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Old 02-12-2019, 12:42 PM
 
Location: northern New England
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weather newbie here - I look at online weather radar maps - like right now, it is showing my whole area covered in blue (snow) for miles in all directions. And yet I look out the window and it is not snowing. And hasn't snowed. Other times the radar show clear and it is definitely snowing. So it is not measuring precip at ground level or what?
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Old 02-12-2019, 12:50 PM
 
Location: Near the Coast SWCT
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Quote:
Originally Posted by VTsnowbird View Post
weather newbie here - I look at online weather radar maps - like right now, it is showing my whole area covered in blue (snow) for miles in all directions. And yet I look out the window and it is not snowing. And hasn't snowed. Other times the radar show clear and it is definitely snowing. So it is not measuring precip at ground level or what?
It's a radar beam across the atmosphere so no, not at ground level. What you're seeing is Virga. Moisture (snow) falling up there in the air but not making it to the ground because it's evaporating .


Dewpoints are in the single digits at the surface right now in Vermont so it's bone dry. Moisture is evaporating falling down.


Bennington and Rutland are reporting light snow now. So it's a matter of time for you.


Gotta get the atmosphere primed up first. (moisten)
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Old 02-12-2019, 12:52 PM
 
Location: Near the Coast SWCT
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Current radar and who is reporting what. See that darker blue shading? When that enters Vermont it will start snowing more. Darker blue shading indicating heavier snow falling


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Old 02-12-2019, 02:31 PM
 
Location: Idaho
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Hummm. Learn something every day. I wouldn't have thought that virga is a New England thing, only an arid southwest desert phenomenon.
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Old 02-12-2019, 03:24 PM
 
Location: Bidford-on-Avon, England
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Often there is a delay between the radar and what's actually happening
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Old 02-12-2019, 03:51 PM
 
Location: Near the Coast SWCT
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Quote:
Originally Posted by volosong View Post
Hummm. Learn something every day. I wouldn't have thought that virga is a New England thing, only an arid southwest desert phenomenon.
Definitely. Especially with how dry we can get here, but Virga can happen anywhere I thought even the south.. Think about the front edge of a storm, moisture doesn't just fall to the surface all the time, No?
So even in the south a storm will be dropping moisture but not reaching the ground until it's heavy enough. I could be wrong about it happening down south. I'm pretty sure Virga doesn't last long if its happening there..

I wonder if there is a surface dew limit though. Like if its humid in the 60s, can there still be Virga? I guess?

Virga can last an hour+ here. it's annoying because it's wasted precip.

Sometimes NWS mentions it in their discussion. Today they didn't use the word but mention the dry air affecting the precip.

https://www.weather.gov/bgm/productText

Quote:
National Weather Service New York NY
748 AM EST Tue Feb 12 2019

Cold air damming continues along the eastern slopes of the
Appalachian mountain chain, extending from high pressure
centered over southeastern Canada and down into the mid
Atlantic. With cold and dry air in place the overrunning
precipitation was moving slowly northeastward. Dew points have
dropped into the single digits and even below zero across
portions of southeastern CT. The onset of precipitation will
likely have to be delayed even further. With this scenario the
warmer air will be moving in aloft over the entrenched cold air.

At this time precipitation does begin as snow, and will rather
quickly transition to sleet as thermal profiles suggest as the
warm air moves in aloft. Along the coast there could be a
prolonged period of sleet from later this morning through this
afternoon. Only a brief period of freezing rain appears to be
possible along the coast before going over to all rain. Farther
inland once the snow transitions to a wintry mix, sleet and
freezing rain will be the main threat. And across portions of
the lower Hudson Valley into northeastern NJ a couple of tenths
of an inch of ice are possible. Have continued with the winter
storm warning across western Passaic county, Orange and
Putnam counties for a combination of snow a sleet potentially
just reaching warning criteria. However, confidence is lower at
this time. The larger threat may be freezing rain, with more
than the quarter inch possible, depending on how quickly the
warmer air can be eroded at the surface as temperatures rise
late this afternoon and tonight.
Found one Cached..

Quote:
National Weather Service Albany NY
1238 AM EST Mon Jan 7 2019

Monday evening into Monday night, shortwave trough marking the
leading edge of broader longwave troughing approaches in
conjunction with the nose of a strong upper level jet digging
into IN/OH/PA. Strong isentropic lift is forecast in advance of
this wave with H850 winds increasing to 45-55 kt from the
southwest after 06Z. There is not a ton of moisture with this
system, but still an adequate amount as PWATs climb to around
0.5 inches. Much of the lift will be lost to saturating a dry
atmosphere initially as seen in the BUFKIT profiles and Sref
BUFKIT profiles, and the time period for deep saturation into
the snow growth region is limited as a midlevel dry slot punches
in around 12Z Tuesday. Model QPF output suggests topography
will dictate much of the precipitation distribution as would be
expected with a system with such a strong southwesterly LLJ,
favoring the southern Adirondacks and southern Greens with
downsloping cutting down totals in the Mid Hudson Valley and
Capital District. Reflectivity forecasts show the initial batch
likely in the form of virga. Then forecast soundings suggest the
profile mainly remains below freezing suggesting snow as the
dominant p-type, although we could see some sleet sneak in from
the Hudson Valley westward as the warm nose flirts with 0C yet
well bulb profiles are near or below freezing. Updated forecast
calls for advisory-level snowfall of generally 3-6" over the
southern Adirondacks/southern Greens, an inch or less in the
Mid-Hudson Valley/Litchfield Hills, and 1-3" elsewhere. The
timing of the snowfall could impact the Tuesday morning commute
as further updates may require advisories for these areas.
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Old 02-12-2019, 05:56 PM
 
Location: Northern Virginia
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Just to clarify one thing here though. Without knowing the source of that radar image..a lot of weather apps and sites nowadays feature a continuous radar 'stream' comprised of past, actually observed radar images and future precipitation radar based on forecast models.



If there is a significant gap between what the models predicted and the reality out there there will be a big break in consistency in that stream between the most recent real observation - often 10-15 minutes old - and the first forecast image (which is usually the image shown as the current radar picture as it takes a number of minutes to update).



This often happens with thunderstorms due to their relative unpredictability in terms of cell genesis etc. but I've seen it with snow and rain in bigger systems as well. In fact I saw it today.
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Old 02-12-2019, 06:20 PM
 
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VT...try this one:
Mike's Weather Page... powered by Sparks Energy!

it gives several looks at the same situation.
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Old 02-13-2019, 06:00 AM
 
Location: northern New England
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Thanks all!! just one more question - where does all this radar come from?
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Old 02-13-2019, 03:39 PM
 
Location: Idaho
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Quote:
Originally Posted by VTsnowbird View Post
Thanks all!! just one more question - where does all this radar come from?
Mostly federal government and university research/monitoring installations. Your tax dollar at work. The NEXRAD system operated by NOAA's NWS has 160 sites mostly in the U.S. and a few overseas.
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Last edited by volosong; 02-13-2019 at 03:48 PM..
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