MillvilleWx

I think he meant dew point of 16 and put 61. So, looks like you guys will at 425% RH. Might be a little high. Saturation to the Ionosphere lol

Dang. I think we might lose you all in Columbia today to liquification Good luck @WxUSAF @Scraff @osfan24

Anywhere here. Lewisburg is probably where I would setup. I see them 20-24" with up to 30" possible

I'm gonna say it now. Someone north of I-70 between 20-00z is gonna see some absolutely insane snowfall. The 85H frontogen is beyond incredible for that area during that timeframe. This reminds me of some of the surprise overperformers in recent past with front end thumps.

They should absolutely. This is a very good setup for MDT on north. I see 14-22" with local to 25" for you up there

The HRRR has been consistent with the northern tier getting absolutely destroyed for several hrs today between 17-21z. Here's an example regional based sounding across the northern areas. This is a great way to get WSW criteria snow before any issues of mixing.

That's the thing that gives me pause. WAA events tend to overperform on the front end, but they also tend to blast further north and west and hold sleet sigs longer for some. I'm still skeptical of the breach of the 81 corridor with it, but it's certainly possible. Meanwhile, HRRR coming in aggressive still with the initial WAA surge lol. What a crazy run of models we've had last 48 hrs. It is 2020. Only fitting. Nothing is easy in the Mid Atlantic

They are actually pretty solid overall, but the issue becomes the dynamic nature of these types of events are complex and little deviations from run to run can have influences on everything from SLP strength/placement, height patterns, and that will have a feedback on mesoscale determination. That's why globals are good for upper level pattern recognition and overall schema of of the aforementioned characteristics, but the short term guidance will be finer in the smaller details in shorter time frames, or at least they should be, and a lot depends on initialization and parameterization of the specific model. They also have their biases and that's where trends and utilizing dprog/dt have benefits. Ensembles are meant to smooth means and give a broader scope of a pattern incoming, but they will absolutely miss some of the nitty gritty details that a full deterministic will see. It's all about using everything together and using blends with analysis of current and past obs to delineate what models are having better handle of a pattern progression in both the synoptic and unfolding mesoscale. That's where people at WPC come into play and assist WFO's for forecasts. They see WAY more guidance and data than we do because we don't have time for all that with other duties on a local scale.

For me, it really depends on the setup. The HRRR does well with 12hr and in and sort of loses some integrity outside that, but I have seen it do well at leads. Typically it does well with mesoscale temp climates inside 8 hrs, especially when it comes to WAA or CAA patterns. This is at least true for west TX. I remember it was unbelievable for one of our snow storms back into the mid 2010's. It was nailing banding structures that other models were struggling with. I was to say it did excellent for 2016 too with banding placement and was solid in the total precip department. It is good with changeover periods too within that 12 hr window. Outside of that, I like the Nam Nest for the mesoscale and short term trends. That's why I'm a little perplexed now given the sharp shift in the 7H and 85H moisture fields which became pretty paltry for most of the sub. The weakened low obviously had impacts on the low level frontogen (850mb) and at 700mb, so that probably had some play. There's a decent PWAT increase on many of the models, so it could be a hiccup, but this would be a bad time for it to sniff something out this close in that would ruin forecasts for many. Hell, it would zap some potential for eastern PA too. Want to see if HRRR continues the course because it's been consistent so far with little deviation in precip structure and increased 850-700mb frontogen placement along and north of I-70. That is fairly textbook too.

@psuhoffman Good to see the Nest coming in with its worst run of the whole storm so far. That was pretty ugly. Lower precip, blasts the boundary layer easterlies to the Alleghany front, and then misses any wrap around love on the back end. Good lord, I hope that's wrong for everyone.

It stopped for Jimmy's Seafood

After mulling over guidance for a bit, decided to make a final update to the forecasted snow for the area and where I think the max will occur. I did trim totals back for many, but I do think areas north of I-70 stay snow or sleet through the duration of the storm. The wrap around from the CCB when the 85H low develops and swings south of Rt50 will help tack some fresh powder on the typical suspects over the northern tier. 2-4" could be had from just that, so it'll be nice after changing to sleet for a while. Time to let it all play out. Here's my projected totals for snow/sleet: East of I-95 near Baltimore: 2-4" with max to 5"; will mix with sleet then change to sleet/rain for a time East of I-95 near DC: T-2" then changing to rain mid afternoon DCA: T-2" IAD: 3-7" BWI: 2-5" Eastern HoCo: 3-7" Western HoCo: 5-9" Southeast MoCo: 2-4" Northern MoCo: 4-9" Loudon: 4-8" with highest across western Loudon NoVA south of DC: T-1" Southern Carroll: 5-9" Northern Carroll (Westminster and north): 7-12" with local to 14" Southern Frederick County: 4-8" Northern Frederick: 8-14" with local to 16" Washington County: 9-15" with local to 18" Winchester: 8-14" with local to 16" WV Panhandle: 9-15" with local to 18" Southern Harford County: 4-9" Northern Harford County: 6-10" Southern Baltimore County: 3-7" Northern Baltimore County: 6-10" with local to 12" Southern PA from Chambersburg to Gettysburg: 10-16" with local to 20" Gettysburg to York: 9-15" with local to 18" Bullseye: Rt 99 in PA down to western MD, moving NE through Central PA into NE PA: 18-24" with local to 30"

Looking at mean soundings over a general area, the best chance for TSSN will be located across eastern PA near the Pocono's, but there is a secondary shot between 00-06z Thu over central PA within the deformation axis. There's a bit of conditional instability for a time between Harrisburg and State College during the height. It's not a slam dunk imo, but I can see some thunder for sure in places.

Brought this back over because I think it's important for anyone checking in why the trends have gone the way they have.

Here's a gif of the Nest run. Keep an eye on the time frame between 03-09z. Watch the band develop out west and then pivot eastward. That is in conjunction to the frontogenic forcing at 85H as the low closes nearby.

I know this won't pertain to everyone in the sub, but there's a little positive note on the 3km NAM which should be something to monitor in Hi Res trends over the next 24 hrs. Here is a snapshot of the 12 hr precip panel until 00z Thu for the Nest. I have drawn a rough outline of where the snow/sleet line is located based on model Composite Reflectivity and some sounding excavation. Anything north of the line drawn was actually all snow in this picture. Usual caveats of snow ratios aside, even with an ~9:1 ratio on the southern periphery of the line drawn, this is a WSW criteria event with over 10" in spots prior to any sleet shift. Another positive is the 85H low location overnight. The Euro, the NAM Nest, and GFS all have 85H close off to the south over VA and trek to the NE between 03-09z. At this point, there is a bit of a curl in the precip delineation due to increased 850mb frontogen over the northern tier of the sub. This is shown on all the guidance I mentioned above. That is important as this will be a higher ratio snowfall as 850mb temps crash on the backside of the system and we go from borderline isothermal to a pure snow sounding with sufficient lift to generate a nice of snow. It sweeps west to east over the overnight period and can add 2-4" in a quick time frame for those north of I-70, perhaps further down 81. I think we'll see some steep gradients in snowfall with this storm with a hard slope SW to NE into PA. Something to watch.

I hate to say it, but that is an unbelievably good catch by Eric. I’ve spent a lot of time focusing on model disco at both the synoptic and mesoscale the last several days, but at a regional parsing, and not at full broad scale look at the impending 5H pattern. Discernible differences like that can make or break a forecast. Those s/w progressions, especially one as small and later developing can have major downstream implications. PSU did a fantastic job outlining the “Death by a 1000 cuts” mantra. It doesn’t take one big thing to ruin a forecast, but small changes in both a synoptic and mesoscale sense can cause shifts in setups like this. High further NE and more western trek of an SLP will cause a large u-vector anomaly of the wind field, allowing the low-level easterlies to protrude way beyond the scope of where we were thinking 24 hrs ago. I’m gonna have to adjust my totals most likely due to the current state of guidance. I do see the 81 crew still doing the best here. HREF is very interesting to say the least, so I’ll have to dig more into that. It’s a phenomenal tool to use for situations like this, for ZR events, and especially for severe. Absolutely good bookmark. I’m really tired. My neighbor set off a car alarm outside and it woke me up a bit earlier than normal for the PM. I’m heading back to bed. I hope my previous discos can sustain merit with the new trends, and don’t sway people to take my posts with a grain salt. I was just reporting my thoughts based on the model trends at hand. I wish I wasn’t on mids so I could delve more into the nitty gritty. Edit: I had to switch to the PC and post this since Tapatalk absolutely butchered this post. Sorry for the repost.

Absolutely died laughing at work

It's no problem. I love doing this and I can't wait to hopefully come back and actually be sitting in the hot seat at Sterling in my career

This is literally my forecast I posted overnight, but in map form lol. Glad to see I jive with Ava. She's a very good met for the area.

Reading the WPC Heavy Snow and Icing Disco and then seeing that map was a little interesting. Maybe they are gonna see what the 12z runs come up with since the 00z north shift was so sudden compared to the last set of runs. I mean, we went front prime fronto over southern PA and the northern tier to central and northern PA. They still have time to adjust totals since the storm is over 24 hrs out

Heavy Snow and Icing Disco from WPC

I'm a little perplexed at that too actually.

NAM Nest came in super aggressive with the 850mb easterlies on the 06z run. That was actually wicked and reminded me a bit of March 2017 unfortunately. The scene will be seriously wintry across the northern half of the sub with 4-8" of snow, followed 1-2" of sleet and some ZR mixed prior to changing back to snow at the end. I was expecting some mixing as I mentioned in my forecast, but blasting out to the I-81 corridor is beyond my expectations. I hope it doesn't turn out like that. I can see LWX cutting totals back for part of the region because even I thought they might be too aggressive in spots, but damn. 7H fronto is stout, but it slings up into PA and never wraps back. That was disheartening to see. This is when the NAM Nest is going to have more weight added to forecasting too, especially the mesoscale. I'm sticking with my current forecast for now, but even I might have to trim if I see more aggressive 85H easterlies.

You guys are going to get an unmitigated beatdown on Wednesday into early Thursday morning. I still like my I-81 corridor with jack over Northern York county. Perhaps up to Selinsgrove, then east to the Poconos as well. Absolute destruction. I can't wait to see pictures. Enjoy y'all