GEOS5ftw

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.

Looking at the hourly maps at http://collaboration.cmc.ec.gc.ca/cmc/cmdn/pcpn_type/pcpn_type_gem_reg.html, 0Z CMC starts between 10-11Z whereas the old run started 13-14Z, so yes, a bit quicker. It does seem like the trend is for the ridge to pump more if anything, so getting precip in quicker is the key to accumulations. With the high scooting off to the east, I don't think the low levels will hold on for long in the urban corridor during the day.

FV3 starts before 6z and heavy by 12z...very NAM-like. Flips to ZR by 18z though...probably 0.6-0.8 qpf before the flip. This according to pivotal maps.

RGEM is a qpf bomb (0.5 from I95 north and west with more to come as the run ends at 0Z) as it spins up a sfc low along the front but a good bit of that is rain especially towards DC. North of 70/west of 83 is mostly snow. Will be interesting to see the HRDPS later. Where's cae with the RGEM ensembles?

One thing about real deal arctic airmasses (capable of setting daily records, where all the airports are lower single digits and outer suburbs below 0) is that they seem to come in with some kind of frozen precip even if just 1-2 inches of snow with the front. Think back to Feb 2015, early Mar 2014, and going back further Jan 94/Feb 96. All were big time arctic outbreaks, the latter two featuring the pool of -40C air in the midwest, similar to what we are seeing on the models for next week (more recent ones were in -34 to -38 range iirc). Maybe someone with a better memory can think of a time when all the airports were near zero w/o snowcover (Jan 2014 might be the best example but there was a 2-4 inch event a few days before the arctic front and I seem to recall that front underperforming the hype a little around here) Now part of this might be a chicken and egg thing - without snow cover the boundary layer probably moderates enough along the trajectory that by the time it reaches our area, temps are out of the extreme range. But I do think it has to do with the nature of the air mass and that true arctic air is a) usually preceded by an cold enough antecendent airmass that any precip produced by the front can be frozen, b) the shallow nature of the cold lends itself a bit more to an anfrontal structure (showmethesnow had a nice explanation of this earlier today). Another point is that to drive those air masses into our area you need to be to the west of the surface low to stay in or near the cold sector and have a strong northerly component after the low passes - a true cutter like the one last weekend or today can't do it because the flow behind the front is has more of a westerly/downslope component which reinforces the dynamic downward motion and maxes out the adiabatic heating. I'm not sure how much of this applies to next week since the core of the cold seems to want to stay north, except on the op EC which of course amplifies the storm in the right place to drive the core of the cold into our region. But I'd be skeptical of forecasts for area-wide temps near zero unless we get some snow cover laid down ahead of time.

Gotta be tough to be a snow weenie in Chicago. Just 3 days after temps in the -20s they get rain per the GFS.

Key is how high the dewpoint gets...low-mid 30s for 6 or even 12 hours won't do much damage. 40s or 50s will decimate it within minutes.

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.

12z HRDPS is impressive - best stuff stays south of the area but even so, has 4-8 inches with more to come after the run ends.

RGEM has a much more impressive precip shield at H54 than the NAM does, although it does seem to be a bias of that model (overestimating precip in dry air - in the 12/9 storm it had the furthest north, and most gradual cutoff). It will be interesting to have the HRDPS in range tomorrow - according to cae it did the best of the meso models for that storm with the cutoff.

CMC shifted north from 12z...6+ line up to DCA/BWI.

cae did a nice writeup about a week ago. CMC did best overall although it didn't nail the tight gradient on the northern edge (HRDPS did in the short range). FV3 was better than GFS and Euro/UKMET/ICON were about on par with FV3 for this one.

Yep, a bit different evolution with a later phase but big step from the 0z run.

Less of a weakness over the lakes though. Should be a good run. Will be interesting to see the trend on the 12z Euro/UKMET for sure.

This is not a terrible look at H144 from the UKMET. That 1036 high arching down from Hudson's Bay is in a good spot and the upper plains s/w looks ready to drop into the southern stream.

Ji must not have seen the JMA yet...extrapolating from 192 is a HECS.

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.

While we wait for Fv3 the GEFS looks to hold with a bit of tightening of the gradient in total precip...this is looking at total precip on TT not snowmaps but I would expect the means to looks the same as 18Z...maybe a bit less towards PA and a bit more in central VA.