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2/19-20 Winter Storm Observations


nj2va
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Updated AFD from LWX as of 8:57pm:

.NEAR TERM /THROUGH WEDNESDAY/... Leading edge of precipitation is moving northward into southern Virginia as of mid-evening, and should arrive just prior to midnight across central Virginia. Precipitation will then move towards the remainder of the area from southwest to northeast overnight. A strong area of frontogenetical forcing will move across the area, and a band of moderate to heavy snow is expected across the area Wednesday morning. Localized rates of 1"+ per hour are possible in this band. Latest guidance and trends suggest the strongest lift in the dendritic snow growth zone will setup over eastern West Virginia, the northern Shenandoah Valley into west-central Maryland Wednesday morning, where 2" per hour snowfall rates are possible. At any given location, a 4-6 hour period of snow is expected before the strong warm air advection pulls in a warm nose aloft causing snow to mix with and change over to sleet during the mid to late morning hours Wednesday for most locations, and early afternoon across northeastern Maryland. An extended period of sleet then appears likely for a good portfrontogenetical forcing will move across the area, and a band of moderate to heavy snow is expected across the area Wednesday morning. Localized rates of 1"+ per hour are possible in this band. Latest guidance and trends suggest the strongest lift in the dendritic snow growth zone will setup over eastern West Virginia, the northern Shenandoah Valley into west-central Maryland Wednesday morning, where 2" per hour snowfall rates are possibleion of the area before warming continues, changing precipitation to freezing rain and then rain. Freezing rain will persist the longest for portions of eastern WV, western MD, and northwestern VA near/west of the Blue Ridge. Of note, even an hour sooner (or later) in changeover from snow to ice could result in notably lower (or higher) snowfall amounts, given the moderately heavy rates expected. Snow and sleet totals from 3-6" appear most probable for locations along/west of I-95, with some areas in excess of 6" near/west of the Blue Ridge and northern MD. 2-4" of snow/sleet likely for areas south/east of I-95. This will then be followed by a light glaze of ice accrual along/east of I-95 with a tenth to two-tenths west of the I-95 corridor. Some locations may see up to three-tenths in the favored locations west of the Blue Ridge. The combined impact of the snow, sleet and freezing rain will create very hazardous travel conditions, and could result in downed trees and isolated power outages even in the absence of strong winds (northeast 10-15 mph, with a few gusts to around 20 mph).

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13 minutes ago, Weather Will said:

OPM has a new policy.  Federal employees no longer get snow days or weather and safety leave as it is now called.  We must either take personal leave or telework.  

It is 2019 telework should be available to the majority of the workforce. My office follows OPM, but we have been telework enabled since 2012.

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2 minutes ago, Grothar of Herndon said:

It is 2019 telework should be available to the major of the workforce. My office follows OPM, but we have been telework enabled since 2012.

Policy where are I is that if you have a TW agreement you are expected to make yourself "Telework Ready" and do so during a weather day.  Or take leave.  Makes good sense. 

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I can’t remember discussions be so far all over the map. 2-7 inches just west of the City? far west of the city? 2-3 change to rain by 4:00 pm?

 Looks like just a mix of slushy 2 inches and then fr rain and rain. Not to be Deb but a whole lot of hoopla for what appears to be an an 11:00 am changeover to sleet and then rain 2 hours later.  Hope the CAD just sits and cold air wins out overperforning. 

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Just now, nj2va said:

00z 3k NAM is about an hour slower on the changeover to sleet in DC from 18z.  Extrapolating about 0.35" would fall as snow through 16z which is when we'd flip (0.27" through 15z and 0.53" through 18z)

If we can keep the heavy rates, I think we can hold off the flip until 18z

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11 minutes ago, snowfan said:

3K NAM is a bit better on the QPF front.....by 15Z, DCA is at .27". By 18Z, DCA is up to .5" and still holding the surface and 850s.

Yes, but not very good dendrites before the flip to sleet according to the Ferrier or rime corrected SLR per the NAM3. I'm hoping we're closer to the 2-3" bar I've set for our BY, which is on the 10-1 NAM3 map, vs. the 1-ish inch per the NAM3 Ferrier.  The Ferrier has consistently been 1 maybe 2 inches for the I-95 corridor. Now it's barely 1" (2nd and 3rd from the bottom blue colors). Hoping it's more of a QPF issue, but as you said, with 0.27" by 15Z, that tells me we've got a) riming problems (less than a 10-1 ratio) and/or b) a quicker transition to a snow-sleet mix (way less than 10-1).

nam3km_asnow_ferrier_neus_18.png

nam3km_asnow_neus_18b.png

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3 minutes ago, WxMan1 said:

Yes, but not very good dendrites before the flip to sleet according to the Ferrier or rime corrected SLR per the NAM3. I'm hoping we're closer to the 2-3" bar I've set for our BY, which is on the 10-1 NAM3 map, vs. the 1-ish inch per the NAM3 Ferrier.  The Ferrier has consistently been 1 maybe 2 inches for the I-95 corridor. Now it's barely 1" (2nd and 3rd from the bottom blue colors). Hoping it's more of a QPF issue, but as you said, with 0.27" by 15Z, that tells me we've got a) riming problems (less than a 10-1 ratio) and/or b) a quicker transition to a snow-sleet mix (way less than 10-1).

nam3km_asnow_ferrier_neus_18.png

nam3km_asnow_neus_18b.png

Yeah, by 15Z, DCA is at -1C at 700, which is less than ideal for snow. Should want something around -2/-3C or we're already mixing.

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2 minutes ago, WxMan1 said:

Yes, but not very good dendrites before the flip to sleet according to the Ferrier or rime corrected SLR per the NAM3. I'm hoping we're closer to the 2-3" bar I've set for our BY, which is on the 10-1 NAM3 map, vs. the 1-ish inch per the NAM3 Ferrier.  The Ferrier has consistently been 1 maybe 2 inches for the I-95 corridor. Now it's barely 1" (2nd and 3rd from the bottom blue colors). Hoping it's more of a QPF issue, but as you said, with 0.27" by 15Z, that tells me we've got a) riming problems (less than a 10-1 ratio) and/or b) a quicker transition to a snow-sleet mix (way less than 10-1).

nam3km_asnow_ferrier_neus_18.png

nam3km_asnow_neus_18b.png

I’d have to think that there is something amiss with the Ferrier scheme in this scenario.  That is implying a sub 5:1 ratio which is really hard to come by.

That said, if you thought the 00z HRRR was bad, 01z upped the bar there.

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