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Possible snow->rain->snow? event, Tuesday-Thursday 3/3-5/2015


famartin

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GFS is solid. About 0.60" as snow at TTN. Seems we've reached near-universal agreement.

Crazy! Still sitting at 37.7 with all this moderate rain, but yet we still have quite a bit of QPF after changeover. Amazing these late winter storms can be with the moisture feed.

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Actually, if the last 5000 feet are at 33 and its saturated, it will be rain at the surface.  5000 feet is a long way for a snowflake and you actually have liquid water droplets at much lower temps than freezing.  If its above freezing and saturated, its not hard to melt the flakes as it runs into all those little droplets.  However, melting takes energy from the atmosphere, so gradually, assuming no advection, the continued melting of snowflakes would slowly cool the column from 5000 feet on down to the surface to 32, at which point it would snow. 

 

The whole "transporting cold air down" is actually mainly based on the heat absorbed from melting snowflakes. 

 

I see what you're saying about flakes at 33F and 5000 feet of saturated column with liquid droplets present - in that case, yeah, the flakes should melt, but as you said that requires heat from the atmosphere, which would eventually cool the column and allow the snowflakes to make it to the ground.  I guess I was thinking more of what happens when the snowfall generation rate in the dendriditc nucleation and growth region, 10-15K feet up at -12 to -18C (let's say -15C, on average, or 5F) is very high, due to intense vertical velocities bringing relatively warm/moist air up from below.  I would think that a very heavy snowfall rate of flakes that are all 5F or so could provide enough momentum to truly drag some of the local cold air down with them through the column, such that maybe only a fraction of the snowflakes would ever reach the melting point and melt while falling through the last few thousand feet at 33F, resulting in snow reaching the ground, especially if the column isn't saturated (not all of the snowflakes are going to melt/evaporate to saturate the column).  

 

Also, as an aside, I'm sure you or others have done this calculation, but it surprises me a little bit that 5F snowflakes falling through a 33F column for ~5000 feet, even if saturated, would result in all of the flakes melting.  I guess I would have thought that the limited time to traverse those 5000 feet (curious to know what that flake velocity is, so one could calculate the residence time of a flake in that 33F column) would not be enough to allow both the warming up of the snowflake from 5F to 32F and then the melting of the flake at 32F. especially since the latter requires a fair amount of heat for the phase change.  Maybe this is not a good comparison, but I think about how long it takes to melt snow on the ground at 33F.  

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I see what you're saying about flakes at 33F and 5000 feet of saturated column with liquid droplets present - in that case, yeah, the flakes should melt, but as you said that requires heat from the atmosphere, which would eventually cool the column and allow the snowflakes to make it to the ground.  I guess I was thinking more of what happens when the snowfall generation rate in the dendriditc nucleation and growth region, 10-15K feet up at -12 to -18C (let's say -15C, on average, or 5F) is very high, due to intense vertical velocities bringing relatively warm/moist air up from below.  I would think that a very heavy snowfall rate of flakes that are all 5F or so could provide enough momentum to truly drag some of the local cold air down with them through the column, such that maybe only a fraction of the snowflakes would ever reach the melting point and melt while falling through the last few thousand feet at 33F, resulting in snow reaching the ground, especially if the column isn't saturated (not all of the snowflakes are going to melt/evaporate to saturate the column).  

 

Also, as an aside, I'm sure you or others have done this calculation, but it surprises me a little bit that 5F snowflakes falling through a 33F column for ~5000 feet, even if saturated, would result in all of the flakes melting.  I guess I would have thought that the limited time to traverse those 5000 feet (curious to know what that flake velocity is, so one could calculate the residence time of a flake in that 33F column) would not be enough to allow both the warming up of the snowflake from 5F to 32F and then the melting of the flake at 32F. especially since the latter requires a fair amount of heat for the phase change.  Maybe this is not a good comparison, but I think about how long it takes to melt snow on the ground at 33F.  

 

Maximum fall speed of snowflakes is about 6 feet per second.  For them to traverse 5000 feet, it would take about 14 minutes.  Many snowflakes fall at slower speeds, with some falling at only 1 foot per second.  Obviously, it depends on the flake's geometry.  In any case, 14 minutes is plenty of time for melting.  Consider that when falling through saturated air, both heat from air and from droplets they strike are absorbed by the flakes.  So, at least at first, melting would be fairly quick.  Of course, if the rate is heavy and advection is minimal, it wouldn't take long for the column to cool enough for snow to reach the ground.

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Maximum fall speed of snowflakes is about 6 feet per second.  For them to traverse 5000 feet, it would take about 14 minutes.  Many snowflakes fall at slower speeds, with some falling at only 1 foot per second.  Obviously, it depends on the flake's geometry.  In any case, 14 minutes is plenty of time for melting.  Consider that when falling through saturated air, both heat from air and from droplets they strike are absorbed by the flakes.  So, at least at first, melting would be fairly quick.  Of course, if the rate is heavy and advection is minimal, it wouldn't take long for the column to cool enough for snow to reach the ground.

 

Interesting comparison regarding the maximum speed at which  a snowflakes falls - 5000 feet in 14 minutes is about 4 mph which is (for me) 2nd gear walking speed. Funny to think that a snowflake falls at the speed at which we walk. Hmmm...

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Maximum fall speed of snowflakes is about 6 feet per second.  For them to traverse 5000 feet, it would take about 14 minutes.  Many snowflakes fall at slower speeds, with some falling at only 1 foot per second.  Obviously, it depends on the flake's geometry.  In any case, 14 minutes is plenty of time for melting.  Consider that when falling through saturated air, both heat from air and from droplets they strike are absorbed by the flakes.  So, at least at first, melting would be fairly quick.  Of course, if the rate is heavy and advection is minimal, it wouldn't take long for the column to cool enough for snow to reach the ground.

Cool stuff - thanks for indulging me.  Didn't think the max speed would be that low - is that due to a higher coefficient of friction factor based on shape, as opposed to a rain drop (more surface area in contact with the air for a snowflake), which I presume would fall faster?  Knew I should've majored in meteorology instead of chem eng'g, lol - might finally be time for some formal education at dear old RU, since I'm only 10 minutes away - I'm guessing I'd be the oldest one there...

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Cool stuff - thanks for indulging me.  Didn't think the max speed would be that low - is that due to a higher coefficient of friction factor based on shape, as opposed to a rain drop (more surface area in contact with the air for a snowflake), which I presume would fall faster?  Knew I should've majored in meteorology instead of chem eng'g, lol - might finally be time for some formal education at dear old RU, since I'm only 10 minutes away - I'm guessing I'd be the oldest one there...

Yes.  Hey, the more RU met grads, the merrier ;)

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Here's my AFD, very busy so not much posting today. 

 

NOW IS IN PROGRESS ACROSS THE REGION WITH AREAS ACROSS SOUTHEAST
NJ AND THE SOUTHERN DELMARVA CHANGING FROM SLEET AND RAIN TO SNOW
IN THE NEXT FEW HOURS. THE BACK EDGE OF THE SNOW IS APPROACHING
THE NORTHWEST PARTS OF THE REGION. CAA WILL CONTINUE TO PUSH INTO
THE REGION BEHIND A COLD FRONT WITH A CONTINUING STREAM OF
OVERRUNNING MOISTURE. TEMPERATURES WILL CONTINUE TO DROP TODAY
ACROSS THE AREA AND ONLY A FEW ADJUSTMENTS WERE MADE WITH THE 9:30
AM UPDATE. PRECIPITATION SHOULD END VERY SLOWLY FROM NORTH TO
SOUTH THIS AFTERNOON. WIND GUSTS AROUND 20 MPH HAVE ALSO BEEN
REPORTED IN SPOTS AND THESE WERE INCREASED WITH THE 9:30 AM
UPDATE.

ONTO SNOW AMOUNTS IN THE ADVISORY AREA A FEW INCHES HAVE ALREADY
FALLEN WITH THE POTENTIAL FOR SOME ADDITIONAL ACCUMULATION WITH
TOTALS STILL LOOKING TO AVERAGE UNDER WARNING CRITERIA.


FURTHER SOUTH TOWARD THE PHILADELPHIA METRO AREA MODERATE TO
HEAVY SNOW BANDS HAVE SETUP ACROSS THIS REGION WITH REPORTS OF 1
TO AS MUCH AS 2 INCHES PER HOUR FOR SHORT INTERVALS. THE RAP AND
HRRR BOTH INDICATE THAT THESE MESOSCALE BANDS WILL BE SOMEWHAT
PROGRESSIVE BUT CONTINUE THROUGHOUT THE DAY TODAY. THESE BANDS ARE
CAUSED BY SIGNIFICANT FRONTGENETICAL FORCING AND AMPLE ISENTROPIC
LIFT. STORM TOTALS OF 6-8 INCHES STILL LOOK ON TRACK ATTM. ALSO
PLEASE REFER TO MCD 124 FROM THE SPC AND THE SPS WE HAVE IN
EFFECT.

IN THE DELMARVA AND SOUTHEAST NJ, THE 9:30 AM UPDATE SLOWS THE
CHANGEOVER AN HOUR OR SO. HOWEVER STILL DO NOT THINK THIS WILL CUT
INTO TOTALS EXTENSIVELY ATTM. THIS IS DUE TO THE SAME MESOSCALE
BANDING SIGNATURES SINKING FURTHER SOUTH FOR THE AFTERNOON HOURS.
THIS IS PICKED UP WELL BY THE RAP, HRRR AND 06Z 800-600 MB
FRONTGENETICAL FORCING ON THE 06Z NAM FROM EARLIER TODAY.
SNOWFALL MAY ALSO FALL AROUND AN INCH FOR A TIME THIS AFTERNOON
LEADING TO 4-8 INCHES OF SNOW BY EVENING.

ROAD CONDITIONS WILL CONTINUE TO DETERIORATE TODAY WITH THE SNOW,
ALLOW EXTRA TIME IF YOU HAVE TO TRAVEL.

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Shoulda just kept my original 5-10 for this one.  I downgraded to 4-8 yesterday.  7.4 verifies both but it fit into the first forecast better.

May not have reached double digits in a single storm this year, but at least we had our biggest snowfall albeit in March. Final total 7 3/4 inches. Did not watch models, and radar today just watched the snow come down during the daylight which was rare this year. As Wederman said I hope the full moon comes out tonight. It will be spectacular.

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Should we revisit the hysteria over the 6Z GFS & 12Z NAM run on 3/4 and learn from the experience of taking the wait and see approach before we start all of the bridge jumping?    Always a good idea for anyone to go back and look at each event to see what did or didn't go wrong.   I am sure the good mets do this regularly in order to perfect their craft. 

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