March big dog storm has been nutered

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HPC

EXTENDED FORECAST DISCUSSION
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
139 PM EST FRI FEB 26 2010

VALID 12Z MON MAR 01 2010 - 12Z FRI MAR 05 2010



...WINTER CONTINUES WITH ANOTHER SNOW THREAT ACROSS THE INTERIOR
SOUTHEAST AND LOWER MID ATLANTIC REGION NEXT WEEK WITH AN
ADDITIONAL WIDESPREAD SNOW THREAT FROM THE SIERRAS INTO THE
ROCKIES AND REACHING THE CENTRAL PLAINS LATE NEXT WEEK...

MEDIUM RANGE GUIDANCE MAINTAINS A FAIRLY ENERGETIC PATTERN WITH
BUNDLES OF ERN PAC TROF ENERGY SEPARATING NEAR THE WEST COAST AS
THEY RUN INTO A PERSISTENT MEAN RIDGE OVER CANADA AND W-CNTRL
CONUS. THE LEADING SRN STREAM SYSTEM OF INTEREST IS FCST TO TRAVEL
FROM THE SRN PLAINS/WRN GULF COAST TO JUST OFF THE EAST COAST IN
THE MON-WED TIME FRAME. WHILE THERE IS SOME SPREAD WITH THIS
FEATURE... THE 00Z GFS AND GEFS MEAN ARE ON THE PROGRESSIVE SIDE
OF THE SPECTRUM WITH THE ECMWF/CMC/UKMET/NOGAPS AND 12Z ECMWF
ENSEMBLE MEAN ALL HOLDING THE SYSTEM CLOSER TO THE COAST BY LATE
DAY 5 WED. THIS SLOWER/WRN CLUSTER IS LIKELY BEST IN LIGHT OF THE
TENDENCY THIS SEASON FOR MODELS TO VERIFY TOO FAR SEWD SEVERAL
DAYS IN ADVANCE. ONE SIGNIFICANT WILD CARD IN THE EVOLUTION OF
THIS SYSTEM IS A BUNDLE OF ENERGY EXPECTED TO TRACK SEWD FROM THE
UPR MS VALLEY MON ONWARD. THUS FAR GUIDANCE HAS NOT BEEN
EXTREMELY AGREEABLE AMONG EACH OTHER AND COMPARED TO SUCCESSIVE
RUNS... BUT AT THE VERY LEAST THE LATEST ENSEMBLE GUIDANCE OFFERS
MORE SUPPORT FOR THE 00Z ECMWF WITH THIS FEATURE VERSUS THE
SLOW/DEEP OUTLIER SOLN OF THE 00Z GFS. MEANWHILE THE GFS/GEFS
MEAN MAY ALSO BE A LITTLE TOO FAST IN BRINGING THE NEXT ERN PAC
TROF TO THE WEST COAST DURING THE FIRST HALF OF NEXT WEEK. THE
GFS/ECMWF ACHIEVE SIMILAR TIMING BY DAY 6 THU THOUGH... AND BY DAY
7 FRI THERE IS MODERATE AGREEMENT UPON A WAVE REACHING THE SRN
PLAINS. FINALLY THE 00Z GFS IS ON THE SRN EDGE OF THE SOLN
ENVELOPE WITH THE CORE OF MID LVL ENERGY AND ASSOC SFC LOW NEARING
THE WEST COAST LATE NEXT WEEK BUT LATEST ENSEMBLE MEANS INDICATE
THE DAY 7 TROF ALOFT MAY BE A LITTLE MORE AMPLIFIED THAN THE 00Z
ECMWF. OVERALL BETTER COMPARISON TO THE FULL RANGE OF GUIDANCE
FAVORS USE OF A 50/50 BLEND OF THE 00Z ECMWF AND GFS AS THE
STARTING POINT FOR THE DAYS 3-7 FCST AN INCREASING PERCENTAGE OF
00Z ECMWF ENS MEAN CONCURRED BY USE OF GFS/EC LAGGED AVERAGE
FORECASTS.


12Z GFS CONTS ITS FAST SFC TREND COMPARED TO ALL OTHER MODELS WITH
THE TIMING OF THE GLFMEX/SERN SEABOARD LOW RESULTING IN A LOW
ABOUT 180 NM NW OF ITS ENS MEAN AND OTHER MODEL POSITIONS AND IS
AGAIN RULED OUT AS A SOLUTION OVER ERN CONUS. UKMET TRACK IS WELL
EAST OF ITS PRIOR RUN WELL OFF THE US COAST WHILE CMC IS ALSO
FASTER AND ABOUT 180 NM NE OF ITS PRIOR POSITION FOR DAY 5. LATEST
12Z ECMWF ALSO FASTER AND ABOUT 200 NM NE OF ITS EARLIER FORECAST.
INDIVIDUAL RUN DETAILS WILL CONTINUE AT THE MID LEVEL AND SFC WELL
INTO THE SHORT RANGE AND THERE IS LITTLE CONFIDENCE IN ANY TRENDS
AT THIS TIME. NO CHANGES TO AFTN FINALS FROM MORNING PRELIMS.

THE SHORTWAVE WITH A STRONG NEG HT ANOMALY MOVING ACROSS TX AND
THE GULF COAST WILL INDUCE HVY RAINS ALONG THE GULF COAST MON AND
TUES AND EWD ACROSS FL INTO GA AND SC. DEEP TROF WITH WELL BELOW
NORMAL HTS/MID/LOW LEVEL TEMPS APPEAR LIKELY TO BRING SNOW ACROSS
THE NRN EDGE OF THE PCPN SHIELD ACROSS NRN AL/GA/SRN AND ERN TN.
AS THE TROF TAKES A MORE NEG TILTED ORIENTATION HEAVIER SNOW FALL
IS POSSIBLE ACROSS THE MTNS OF SC/NC/SWRN VA AND SRN WV. HVY
COASTAL RAINS AND ONSHORE GALE FORCE WINDS WITH HIGH SURF AND
BEACH EROSION/COASTAL FLOODING THREAT CAN BE EXPECTED TUES INTO
THURSDAY ALONG THE SOUTHEASTERN SEABOARD. AS THE MOVES UP EWD
ALONG THE COAST LOW DEEPENS LOWERING 1000-500 MB THICKNESS VALUES
AND H850 TEMPS IMPLY A SIG VERY POSSIBLY HVY SNOW THREAT FROM THE
NC MTNS AND FOOTHILLS INTO THE PIEDMONT AND NEWD INTO THE LOWER VA
PIEDMONT/TIDEWATER AREAS OF VA AND POSSIBLY LOWER SRN MD AND
DELMARVA. STRONG N-NE WINDS WILL EXTEND NWD FROM THE MID ATLC
REGION ALONG THE NEW ENG COAST. DETAILS OF PCPN TYPE/AMOUNTS AND
COVERAGE ALONG WITH WIND FIELDS WILL BECOME BETTER DEFINED AS THE
EVENT CLOSES TO WITHIN A 72 HR OR LESS TIME FRAME.

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Heavy Snow Potential: The heaviest banding of snow within this system will be solely dependent upon where the greatest vertical velocity/highest uplift. Simply put, this will be a situation where surface temperatures don’t have to be 32 degrees to snow… and as we have seen happen before around here. This will be a dynamic cooling event where ice crystals will literally pull or drag down cold air from the middle and upper levels due to the amount of uplift and vertical motion involved. The amount of vertical motion will depend on how much interaction/phasing there is between the southern stream storm coming out of Texas and the northern stream energy from the polar jet. As of now, there is only partial phasing shown on model projections, but that is still enough to create a large amount of dyamics which will cool low-level temperatures quickly within the heavy bands of precipitation that form on the storms north and northwestern side.

Snow Ratios: This isn’t a forecast for snow totals or ratios, but I want to give you an idea on ratios as projections stand now. Due to 850mb/5,000ft temperatures starting out close to 0C to -1C in the Upstate, rain and snow would initially mix together and it would be an all rain start for some. As the storm strengthened, mid-level temperatures wrapping around the storm would cool quickly on the order of -1C to -3C throgh the day Tuesday. As of now, it doesn’t look like 5,000ft temperatures would dip lower than -3C, except for areas in Western North Carolina running -4C to -5C in the mid-levels.

Snow ratios at -1C or -2C would be close to 10:1. This means 10″ of snow could be produced if liquid equivalent moisture reaches 1-inch. In Western North Carolina, ratios would be closer to 12:1 and 15:1.

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BMX

BIG PICTURE...THE FAIRLY INTENSE TROUGH CURRENTLY OVER THE WEST
COAST WILL PRESS EASTWARD THROUGH THE WEEKEND...AND BY MONDAY
NIGHT...SPIN UP ANOTHER GULF LOW FOR OUR ENJOYMENT. MODELS ARE IN
FAIRLY GOOD AGREEMENT ON THE TIMING AND STRENGTH OF THE LOW...BUT
THE COLD AIR REMAINS TO BE THE BIGGEST QUESTION. WITH THE WARM UP
WE WILL SEE OVER THE WEEKEND...ITS GOING TO TAKE A PUSH OF COLD AIR
TO DIP TEMPERATURES LOW ENOUGH MONDAY NIGHT TO SEE THE FROZEN
STUFF. UPPER LEVEL MODEL SOUNDINGS SHOW TEMPERATURES PROFILES
CONDUCIVE TO CRYSTAL GROWTH...BUT THE LOWEST LAYERS SOUTH OF A LINE
FROM DEMOPOLIS TO ALEXANDER CITY LOOK TO BE JUST A LITTLE TOO WARM
TO SUPPORT SNOW REACHING THE GROUND. SOME WET-BULBING COULD BE
SEEN...BUT RIGHT NOW...WET-BULB PROFILES ARE SLIGHTLY ABOVE FREEZING
AS WELL. BEING IT A NIGHT 3/DAY 4 FORECAST...AND THE FACT THAT
MODELS ARE IN PRETTY GOOD AGREEMENT WITH TIMING...THE BIGGEST CHANGE
I MADE TO THE FORECAST THIS MORNING WAS DROPPING THE POTENTIAL
RAIN/SNOW LINE A LITTLE FURTHER TO THE SOUTH...MAINLY BECAUSE OF
MODEL THICKNESS VALUES...WHICH SUPPORT AT LEAST FLURRIES WITH
CURRENT QPF GUIDANCE ON THE LIGHT SIDE...ESPECIALLY ACROSS THE
NORTHERN HALF OF THE CWA. BECAUSE OF THE FACT THAT FREEZING
TEMPERATURES ARE NEXT EXPECTED TO ALREADY BE IN PLACE...LIKE WE SAW
ON THE FEB 12 EVENT...AND THAT THE COLDER AIR NEEDS TO CREEP IN
AFTER THE RAIN STARTS MONDAY EVENING...I HAVE LEFT /RAIN OR SNOW/
FOR ONE MORE NIGHT...AND HOPE FOR MODEL CONSISTENCY TOMORROW NIGHT.

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HPC

.SYSTEM INITIALLY NEAR THE WEST COAST...

PREFERENCE: 12Z GFS AND 12Z ECMWF

THE 27/12Z NAM WAS ALSO ON THE SLOWER SIDE OF THE PREVIOUS
OPERATIONAL AND ENSEMBLE GUIDANCE BY MAINTAINING A SPEED
CONSISTENT WITH ITS 27/00Z RUN. THE GFS HAD BETTER CONTINUITY
THAN THE NAM THROUGH THE END OF DAY 2...WHICH KEEPS THE MID LEVEL
CENTER/TROF AXIS AHEAD/EAST OF THE 12Z NAM. THE RESULTING SFC
REFLECTION SHOWED A SIMILAR EASTWARD SHIFT AND DEEPER LOW IN THE
GFS COMPARED WITH THE NAM. THE GFS IDEA OF A FASTER SOLN WAS
SUPPORTED BY THE 12Z UKMET AND CANADIAN. THERE IS A PERIOD LATE ON
DAY 1 INTO EARLY DAY 2 WHEN THE 12Z ECMWF GAVE LESS SUPPORT TO THE
FASTER GFS SOLN...BUT IT CAME INTO GOOD AGREEMENT WITH THE FASTER
SOLN AS THE WAVE PROPAGATES EASTWARD AND WAS CLOSER TO THE
NAM/ENSEMBLE MEAN FOR MOST OF DAY 2 AND THROUGHOUT DAY 3. THUS A
GFS/ECMWF SOLN LOOKS REASONABLE.

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Resized to 83% (was 1516 x 1060) - Click image to enlarge

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MOB

There could be a time window a
couple of hours either side of 12z Tuesday where some of the precipitation
over our northwestern zones could potentially be of the frozen
type...primarily along and north of a Waynesboro...Grove
Hill...Camden line. Have not included any mention of wintry precipitation at
this time as surface min temperatures are forecast to be in the 34 to 38 degree
range and the boundary layer overall appears to be a little too
warm. In addition...wet bulb zero heights are noted to be around
1500-1700 feet. With only a few minor deviations a wintry mix of
precipitation could be possible over the northwestern zones however...and we
will have to monitor this for any possible changes.

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NAM 42 Hours




NAM 48 Hours


NAM 54 Hours


NAM 60 Hours

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From Tim Coleman

Snow analysis – 930 am

February 28, 2010, 9:28 am | Dr. Tim Coleman | Forecast Discussion


Dynamically, the setup over the next 48 to 60 hours looks *almost* classic for snow here in central Alabama. An upper level disturbance will come in from the west, and interact with a thermal boundary near the Gulf Coast, producing a low pressure area due to divergence aloft lowering the surface pressure, and the warm air moving northward ahead of the low (that is less dense) causing faster pressure falls along the front. The models differ quite a bit on the northward extent of heavy precipitation, but the consensus is that about 0.25″ of liquid water will fall near BHM, with higher amounts over south Alabama.
The air will also be cold enough aloft so that the precipitation will fall as snow, at least aloft. Even though surface temperatures Monday afternoon may reach 50 degrees, snow falling should melt as it gets below the freezing level (near 4,000 feet at BHM Monday afternoon), using up heat and cooling the air more. So, the precipitation should begin as rain Monday afternoon, then change over to snow Monday night.
This system is particularly difficult because it will be moving slower than most systems (see the 500 mb loop), allowing warm air to spread northward ahead of it today and tomorrow. We will likely reach the 50s with sunshine today, and even with clouds moving in, reach well into the 40s Monday. Our days are longer now (over 11 hours of daylight), and the sun angle is higher in the sky, meaning at least 50% more heat is coming in from the sun now than it was in late December. And, there is no extreme Arctic air to our north (like there was in 1993). Take a look at model surface temperatures Monday afternoon.

The closest freezing temperatures at the surface will be near Lexington, Kentucky. So, the cold has to be brought down with the snow. It will snow, probably quite a bit, 3,000 feet above the ground. The heavier the precipitation is, the more cooling, and more snow will reach the ground. It is likely that 1″ or more of snow will fall in BHM, with some places south of I-20 getting 2-3″. But, then we have the problem of warm ground temperatures, after sunny skies tomorrow. With surface temperatures struggling to go below freezing (unless we get very heavy precipitation, a small but important possibility), it is likely that little accumulation will occur, with the 1″ of snow melting as it falls or shortly afterwards. There could still be road problems Tuesday night, as melted snow freezes on roads.
So, the bottom line is that it will take a lot of precipitation (on the order of 0.50″ or more) to bring enough cold down to the ground to allow significant accumulation. That is possible, and it depends on the strength of the system. The most likely event will be rain changing to snow Monday night, and the snow may even be heavy at times. However, warm temperatures near the ground will help the snow to melt, leaving 1″ or less on the ground by the time it tapers off Tuesday afternoon. I.e., it will be pretty as it falls, but not stick much. If the system is more intense than the consensus, the mass of snow falling could bring temperatures down to freezing Tuesday morning, especially on ridge tops in east Alabama, producing snow accumulations and icy roads.
A lot can change with this system as it comes out of the mountains today and tonight, so keep an eye on this blog for updates.

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BMX

WINTER PRECIPITATION POSSIBLE MONDAY NIGHT INTO TUESDAY...

A STRENGTHENING LOW PRESSURE SYSTEM WILL CROSS THE NORTHERN GULF
OF MEXICO MONDAY AND ACROSS NORTHERN FLORIDA BY TUESDAY AFTERNOON.
AHEAD OF THIS SYSTEM...MOISTURE WILL SPREAD INTO CENTRAL ALABAMA
MONDAY AFTERNOON RESULTING IN WIDESPREAD RAINFALL. COLDER AIR WILL
FILTER INTO THE AREA MONDAY NIGHT FROM THE NORTHWEST...RESULTING
IN THE POSSIBILITY OF WINTER WEATHER.

AT THIS TIME...RAIN IS EXPECTED TO TRANSITION TO SNOW ALONG AND
NORTH OF A LINE FROM EUTAW TO CLANTON TO ROANOKE EARLY TUESDAY
MORNING...CONTINUING THROUGH THE MORNING HOURS AND ENDING FROM
WEST TO EAST DURING THE EARLY AFTERNOON. DUE TO WARM GROUND
TEMPERATURES...SNOWFALL ACCUMULATIONS MAY BE LIMITED...WITH LESS
THAN AN INCH EXPECTED ACROSS THE AREA. IN THE HIGHER ELEVATIONS OF
NORTHEAST CENTRAL ALABAMA...AROUND ONE INCH WILL BE POSSIBLE.

CONSIDERABLE UNCERTAINTY REMAINS WITH THIS SYSTEM...AND MINOR
CHANGES IN STORM TRACK AND INTENSITY COULD HAVE A SIGNIFICANT
IMPACT ON THE POSSIBILITY OF WINTER PRECIPITATION. THE RAIN SNOW
TRANSITION LINE AND ACCUMULATION TOTALS WILL LIKELY NEED TO BE
REFINED WITH FUTURE FORECASTS. CENTRAL ALABAMA SHOULD STAY TUNED
FOR UPDATES ON THIS DEVELOPING WINTER WEATHER SITUATION.