December 16/17 Winter Event

[Zde4]

Rgem came in NOT west.

[yoda]

@psuhoffman @MillvilleWx 

I'd take the 06z RGEM and run

[ravensrule]

I’m way too old to be doing these late nights. If the GFS goes West I’m going to pull a JI and lose my shit. We better get 6” from this. 

[Ruin]

Just now, ravensrule said:

I’m way too old to be doing these late nights. If the GFS goes West I’m going to pull a JI and lose my shit. We better get 6” from this. 

lol im getting up in age to but my job I work 10 hours second shift till 2am so im use to it.

[psuhoffman]

3 minutes ago, yoda said:

@psuhoffman @MillvilleWx 

I'd take the 06z RGEM and run

I wouldn’t worry about the NAM. I can’t tell you how many times in the past few years at 36+ hours it’s given me snow on the back end of a storm that ended up no where close because the system was way more progressive.  Gonna get some sleep. 

[yoda]

06z RGEM... good move SE with the snow accums compared to 00z

[nw baltimore wx]

Winter Storm Watches issued west of the fall line central MD. A little further west for DC metro.

[yoda]

06z GFS says it's still lightly snowing at 12z THUR in N VA into MD

[ravensrule]

URGENT - WINTER WEATHER MESSAGE
National Weather Service Baltimore MD/Washington DC
443 AM EST Mon Dec 14 2020

MDZ006-008-011-503-505>508-VAZ025-026-040-501-505-506-141745-
/O.NEW.KLWX.WS.A.0002.201216T1200Z-201217T0900Z/
Northern Baltimore-Cecil-Southern Baltimore-Northwest Montgomery-
Northwest Howard-Central and Southeast Howard-Northwest Harford-
Southeast Harford-Augusta-Rockingham-Rappahannock-
Northern Fauquier-Western Loudoun-Eastern Loudoun-
443 AM EST Mon Dec 14 2020

...WINTER STORM WATCH IN EFFECT FROM WEDNESDAY MORNING THROUGH
LATE WEDNESDAY NIGHT...

* WHAT...Heavy snow possible. Total snow accumulations of greater
  than 7 inches possible.

* WHERE...Portions of north central and northeast Maryland and
  northern Virginia and the Shenandoah Valley.

* WHEN...From Wednesday morning through late Wednesday night.

* IMPACTS...Travel could be very difficult.

* ADDITIONAL DETAILS...A wintry mix could affect accumulations
  over portions of the watch area and result in a sharp gradient
  of snow totals across a small distance.

[Weather Will]

WB 6Z GFS, 925 did tick NW at Wed 7pm

[Weather Will]

WB 6z GFS snow map did tick NW around DC due to mixing/rain 7pm-10pm

[Weather Will]

WB 6Z GFS 925 7pm, 10 pm

[yoda]

Updated portion of the LWX AFD after the watches issued 

After collaborating with our neighboring offices, have
issued a Winter Storm Watch for the areas that have the highest
probabilities of significant snow. As is the case with an
uncertain system with a sharp gradient, some areas may need to
be added to the watch (or eventual warning), or portions of the
watch may eventually be resolved to an advisory, based on
forecast trends. It looks like precipitation should largely exit
by Thursday morning.

[CAPE]

The way things are looking I may have to take a drive to Chestertown and hangout.

[MillvilleWx]

7 minutes ago, Weather Will said:

WB 6Z GFS, 925 did tick NW at Wed 7pm

This makes sense because there was a slight tick west with the surface reflection, a bit stronger and 925mb winds were able to make it to the fall line. The fall line and east will very likely mix during this storm. Perhaps slightly beyond it too, but for how long, and then where is the deformation axis established, and is there a TROWAL. I've seen that feature now on most model runs. A lot of times in SoPA, so something to watch because it's not far away from the northern folks and that would give some crushing totals whoever gets under it.

[Weather Will]

WB 6Z GFS surface temps upper 30s inside Beltway 8pm

[CAPE]

For those on the edge (not the ledge) the only hope now is the coastal low makes the eastward turn a little sooner(further south). That would cool the lower levels quicker and allow for a longer duration of accumulating snow on the backside.

[Weather Will]

6Z GEFS mean low locations does not support NAM.

[jayyy]

6 hours ago, clskinsfan said:

DCA almost 10 inches. GFS is good for all. 

Me likey!!! 

[jayyy]

RGEM & GFS look MUCH better this morning for Baltimore metro. Noticeable tick SE with snowfall amounts. 
 

looking like a solid lock 6-12+ up here in Carroll county. Man... I’d take that 6z GFS for the entire forum!!

[WVclimo]

Latest forecast from NWS:

[WVclimo]

[H2O]

That says 1-2" for me.  Tops.  Oh well.  Time to look for the next one

[MillvilleWx]

26 minutes ago, WVclimo said:

Not a bad first map tbh. You have to play it conservative right now. In the meantime, the fact they have totals already flirting with WSW criteria this far out is a solid place to start. I don't blame them at all. 

Edit: I need to look at the other maps, so disregard my tired brain. 

[aldie 22]

1 minute ago, MillvilleWx said:

Not a bad first map tbh. You have to play it conservative right now. In the meantime, the fact they have totals already flirting with WSW criteria this far out is a solid place to start. I don't blame them at all. 

So that map says 7pm to 1am Wednesday to Thursday does that mean they think no accumulation prior?

[mappy]

1 minute ago, MillvilleWx said:

Not a bad first map tbh. You have to play it conservative right now. In the meantime, the fact they have totals already flirting with WSW criteria this far out is a solid place to start. I don't blame them at all. 

well its three sets of maps over three time periods

for example, my backyard: 

7am - 1pm: < 1

1pm - 7pm: 6-8

7pm to 1am: 4-6

and then more should fall after that

[yoda]

11 minutes ago, H2O said:

That says 1-2" for me.  Tops.  Oh well.  Time to look for the next one

They are 6 hour time period maps... not total event snowfall maps

[EastCoast NPZ]

7 minutes ago, MillvilleWx said:

Not a bad first map tbh. You have to play it conservative right now. In the meantime, the fact they have totals already flirting with WSW criteria this far out is a solid place to start. I don't blame them at all. 

11- 16" is conservative?  

[MillvilleWx]

4 minutes ago, leesburg 04 said:

So that map says 7pm to 1am Wednesday to Thursday does that mean they think no accumulation prior?

To be honest, I'm not sure. I was a little surprised on the time frame posted. I would've thought they would have something at least starting at 3PM like guidance has. And southern stream moisture tends to arrive faster than modeled. They might have just wanted to cover the period where they think snow is gonna become heaviest. 

 

6 minutes ago, mappy said:

well its three sets of maps over three time periods

for example, my backyard: 

7am - 1pm: < 1

1pm - 7pm: 6-8

7pm to 1am: 4-6

and then more should fall after that

Ahhhhhhh. Okay, that makes more sense. They are breaking up into 6 hr increments. That forecast literally matches mine, so I'm good with it. Thanks for the clarification. I had no idea if there were more maps since I didn't check. 

 

1 minute ago, EastCoast NPZ said:

11- 16" is conservative?  

I didn't know they made more maps haha. Didn't check the grids for LWX this morning. Been trying to stay awake tonight by listening to heavy metal 

[MillvilleWx]

3 hours ago, MillvilleWx said:

Okay y'all. So, here are my thoughts with the 00z runs and why the trend towards a more tucked solution is becoming favored, but it's not going to be the end all for everyone when it comes to snow chances. 

So, looking at MSLP plots for each of the 3 major globals (ECMWF/GFS/CMC), the models are pretty much in lock step with each other until the storm begins to breach the latitude of Hatteras and that's where we see some separation from the CMC/Euro combo and the GFS. The GFS is the flattest solution as it's not as aggressive with the digging jet associated with the s/w trough located over the Mississippi valley. Height rises out ahead of the trough are more subdued, so the storm doesn't amplify to the degree of the CMC or Euro. Both the CMC/Euro combo are fairly similar in the depth of the trough and relevant height rises along the Atlantic seaboard. This allows for SLP genesis to gain latitude a bit more with a northerly movement before becoming caught up by the negatively tilted 5H trough progressing through the Tennessee Valley. The GFS is a more NNE trajectory with less amplification and slower strengthening due to the lower dynamic field played by the weaker 3H jet alignment. In turn, by Thursday at 00z, there is a sub 1000mb low on both the CMC and Euro tucked into the Delmarva coast where the GFS is a 1003 about 100 miles off ORF. I think we'll see a correction in the GFS with a more tucked solution, but perhaps not to the degree of the CMC/Euro tandem that have it basically onshore at Lewes in the same light. After 00z Thursday, the CMC is much more progressive with the 5H trough and actually motions to the NE prior to weakening with a due easterly motion. Euro becomes stacked right south of Cape May and then begins to pull eastward, leading the QPF shield to slowly wane from the sub and end by Thursday AM. CMC is a little more aggressive with ending the precip as it punches the dry conveyor overhead overnight due to it's NE movement away from the coast. A general compromise would be a slight backing from the Atlantic seaboard which would keep the 925/850mb easterlies from overtaking the boundary layer back towards the fall line. This would lead to less sleet/rain and more snow, albeit lower ratios due to the warm tongue that will still creep into the boundary layer.

Now the dynamics side of the precip field. The first round of precip will come in from SW flow aloft bringing in moisture off the gulf with qpf field expansion due to exit region of the 3H jet pivoting north into the area. There's a fairly robust area of diffluence ahead of the negatively tilted trough moving into the Tennessee valley on Wednesday, so regional lift is maximized, which is pretty standard for setups like this. We know this precip typically arrives a little sooner than modeled, and the current dew point depressions leading in are not insane and should be overcome fairly quickly with wet bulbs near and below freezing for most of the sub-forum. Areas along and SE of I-95 will likely be a touch on the warm side, but upper levels are still supportive of snow for a time west of the bay, so there will be flakes flying, at first. MSLP will develop along the SC coast and motion to the north, allowing for increasing low level dynamics to take shape with SFC and 850mb frontogen increasing in intensity as it gains latitude. Low level easterlies on the northern edge of the emerging surface low will begin throwing more Atlantic moisture into the mix as the WCB is fully established at + Theta-E advection noses along the Atlantic coast into the mouth of the Chesapeake. At this point, the 7H trough will be pivoting eastward with increased mid-level forcing focused over the Mid-Atlantic, allowing for increased banding potential over NoVA and Central-Western MD. Right now, current frontogen placement has the best mid-level ascent focused over the I-81 corridor over into central MD where areas of heavy precip will likely develop with embedded banding structures becoming more pronounced.

By 00z Thursday, a classic pivot of the precip field will become recognized as the 5H trough axis begins to close off overhead and the added torque in the upper jet will take the SLP and draw it closer to the coast. This is obviously subject to some fluctuations pending the total evolution of the 500mb trough, but it's looking more likely with ensemble guidance indicating a bit more "tucked" solution, which the NAM12km (Yeah, I hate it) has as well. I don't see it being that amplified however, so the bias of the NAM is likely being shown and should draw away from the inland runner solution. In any case, as the 7H trough moves overhead, models show the trough closing off over the Delmarva which would lead to enhancement of the 700mb frontogenesis field with a more defined deformation axis becoming established. There's some indications of a TROWAL developing on some guidance as the 7H Theta-E analysis begins to take the classic "S" shape on the NW focus of the surface reflection, and is in conjunction with the closed 7H reflection. Pending the extension of the S curve will be where there's potential for a region of enhanced precip within the heart, or just north along the strongest 700mb frontogenesis. This is something to never overlook when it comes to heavy precip potential, and is generally a good indication of where the heaviest rates will occur within the developed CCB. On the Euro, there's excellent lift in two regions; the first is along I-81 where the strongest mid-level fronto is positioned. There's also a secondary precip enhancement near I-95 where 850mb frontogenesis is at it's max. Looking at 850mb temps, you can see the crash between 21-00z on the run when areas of CMD see temps in the lower boundary layer fall from +1-2 to -3-5C in a few hrs. That the CAA pattern typically associated with maturing cyclones on the west side of the SLP due to CCB as air cools considerably due to the deformation element. This will lead to a secondary banding structure that can lay down a solid amount of snow in time, and that's what the Euro has for our northern tier east of Carroll on this current 00z run.

The CMC is a bit more drawn as the TROWAL is located a bit further north and intersects the incredibly strong 7H frontogenesis that is present over PA, extending into the northern portions of the sub-forum. The TROWAL is more pronounced than on the Euro, and is maxed over York County into the Coal country of eastern PA. That's why the clown maps show insane totals for that general region. The Euro shows the best axis of heavy snow out towards 81 with secondary max displaced to the east over Pars Ridge as they get in on the 85H frontgen band and rip for several hrs before the storm pulls away.

General QPF distribution is similar for all guidance, but the Euro and Canadian show distinct maxes intersecting the areas where lift is maximized, as well as where the WCB pushes ashore along the Delmarva coast. PWATs are well above normal with Euro coming in between 1.5-2.5 StD above normal on Wednesday and Wednesday night with up to 3 StD where the deformation axis is located, so the precip potential is absolutely there to breach 1.5" with local 2" spots possible within the best areas of lift.

Bringing this all together, I think we're beginning to see where the max potential will be realized for the sub-forum, and the storm overall. The Blue Ridge from Front Royal on north along I-81 over into south-central PA will likely be ground zero with a relative axis of heaviest snowfall extending east into the Catoctins, Northern Carroll, and perhaps northern Balt county, pending the deformation extension to the east. 10-16" of snow with local 20"+ totals will be realized in parts of the sub with incremental decreases further east from the line I mentioned. The urban corridors will likely mix and end up either high end Advisory or low end Warning criteria snowfall. Pars Ridge will have a secondary max of 8-14" with potential for more pending localized banding structures. I'll have a final call Tuesday night into early Wednesday. 

If anyone is interested, I wrote a book earlier this evening on my thoughts. Feel free to critique. Thanks!