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The March 6 Storm- The (last) Great White Hope, Part II


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Wow...very cold 850mb temps on the Euro (relative to GFS and NAM). Assuming the warm BL bias still exists, I can explain away the surface temps being ~2C and go for predominantly or entirely snow with the Euro?

Yes. Which definitely should give us pause when looking at the EC verbatim with it's surface temps. The sfc low track, 850 temps, and extrapolation of those temps down to the surface should leave us to believe the p-type would be mainly/predominately snow for DCA. Let's not forget that the strong diabatic cooling via precip rates would drag that colder layer down and lead to a more isothermal profile near 0C in the lowest level, *not* going from -3.3ish at 850 to over 2C at the surface.

I noted earlier how at 00Z the 50 EC ensemble members had about 5-7" for DCA and immediate areas, yet not one member was over 9". That's incredible consistency among the EC members, to get that high of a mean without a 10+ member. That really stands out to me. Granted, the EC ends the accumulating snows earlier, which given the transfer offshore may end up happening.

This time of year a 4-8" forecast for immediate DC is a good one, because the low and high end are as far apart as you're going to get in terms of impact compared to other snow ranges. 6-10" is also a 4 inch snow range, but 6 isn't as different to 10 impact-wise as 4 is to 8. Go with 4-8 and hope for the best. Like we saw in the central plains the other week, convection will certainly make totals more binary, or unevenly distributed from location to nearby location. Someone posted the soundings from the NAM and GFS earlier...you can see some potential instsbility with that dry punch above the -10C isotherm. That's where you want it.

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 Let's not forget that the strong diabatic cooling via precip rates would drag that colder layer down and lead to a more isothermal profile near 0C in the lowest level, *not* going from -3.3ish at 850 to over 2C at the surface.

 

Great post.  Euro is showing something like ~6-7C difference between the cold 850mb temps which is not far off the moist adiabatic lapse rate of ~8-9C for a surface temp of around 0C.  With heavy precip, the profile should be closer to isothermal as you say.  

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Here's my benchmark...if we are productive tonight into the early AM...that is, if we go to snow before 4 am or so, I'll feel good.

If we have a little drizzle and keep immediately over to snow, I'm popping out the champagne.

For people aob 500' in elevation near the cities one of the keys will be how much can fall and how long can it hang on before trouble comes. Once mixing starts you know the column overhead is in obvious trouble and it will take time to get things right again. One of the nerveracking parts of nowcasting. Things can go either way for us. I don't think anyone can say with any confidence that they are sure what's going to happen.

The later mixing can hold off the better (obviously). I hope the transition and probably lull going from heavier waa to heavier ccb is quick and clean and not drawn out and dirty.

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Yes. Which definitely should give us pause when looking at the EC verbatim with it's surface temps. The sfc low track, 850 temps, and extrapolation of those temps down to the surface should leave us to believe the p-type would be mainly/predominately snow for DCA. Let's not forget that the strong diabatic cooling via precip rates would drag that colder layer down and lead to a more isothermal profile near 0C in the lowest level, *not* going from -3.3ish at 850 to over 2C at the surface.

I noted earlier how at 00Z the 50 EC ensemble members had about 5-7" for DCA and immediate areas, yet not one member was over 9". That's incredible consistency among the EC members, to get that high of a mean without a 10+ member. That really stands out to me. Granted, the EC ends the accumulating snows earlier, which given the transfer offshore may end up happening.

This time of year a 4-8" forecast for immediate DC is a good one, because the low and high end are as far apart as you're going to get in terms of impact compared to other snow ranges. 6-10" is also a 4 inch snow range, but 6 isn't as different to 10 impact-wise as 4 is to 8. Go with 4-8 and hope for the best. Like we saw in the central plains the other week, convection will certainly make totals more binary, or unevenly distributed from location to nearby location. Someone posted the soundings from the NAM and GFS earlier...you can see some potential instsbility with that dry punch above the -10C isotherm. That's where you want it.

 

Great post and reasoning.  Agree with the 4-8 call for immediate DC.

 

Question: regarding the dry punch above -10c...what exactly do you want to see there?  Is it a case where you want to see just a touch of drying there for increased instability, but you still want to keep the -10 to -20 layer saturated with respect to ice?

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I liked the CWG post on analogs the other day. What worries me about it is that history says this will be a big storm but DC will get the short end of the stick. Obv I'm thinking Commutaggedon in particular. But selfishly I'm curious about whether that analog should be extrapolated to just the immediate city, or anything inside the beltway. This is my first time living in DC (Chevy Chase to be exact, grew up in Ellicott City), and I'm used to that EC elevation just west of Baltimore where we KILLED in pretty much every storm as relative to DC or even Bmore.

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18z RAP is encouraging...gets the heavy stuff in here quickly after it starts...thermals on the RAP are unreliable but suggests the diciest time would be around 3-4am..so if we can through that maybe we are ok

 

I like that the first wave through cools us down, and then we are set by midnight.  Would love to see it play out like that.

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So after 3 to 4 am time slot does DCA go to rain? Or expecting all snow from that point? Also do you think this will wrap up quicker as per Euro?

rap is showing saturated with marginal snow sounding at midnight in dc. Couple hours in the wee morning look to be rain mix. By 6am or so everything looks good. Last available sounding @ 7am looks really good.

It's pretty wet on the model too. shows dc w/ .6 by 7am. Winwxluver already needs a snorkle. precip looks way overdone imo. Nasty band sets up over md to pa border. not sure I buy that either.

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rap is showing saturated with marginal snow sounding at midnight in dc. Couple hours in the wee morning look to be rain mix. By 6am or so everything looks good. Last available sounding @ 7am looks really good.

It's pretty wet on the model too. shows dc w/ .6 by 7am. Winwxluver already needs a snorkle. precip looks way overdone imo. Nasty band sets up over md to pa border. not sure I buy that either.

 

Geography/climo does favor good bands setting up in that area, it tends to happen in northern MD (especially psuhoffman land)

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Geography/climo does favor good bands setting up in that area, it tends to happen in northern MD (especially psuhoffman land)

agreed. I just thought it was a little early in the day to see that look on sim. Also shows dc like .7+ by 7am too. Thats seems awful high.

I shouldn't even be looking at 18hr rap stuff anyways.

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DCZ001-MDZ005>007-009>011-VAZ052>056-060400-

/O.UPG.KLWX.WS.A.0004.130305T2300Z-130307T0500Z/

/O.EXB.KLWX.WS.W.0005.130306T0500Z-130307T0800Z/

DISTRICT OF COLUMBIA-CARROLL-NORTHERN BALTIMORE-HARFORD-

MONTGOMERY-HOWARD-SOUTHERN BALTIMORE-

PRINCE WILLIAM/MANASSAS/MANASSAS PARK-FAIRFAX-

ARLINGTON/FALLS CHURCH/ALEXANDRIA-STAFFORD-SPOTSYLVANIA-

INCLUDING THE CITIES OF...WASHINGTON...WESTMINSTER...

GAITHERSBURG...COLUMBIA...BALTIMORE...MANASSAS...MANASSAS PARK...

FAIRFAX...ALEXANDRIA...FALLS CHURCH...FREDERICKSBURG

248 PM EST TUE MAR 5 2013

...WINTER STORM WARNING IN EFFECT FROM MIDNIGHT TONIGHT TO 3 AM

EST THURSDAY...

THE NATIONAL WEATHER SERVICE IN BALTIMORE MD/WASHINGTON HAS

ISSUED A WINTER STORM WARNING FOR SNOW...WHICH

IS IN EFFECT FROM MIDNIGHT TONIGHT TO 3 AM EST THURSDAY. THE

WINTER STORM WATCH IS NO LONGER IN EFFECT.

* PRECIPITATION TYPE...SNOW.

* ACCUMULATIONS...4 TO 8 INCHES WITH THE HIGHER AMOUNTS WEST OF

INTERSTATE 95.

* TIMING...RAIN MIXING WITH AND CHANGING TO WET SNOW FROM

SOUTHWEST TO NORTHEAST TONIGHT AND WEDNESDAY MORNING. SNOW MAY

BE MODERATE TO HEAVY AT TIMES WEDNESDAY. SNOW TAPERS OFF WEDNESDAY

NIGHT.

* TEMPERATURES...IN THE MID 30S.

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