Phil

Well, the answer is no, the dewpoint doesn't have to be below 32F for frost to form. The frost-point is always higher than the dewpoint (stronger molecular bonding requires a higher temperature to complete a full break cycle at/below that threshold relative to surrounding H^2O molecules). This is why aircraft often accumulates ice when flying through thunderheads.

Well, it's the surfaces accreting frost that cool below 32F, not necessarily the air itself. That's why you don't usually see frost on cloudy nights (radiative exchange between the clouds/nucleated aerosols and the surface prevents the surface from cooling below the LTE threshold). This is also why you'll often see frost on open fields but not under trees, thanks to backradiation preventing the ground from radiating efficiently. If it were just the temperature cooling to the dewpoint below 32F, you'd have freezing fog, or just regular fog. There's a difference here. The dewpoint doesn't have to drop below 32F.

Yeah, I agree that we need some form of high latitude blocking this winter. I don't think it has to be a -AO, but a raging +AO/+EPO like last December won't work, obviously. Perhaps even a +EPO could work under a healthy -NAO/-PNA, neutral AO? Or a deep -EPO/-PNA/+NAO can work too, though that's more fragile I think, pattern wise...kinda hit & run, so to peak.

That's not how the PNA is calculated. See the link in my previous post. No real -PNA to speak of during last December..just a behemoth GOA vortex and equatorward Pacific Jet.

That's the monthly average, yes. The daily averages are here if you're interested: ftp://ftp.cpc.ncep.noaa.gov/cwlinks/norm.daily.pna.index.b500101.current.ascii There was a weak -PNA stretch the lasted 5 days. Less than one standard deviation below average..basically a non signal relative to the EPO/NAM.

January 1943 @ 500mb. Block party, with -NAO dominance.

Oh, 1942-43 also had a massive -NAO in January. Second strongest -NAO of any Niña winter, actually. The strongest -NAO winter of all time is 1959-60. Here's what it looked like, temperature wise: http://i724.photobucket.com/albums/ww243/phillywillie/Mobile%20Uploads/9F624EB5-F4A1-4A02-81AF-9894D0543A01_zpsargshgwb.png

For reference: http://www.cpc.ncep.noaa.gov/products/precip/CWlink/pna/pna.timeseries.gif

The index went weakly negative for 5-6 days, but really that wasn't a real -PNA, just an extremely contracted, fully coupled polar vortex and corresponding flat height rises over the mid-latitude Pacific. Obviously, a pattern like that won't ever do the trick in the lowlands.

The winters of 2009/10, 2010/11, and 2012/13 were -NAO, while 2008/09 was neutral NAO. The winters of 2011-12, 2014/15, and 2015/16 were +NAO.

The PNA wasn't negative last December. It was quite positive, actually. Last December was a strong +EPO/+NAM, with that flat equatorward NPAC ridge representing the climatological loading pattern for a +EPO. Definitely not a -PNA.

Based on which features, exactly? The computed PNA values reflect a weak negative PNA in Dec/Jan, followed by a heavily negative PNA in February.

Yeah, when juxtaposed with a -PNA, the -NAO can actually assist in keeping troughing in the west, as opposed to a January 2014 type deal. The prolonged +NAO in recent winters I think has been a contributing factor to the warmth in the western US, given that a +NAO constructively teleconnects with a +PNA, in most cases. Meanwhile, the -NAO stretch from 2008-2013 featured improved winters over the western US as a whole.

Yeah, recent history suggests a -NAO is somewhat more favorable for precipitation/snowfall during sharp -PNA events.

I have an academic account on StormVista.

Slow down there, sparky. I'm just describing a model forecast. Did I say I "knew" what was going to happen? Nope.

Where are you seeing the +PNA? I'm seeing a neutral to slightly negative PNA in December and January, followed by a very negative PNA in February.

December on the monthlies is crazy looking. Like a blend of January 1979 and December 2000.

New Euro monthlies are a block party. December: Huge -NAO and -EPO blocks...they actually form a "ridge bridge" across the entire Arctic domain. Looks like a December version of October 2009, probably even more extreme, if anything. January: Crazy -NAO continues, however we lose the EPO, looks like a neutral EPO/PNA..not much of a signal either way in the western US. February: More classic Niña (-PNA/+EPO) overall, however that big -NAO continues unrelenting. Trough centered in the west. Talk about a crazy pattern.

Weird, that first sentence didn't show up on my safari browser before I quoted the post. I see it now, though.

Haha, right after I bring it up, the 18z GFS decides to juice up the stratPV and couple it w/ the tropospheric NAM following the secession of the cyclonic breaking out of the NPAC. Result is a blowtorch. Hopefully just a blip.

Wait, are you @blizzardof96 (Severe Weather Blog) on Twitter? If so, aren't you a student at the University of Western Ontario?

I've spent the morning analyzing the NAM state from 10mb to 1000mb over the last few months, and I have to say, the current situation is quite peculiar for a -ENSO/+QBO year. Polar cap heights have been well above average since March, and though it's early, the PV looks to begin the cold season weaker than it has ever been for mid-October in a Niña/+QBO (during the satellite era), except for maybe 2009 which was a Niño/-QBO (hence weak early-winter PV is expected in that case). Looking back, I can only find a few years with even remotely similar stratospheric profiles under similar ENSO/QBO background states. One of these years is 1959/60, but this was a cold neutral/high solar year. There are a few other years, like 2010/11, that hold some similarity at/below 40mb, but nothing like 2016/17 so far.

Looks like DCA gusted to 42mph. So far I've got 27mph through the trees. Dewpoints dropping into the upper 30s and low 40s, perfect for a midday jog. I love downsloping days.

Currently 57.7/49 early this morning with a light drizzle falling intermittently. Can hear the breeze outside from in our bedroom. Probably hitting 30-35mph out of the north, but unfortunately my anemometer is blocked by pine trees from that direction, so the peak gust is only 20mph so far.