
DC/BALT AREA TO BOS AND CONTINUING UP INTO MAINE. THE EXACT
AMOUNTS ON THURS ARE STILL SOMEWHAT UNCERTAIN BUT A STRIPE OF 6 TO
12 INCHES WILL BE POTENTIALLY POSSIBLE FROM NERN PA/NWRN NJ
THROUGH DOWN EAST MAINE. 
 
...PAC NW TO NRN INTER-MOUNTAIN WEST... 
 
AN UPPER VORTEX BREAKING DOWN OVER THE NRN PAC WILL RESULT IN A
STEADY STREAM OF UPPER DYNAMICS PROGRESSING INTO THE NW QUAD OF
THE COUNTRY FOR THE NEXT SEVERAL DAYS. A BATCH OF POWERFUL SHORT
WAVE DYNAMICS WILL APPROACH THE PAC NW ON TUES... UPPER HEIGHTS
WILL RISE SOME BEFORE A STRONG OCCLUDED SURFACE COLD FRONT PLOWS
INTO WA/OR. THIS SHOULD RESULT IN HEAVY SNOW FROM THE WA CASCADES
INTO THE NRN INTER-MOUNTAIN WEST. THEN ON WED... A SHORT WAVE
ORIGINATING FROM THE BASE OF A LONG WAVE TROUGH OVER THE PAC WILL
REACH THE NW. THIS SYSTEM APPEARS TO CAPITALIZE ON THE GREATEST
MOISTURE TRANSPORT RESULTING IN WIDESPREAD HEAVY OROGRAPHIC
SNOWFALL FROM THE WA CASCADES INTO THE NRN ROCKIES/BITTERROOTS AND
TETONS. FINALLY ON THURS... A CLOSED UPPER LOW NEAR THE TAIL END
OF THE ALEUTIANS WILL ROUND THE UPPER TROUGH AND APPROACH THE NW.
THIS SHOULD SPELL MORE HEAVY SNOWFALL FOR THE TERRAIN.


MUSHER

(source URL: http://www.hpc.ncep.noaa.gov/discussions/qpfhsd.html)


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Oh, yeah. love the scientific discussion behind the terse forecasts. On WeatherUnderground, you can see the NWS in Peachtree City discussing the local situation.
http://classic.wunderground.com/DisplayDisc.asp?DiscussionCode=FFC&StateCode=GA&SafeCityName=Doraville

Ah, I thought that looked familiar. It's the AFD - Area Forecast Discussion - which is what I usually end up posting from when I blather about wx :)

It's nice being able to get the background behind the forecast instead of simply the "what we expect to happen" summary.

The FFC NWS URL for it is http://forecast.weather.gov/product.php?site=NWS&issuedby=FFC&product=AFD&format=CI&version=1&glossary=1

Stuff's moving in on the radar - the Dual-Pol upgrade that FFC got offers HCA now too! - but I'm not sure how much is making it to the ground as anything other than rain.

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