GEOS5ftw

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)

12Z IAD sounding shows a decent dry layer just above the sfc, dewpoints a few degrees lower than the 6z NAM forecast fwiw. Makes me feel a little better about my 39/29 pws reading knowing there is some drier air just above the surface. ------------------------------------------------------------------------------- LEV PRES HGHT TEMP DEWP RH DD WETB DIR SPD THETA THE-V THE-W THE-E W mb m C C % C C deg knt K K K K g/kg ------------------------------------------------------------------------------- SFC 1010 98 4.2 -2.8 60 7.0 1.4 10 7 276.6 277.1 274.0 285.1 3.07 1 1000 169 3.4 -5.6 52 9.0 0.1 10 8 276.5 277.0 273.2 283.6 2.51 2 944 633 -0.7 -7.7 59 7.0 -3.1 358 11 277.0 277.4 273.1 283.3 2.26 3 925 795 -1.7 -6.5 70 4.8 -3.4 340 7 277.6 278.0 273.8 284.7 2.54 4 896 1048 -3.7 -5.6 87 1.9 -4.4 258 11 278.0 278.5 274.5 285.9 2.81 5 872 1263 -2.1 -2.6 96 0.5 -2.3 231 18 281.9 282.5 277.7 292.0 3.62 6 850 1466 -2.7 -3.0 98 0.3 -2.9 235 20 283.3 283.9 278.4 293.5 3.60

Just did a fun measurement with the kitchen scale - put out a large plate and measured 52 grams in 15 minutes. Dividing by the area of the plate gives a mass change of 0.34 g/hr/cm^2 which is 3.4 mm/hr, close to what some of the mesos were predicting in the narrow bands. Fun stuff. eta: Just measured 2" on the ground here in Wheaton. Pretty impressive consdiering first flakes were a little more than a hour ago. My bar of 3" looks like it will easily be met.

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.

FWIW (less than nothing since it's extrapolating the NAM), the northern edge of the precip shield on the 3km at hr 60 is about 40mi furthern N than the 12km at the same time...interpret this as you choose.