cold front intrusion amid strong southwesterly flow

[samuel stone]

I noticed this year alot, dating back into febuary plains snow storms even, that cold air has remained in place, or crashed in early, many times even when while the main trough or cutoff upper low was still well to the west and upper flow was still southwesterly on the front range. a situation where normally lee cyclogenesis would draw up warmer air. I have not personally witnessed this happening so many times in the same winter/spring. I was wondering if any of the mets on this board could give ome sort of explanation? does it have anything to do w/ persistent upper troughing in southeast canada? thanks for any help

 

[Stephen Levine]

Shallow cold air, dense and acting like a wedge rushes southwards across the Plains, undercutting the warm moist air. Due to it's density/weight it moves of it's own accord, sometimes even outsmarting computer based outlooks.
The density, perhaps 3K feet or even less is enough to plummet the temperatures at ground level. However with the main trough and/or upper level low pressure system still located to the west, the unstable warm, moist air over-rides the cold dense wedge, producing precipitation up to and including convection/thundersnow and even hail.
In situations like this, low pressure systems rolling across the northern Plains, too far north to directly affect our locale with precipitation never-the-less swings the shallow arctic front southward way beyond it's immediate domain.

 

[Mike Johnston]

Yes, Stephen, and I might add that the upper-level troughs that have come through recently have taken on quite a positive tilt - seen in the models in the couple of days leading up to the forecast event and especially on the day of. For seriously severe, you want at least a neutral tilt and ideally a negative tilt to the upper trough. Southwest upper flow may persist in any event, but instability is shunted in the case of a strong positive tilt, and a crashing cold front at the surface becomes more and more likely.

 

[samuel stone]

i know all about the density of cold air, and i know a poitively tilted trough is much more likely to allow dense cold air to stay in place. but, I feel like there's more too this. i have seen negatively tilted troughs this year that have not advected as much warm air as some positively tilted troughs ive seen in years past (9 years or so of close attention) seams like theres another factor is in play?, maybe the last couple springs have affected my judgement more than i know.

 

[Logan Karsten]

One thing I would add to this is look at the current snow pack across the US - http://www.nohrsc.noaa.gov/interactive/html/map.html. Any air mass originating from Canada right now isn't going to really start modify until after it hits Nebraska where it's snow free. While one usually expects residual snow pack across the Northern Plains going into April, this year's snow pack is unusually deep and persistent given the wet and cool pattern we have been experiencing this past couple months. If you use the drop down menu on the left and look at previous years, you will see this year is highly unusual with the deep snow pack still around. I think this is having a large impact on what we are seeing across the plains. Usually, the Northern Plains is mostly snow free, allowing for strong air mass modification before getting to the Southern Plains. However, this year is just not the case. I think the models are struggling with this, given the time of the year, and the strong heating that is taking place now across the Southern half of the country. It has lead to a very unique situation. Obviously, the snow will melt away, but until that happens, one should probably be cautious of this while looking at forecast temps from the models.