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Pattern January 2021 - Joyless January

The back half of GSP's long term disc this morning
A brief period of quasi-zonal flow then is progged to begin as the
synoptic trough pulls away, but another deep shortwave will swing
through our area Sunday night and Monday. The models are not yet in
good agreement as to the depth of this wave, and accordingly whether
it produces precip as it blows thru the CWA, though they are
currently reasonably close on timing. We almost certainly would have
sufficiently cold profiles to expect snow to predominate as p-type
over the majority of the zones, if the wave can muster enough
moisture. While a few of the NAEFS ensemble members depict the
shortwave as kicking up a coastal low and bringing us substantially
more precip, this isn`t a strong enough signal to affect the means
very much, so it looks wise to keep PoPs very low for this part of
the forecast.
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12z ICON doesn't look bad setup-wise at day 5. A big sfc low & accompanying mid-level vortex over New England is gonna help suppress the next storm coming in stage left over TX & provides significant cold air for this setup.

Don't necessarily need a big cold high to the north to provide cold air, low-level cold advection works for both sfc highs and lows. They both just need to be in the right juxtaposition.

1610465248795.png
 
Webberweather53 said:
The 12z ICON is trending towards a slower/more amped storm this weekend over New England = more cold air/suppression for the coming southern slider.

Definitely a fan of this trend here in the Carolinas.

View attachment 65192
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Where's the cold HP in this pic though Webb? I really do not see a source area other than some back wrap from the NE LP.
 
Interesting pattern shown late January. I love the strong -50/50 low signature, key to some nice cold wedges. Love the pacific ridge going up to the pole too. Translates to cold enough for us to have a good time.

Will it show up? And will the SE ridge be muted enough to keep things to our south? At least it's something to watch.

1610464436616.png

1610464488802.png
 
CaryWx said:
Isn't that cruising out of the picture though? Transient?
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It's also a dry, downsloping westerly wind vs a north or northeasterly wind from a parent high to the north, which would then interact with a moist south or southwesterly flow aloft to create overrunning. I'm not sure how an overrunning winter weather event occurs with downsloping west-southwesterly winds east of the Apps.
 
CaryWx said:
Isn't that cruising out of the picture though? Transient?
Click to expand...

It's been persisting more in subsequent runs. Cold sfc highs are related to but not directly correlated w/ low-level cold air, synoptic-scale sinking not forced hydrostatically by low-level cold is just as if not more important in generating big sfc highs.

sdm47 said:
It's also a dry, downsloping westerly wind vs a north or northeasterly wind from a parent high to the north, which would then interact with a moist south or southwesterly flow aloft to create overrunning. I'm not sure an overrunning winter weather event occurs with downsloping west-southwesterly winds east of the Apps.
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Well, no that would be cold air damming, not overrunning. Overrunning is analogous to anafront setups and involves a weak/ill-defined sfc wave riding along a boundary and throwing warm/moist air back into the cold side of the front leading to freezing precip. Like CAD though, there's some element of isentropic upglide that helps force ascent but it can occur in the absence of the Apps and basically anywhere in the SE US. You don't necessarily need a cold high to the north to make overrunning happen, it happens pretty frequently right along cold fronts in their absence.

The early December 2017 storm in Atlanta is a good example of a big overrunning event happening near the tail end of the winter season w/o a cold high to the north. Cold sfc highs are often present but they're not necessary as this case shows:

1610466837243.png
 
One notably difference between the 00z & 12z icon is the s/s wave that comes on shore in northern Cali is significantly weaker and when it interacts with N/s it essentially shears out instead of phasing properly thus you are left with is dying precip over the se instead of a gulf low forming..
 
Looks like that one (Atl) occurred with a GL LP in place as well. Unusual set up I would think
 
CaryWx said:
Looks like that one (Atl) occurred with a GL LP in place as well. Unusual set up I would think
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I guess the point I'm making here is that big cold highs to the north aren't necessary at all in overrunning. This one occurred in early December and south of NC in a place w/ much crappier snow climo. Thus, we can easily get an overrunning event in NC w/o cold high to the north in the heart of winter
 
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