GEOS5ftw

Yeah, that's the main difference I see on this run - the southern shortwave speeds up and is a bit more neutral than positive tilted, which leads to a strong sfc reflection. The NS didn't change that much, but this progression gives cold a little less time to press in from the N.

I was also questioning the negativity earlier. The pattern doesn't really change until we get high pressure dominating in Canada. That doesn't happen until the northern stream low moves through days 5-6. There's been nothing but low pressure up there since the pattern broke down after the mid-Dec storm. That's a direct result of the Pacific torch but all of the ensembles have been forecasting a complete reversal and hasn't been pushed back in time for several days. That's a direct result of the block and favorable changes in the Pacific as the flow blasting onshore weakens and reverses. That should give every wave a better track and cold air to work with. Also, I was compiling some climo for MBY from my own records and nearby CoCoRAHS which should represent most of the near-beltway suburbs since 2005. Pretty much every year has featured a 3-6 week stretch without ANY accumulating (>T) snow in mid winter...usually from mid-Dec to mid-Jan, or early-late Jan, or didn't even get going until mid-January. So this isn't a new or unusual pattern. Maybe different if you go further back but it's typical the past 15 years, not just since 2016.

Nice recollection. That first system (Jan 20?) is overlooked because of what came after, but a very underrated storm IMO. It was a pure NS wave that dropped far enough south to work for us. Then it blew up into a huge bomb offshore (like 950mb) and hit Nova Scotia pretty hard, but missed NE. That's why it was so windy afterwards, some of the most impressive blowing and drifting I have seen in this area especially considering the storm total was only 3-6 as you mention. I wonder if the look being advertised day10+ could lead to this type of opportunity.

Since I don't think it has been answered yet, the 500mb height is a function of the surface pressure and thickness (which is proportional to temperature averaged over the layer). So, BN heights can be the result of BN temperatures from the sfc to 500mb, BN surface pressure, or some combination. When the 500mb height is BN but sfc is AN, that tells me the sfc pressures are low and/or the sfc temp is higher than would be expected for a given thickness. This makes sense since Pacific maritime air masses have steeper lapse rates (warm sfc, cold aloft) than continental/arctic origin air (cold sfc, cold aloft). Think about it - air coming from the Pacific more or less assumes the SST after many days. There isn't enough time when these travel over North America to cool radiatively at the sfc. Also, the latent heat added from orographic precip over the Rockies actually results in a warming of these air masses (chinook effect). Contrast that with Arctic air masses - the cooling is strongest at the surface (surface radiates heat much more effectively than atmosphere), so these tend to have a strong inversion. Further, when you get an EPO ridge the cold air doesn't have to cross the Rockies, it comes down the eastern slope from the north so no latent heat gets added (not that there would be much considering how dry these air masses are). Models are really good at moving air around - I'm not sure how well they represent the radiative heating/cooling though. It's pretty complex and depends on the surface properties, water vapor, etc. I remember reading a paper about the formation of deep cold air masses, and the authors found that you get "diamond dust" ice crystals - basically very light snow w/o clouds - forming below -40C (or something really cold), and these radiate heat more effectively than the air itself. At really long lead times if these processes are not represented properly it could result in a odd looking temperature for a given flow pattern. There are modeling experts on this forum who know way more than me (I'm from more of a radar/remote sensing background) , so I'm happy to be corrected.

0Z RGEM out to 48 hours on this site: https://collaboration.cmc.ec.gc.ca/cmc/cmdn/pcpn_type/pcpn_type_gem_reg.html (be sure to select 0Z 12/15 run) Seems to be colder with a bigger thump in front (4-5 hrs of snow for DC proper), IAD looks to barely stay all frozen through 0Z (based on sfc low position, that should be the max extent of mixing). Should be interesting when full run is in.

My PWS records only go back 4 years but it does seem like we stepped up into a higher summer dewpoint regime in 2018. I'd be curious how previous years compare. Monthly avg dewpoints, 2016-2020 July: 67.2, 66.7, 69.9, 70.4, 70.4 August: 67.7, 64.5, 71.8, 68.0, 72.0 (so far)

Just measured 11.8 inches on the top of the car, 5 new since the heavy stuff started late afternoon. Seems to agree with other reports around me (Weather53, 1N four corners in the PNS). From the looks of radar we may be able to eke out those last few tenths to get over an inch. BTW melted a core sample earlier today, ratio was probably 11:1 so not outrageously high. Will take another tomorrow morning.

Didn't realize there were so many of us in close proximity. I'm near N Belgrade/Hillsboro and measured 1" at around 7:15, roads and sidewalks around me were covered at that point.