DarkSharkWX

12/-2 with -7 wind chill in Gaithersburg

28/25 gaithersburg

CAD over preforming rn

That would be putting stress on PV, with the Scandinavian ridging/high and low near the Aleutians. Good pattern for PV disruptions.

GEFS has the signal around 27th now

I wouldn't fully give up on this system yet, there is front-end thump potential as well as some potential for some backend snow. If this threat doesn't work out, we also have the 27th to look to

NPJ gets too extended which brings warmer period and pacific puke, however maybe the 27th threat could work out before we switch to a warmer period

New WPC forecast shows 10-50% of exceeding .25" of snow/sleet(liquid equivalent) for much of the mid-Atlantic/northeast, and even extending down into the southeast.

FCPS will most definitely close because of the widespread icing that is expected to continue(as well as the ISW), and MCPS might close as well.

Here's a wetbulb calculator if anyone needs it(<32 Tw freezing rain, >32 Tw rain): https://www.weather.gov/epz/wxcalc_rh

Looks quite icy outside here in northern MoCo. 32/30 with Tw in 30-32F range

Does anyone have FRAM on pivotal for the latest HRRR?

Wetbulb temperatures for DC and north are already 32 or lower. A study found out that the best wetbulb temperature for ZR is 27-30F, which isn't that off and will reach further south as temperatures drop.

Got to a low of 18 here in Gaitherburg, colder than I had expected. Right now 37/17

Think everyone in advisory will close/go virtual or at the very least delay

This is what a study found about precipitation rate, wind speed, wetbulb temperatures, and how they correlated to ILR. Freezing rain can occur even with temperatures above freezing, but if the wetbulb temperature is above freezing, freezing rain can't occur. The study generally found that wetbulb temperatures between 27-30F, greater windspeeds, and lighter precipitation rates all contribute to more efficient ice accretion. Here is the link if anyone is interested in it: https://training.weather.gov/wdtd/courses/woc/winter/fcst-hzds/ice-storm-accum/presentation_html5.html

The best conditions for optimal ice accretion are greater wind speeds(takes away hear from surfaces and advects cooler air), small droplets falling through a shallow melting layer, lower wetbulb temperatures, and lighter precipitation rates(heavier precipitation rates lead to more runoff and less efficient accretion).

FWIW, GFS has a better run with lower heights over eastern 1/3 of US, less seperation between departing 50/50 and the ULL, and more confluence(heights over Canada further south). This all helps bring a better CAD signature and a further south ULL.

RGEM has a further west TPV and weaker ridging over Ohio Valley/Eastern 1/3 of US(based on looking at vorticity and isobars). This is likely why we saw a colder solution this run, with colder temps at SFC and 925mb. We did however have warmer 850mb temps. We will know more tomorrow as mesoscale models get into their better ranges and more things get sorted out.

GFS has the ULL negatilt earlier this run with a further west TPV and more western ridging leading to a colder solution(SFC ptype map just used to show that GFS has colder solution)

With all favorable teleconnections(-EPO/-NAO/-AO/+PNA), we don't have to worry about getting good cold air. However, this pattern will also be active in terms of storms. The -EPO will deliver polar flow and allow cold air to flood into the CONUS as well as bring strong NS shortwaves into the US. This is evident on the 250mb wind maps where we see an active northern stream. With an active STJ(as shown on 250mb wind maps), providing southern stream shortwaves, the northern stream shortwaves dive down and potentially phase with the southern stream shortwaves, which could bring large storms along with the cold air.

GFS slightly better this run with what appears to be a further west TPV, less weakness in the heights over Canada, and weaker ridging over Great Lakes/OH Valley.

This Euro run we saw more separation and less interaction with ULL and PAC trough. This lead to more western ridging building in earlier, and also means that the ULL doesn't negatilt early enough, and stays positive tilt for longer which isn't good. Additionally, the TPV seemed more east this run. Now we want more PAC trough interaction with ULL so that it negatilts faster, but not so much that it gets sheared out.

GEFS has been trending with a stronger CAD push for the 16th-17th storm. Encouraging trends to see

One reason today's afternoon EPS was worse than 0z is due to the placement and strength of the Aleutian low. Comparing 12z to 0z, we see that the Aleutian low in 12z is weaker and further west. A stronger + further east Aleutian low allows for a more poleward western ridge to setup, and along with the -EPO and -NAO/-AO, allows for cold to press on the EC and gives us the great look. Note: The 12z EPS today is certainly not bad at all, and is still a good look, just not as good as previous runs and not the "textbook" setup.