wxmvpete

To see the Probabilistic Winter Storm Severity Index (WSSI-P) showing moderate-to-high chances (50-70%) for most Minor Impacts for most on this forum should instill some confidence that we're dealing with a disruptive event Sunday night into Monday. It's a good signal for 4-5 days out on this product that we generate at WPC.

A little bit of a break down from the 12Z and 18Z guidance. The main discrepancy remains the strength/tilt of the 500mb trough, although the ridging along the West Coast feature differences too. Using 12Z Sunday as the time of interest, the ECMWF/CMC are most bullish on the 500mb trough while the GFS/ICON are weaker and more positively tilted in the 500mb comparison loop. This is also playing a big role in the strength of the emerging LLJ that is drawing up GoM moisture. Guidance like the ECMWF/UKMET/CMC are a little slower and not only generally deeper, but also positioned at a more neutral tilt. The GFS/ICON are more positively tilted and weaker in general. This has ramifications with the LLJ (850mb heights and winds comparison loop) as the strong SWrly flow shown on the ECMWF/CMC/UKMET yields not only more moisture/QPF, but a strong frontogentical component as well. Now one caveat is watching the 500mb trough over Lake Superior and western Ontario. That feature, should it make it far enough south and trend stronger, could act to suppress the tilt of the 500mb trough in the Plains and Mid-South on Sunday. Should that happen we would see not only a flatter system, but the moisture source is directed out of the WSW more and yields weaker 850mb FGEN. That can be give an take though, right? Seeing the 18Z ECMWF for example, it has a 70kt 850mb jet over northern AL. While that aids in exceptional moisture transport, that warm nose would also be a problem and make ice a bigger story than snow. This setup would be an exceptional FGEN maker, but at dry slotting would be a problem as the upper trough got closer. The NAEFS (last image) shows that even the GEFS/GEPS situational awareness tool shows exceptional moisture transport. That could be a sign of potential trends to come inside of 60 hours, because if that kind of moisture transport exists and more instability can come to fruition, that could sharpen up that IVT more at the Mid-Atlantic. Now most of it is oriented at the Southeast, but should the 500mb trough in the Plains trend stronger in the coming days, that orientation could lead to a better plume of moisture and more QPF to work with. On the flip side, this could cause more ice in parts of the VA Piedmont. Remember the ECMWF is the stronger outlier at the moment, and while the GFS and ICON may be not most folks cup of tea, it is what could happen if this system trends weaker. Lastly, I wanted to share the differences at 500mb from the 18Z GEFS vs the 12Z GEFS. We're still seeing some notable changes just 36 hours into the period, even before the Pacific disturbance arrives. While its becoming more likely we are seeing in some capacity a measurable snow event, these changes on an ensemble still show there's much more to resolve.

I'm up to a storm total 1.8" located 6mi northeast of Westminster.

Yeah I'm at 2.7" 6 miles northeast of Westminster. A light but pretty snow falling. One of the negatives with such a good band to the south is the air has to sink somewhere. Looks like northern Carroll and most of Frederick county is dealing with that at the moment.

Got about 1.25" northeast of Westminster so far and sitting at 24F.

Latest 02Z HRRR at 6AM-- area averaged sounding including most of the DCA-BWI corridor shows a sufficient sounding for some great looking dendrites. We've had more saturated soundings, stronger omega, deeper DGZs in bigger events, but this will still produce some locally heavy snowfall rates and lead to some pretty tough travel conditions. Looks like snow lovers will want to enjoy this one. May not get another real good shot at this again for several weeks.

Absolutely. For just about everyone along and west of I-95 (including the metros), WPC 6-hr averaged SLRs are 10-12:1 generally between 06-12Z, but 12-18Z are 13-15:1 (Catoctins could approach 16:1), then 18-00Z tomorrow are 15-17:1. Question becomes how much moisture & lift can stick around the region that late.

Josh is on the grids today and I'm the afternoon winter shift, so I'll shoot him a message

Living on the edge of your Zone C and Zone D. I'd happily take another 3" as would my kids!

However, take a look at early this morning up near the Lower Susquehanna Valley- that sounding regains some of that moisture at mid-upper levels with increasing VVs through the depth of the column. That could be pretty impressive over eastern PA if that can materialize Tuesday AM.

Yep good catch, that is probably the primary reason this band isn't a bigger 1-2"/hr setup. The lowest layer of the DGZ on down to the boundary layer is sufficient to give folks an advisory level event (pockets of warning criteria still possible). You want that DGZ zone layer fully saturated with strong VVs. Still, this is enough to produce a nice event here.

Really starting to see returns filling in at higher radar tilts in western VA. What originally featured drier returns around 4,800ft out near the I-66/I-81 merger (around 850mb) is showing brighter returns. For those who have patiently waited in Northern MD, those heavier rates will be realized soon.

850mb warm air advection continues to increase with 850mb frontogenesis that will support a band of heavier snow N&W of DC in the coming hours. What becomes the wildcard is the approach of a 500mb jet streak over the OH Valley overnight. At the nose of the jet, divergent flow atop the atmosphere will support healthy vertical velocities 12Z Tuesday. This is driving the snowier solutions from northern MD into eastern PA. The new NAM seems to feature similar setup.

I also have a half inch northeast of Westminster. Quite a few cars spun out on the road going home.

As a heads up, these maps are resent to include our hemispheric fronts created from our Alaska desk. The fronts over the CONUS by our day shift medium range forecaster (Rausch, if you look at the Fronts tab on the homepage) is identical compared to this one. The new fronts and pressure from night shift likely won't make the web for a couple more hours, and the overnight forecaster would use the 12/18Z guidance as their foundation for this next set of fronts/pressures forecasts. Usually these fronts and pressures have to be sent out before the new suite of 00Z guidance comes in.

January 2014 event- the week before KIAD got 8.5". For those dates, cold with 0.3". January 2011 dates- KIAD 1.2" January 1992 dates-- cold and dry March 9-10, 1960 dates-- 5.8" at DCA (Bigger storm here to open that month) March 11-12, 1960-- cold and dry at DCA December 11-12, 1958-- cold and a Trace at KDCA March 24-25, 1940-- cold and a Trace in DC January 23-24, 1940-- big storm, 9.5" in DC and cold January 29-31, 1936-- Cold and 0.2" in DC December 28-29, 1935-- Cold and 6.3" of snow in that period. So we know it's cold, but it can give us a decent event based on the event.

May be pulling up xmACIS to check what they did.

Ahh it did start to try to hint at it on the 18Z ECMWF. Hadn't gotten a chance to look at it. Will be curious to see if it keeps making that adjustment at 00Z.

This is a great catch. Just comping a couple other CAMs at 23Z Saturday, the HRRR and ARW remain more southeasterly and don't pinch off that 850mb low as far south as the 3kmNAM. While it isn't to say the 3kmNAM isn't on to something, there isn't any guidance showing an 850mb low evolution quite like what the 00Z 3kmNAM just did. It's something you keep in the back of your mind, but going to need to see it get more support before buying in (and that is assuming if it even still maintains this in future runs).

Something that has been on my mind, especially for those to the west maybe more along Parrs Ridge, is are we seeing guidance bring that warm nose in as-advertised, but also not capturing just how cold the low levels will be once they wet bulb initially? I can see an avenue that still certainly result in a less snowy regime. But if those N-NE winds at low levels holds, some areas may end up more icy than just rain. That will be worth watching on the CAMs in future runs. And the CAMs will be more included in the WPC PWPF now that we are inside of 60 hours for this event.

3kmNAM really went full blown NAM'd here. A notable change was the stronger frontogenesis at 850mb and 700mb (attached). Taking a sample sounding (it is basically eastern Frederick County, all of Carroll, and western Baltimore County, as far south as I-70 and on north to the M-D), the intense vertical velocities lineup well with the DGZ. This stronger FGEN held on for a good 6 hour span. This is the way to an over performer. Now, whether that actually occurs or not, well, we still have time to see.

The 00Z trends on paper show a colder/more south solution for the weekend winter storm. But there's still plenty of time for more changes. Just using the GEFS for this exercise. Here is a Tomer Burg plot showing GEFS probabilities for 500mb troughs. It's a good thing to see a lack of even moderate probabilities (topping out around 20-30%) for toughing in Quebec, suggesting the higher probability for confluence there. However, note the primary shortwave trough in the Southern Plains. As was mentioned a couple times on the forum, part of this is less phasing/interaction with a northern stream shortwave. However, some of this signal could also be a signal flattening of the shortwave. Part is this is also growing dispersion in the GEFS (more ensembles showing more and more different potential amplitudes/positions of the trough drowning out the signal), but it likely includes some flatter solutions. We can get away with slightly less amplified features, particularly in an El Nino when the STJ is super charged and these southern stream disturbances are loaded with moisture. However, the need for strong upper level ascent out ahead of the trough is still important in generating both heavier snowfall rates and dynamic cooling. In short, there are still signs where this trough could weaken, but there are enough solutions to suggest this system remains quite strong. It is simply too early to tell. But we could get by with still a meaningful and impactful event even with a weaker shortwave. A good example was February 26-28, 2003. Note on the reanalysis image for Feb 27 at 06Z-- the surface low along the Gulf Coast is weak, but there is an anchored high to the north and some tighter 1000-500mb thickness packing over VA. On the archived 500mb chart centered on 06Z Feb 28, 2003, the 500mb trough didn't exactly look that impressive. Still, they produced a swath of 4-8" (CIPS image), which given the snow drought we've experienced, just getting an event like that would be a start. Dulles managed 6.4" from this event, DCA 5.3", and BWI 4.5". We'll see how trends go in the coming days. Seems like a storm is coming. But to what extent is still up for debate.

Yep. We get to work remotely a couple times a pay period. But otherwise, I commute to College Park. On a great day, I can get here in just over 1 hour. My wife works in the Hereford Zone and my in-laws live near Freeland. So it's closer to family and my wife's job which is what we wanted (originally lived near Sykesville). Farther drive for me, but I remind myself on those 90+ minute commutes home that the house/price was worth it, and living at 1,000ft will occasionally have its perks in winter.

That MillvilleWx guy is a pretty good dude also. Knows his stuff pretty darn well. P.S.A. for all the mets that contribute on here. It takes time away from their day (and nights depending on what your job is), while balancing being a significant other/spouse and possible father/mother that many of the mets make to contribute on here. I wish when I was in middle/high school that I had the kind of access to degreed mets (and even those non-mets that have learned a ton over the years on forums such as these) that younger folks have today. It's going to be a long 5-6 days for this system and still plenty of winter to go. Be courteous and treat others how you'd like to be treated. Sprinkle in a couple good snowstorms and this place ought to be fun.

There are many variables to watch for next weekend's winter storm. The first glaring one that stands out is northern Quebec. The ECMWF does NOT buy what the GFS is selling with the upper low aiding in stronger confluence. The CMC is more of a blend of the two. Outside of the upper level feature in the South, that upper trough the GFS is showing in northern Quebec will be something I'm monitoring in the coming days.