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The feb 4-5 storm (Obs/Discussions)


Ji

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we were looking at a property in Harpers Ferry late this morning and came home just as the precip started. even then snow was mixed in, and I have been out on the porch a lot today/tonight just looking at how pretty it is here. the only bad thing is that when it precipitates I lose the view of the Apps from our bedroom, but at least today it's been snow. I spent almost 20 years in DC wishing I were out here for winters like this and even though it's a low snowfall winter, I have really enjoyed this winter so far (albeit with appropriately lowered expectations).

but still, as I said earlier this winter, Shenandoah Valley winter!

Zwyts promised me a mid-March bomb last night.. :sled:

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Weatherbug's are not always accurate...33 seems low to me based on surrounding obs, but maybe you were close....point is you were going to go to snow (you were already mixing) assuming we actually had more precip....I don't ever recall an event in mid winter where the column from 875-900 mb and up was below freezing and it was a prolonged synoptic rain event with temps in the mid 30s....and this event was no different.....nobody was going to get hour after hour of rain and 34 with this setup...I'm not saying anyone advanced that idea, but when it looked like more of a prolonged event into the overnight hours, anyone suggesting an all rain event with temps in the low to mid 30s would have busted

This goes along with what we mentioned last night with the model soundings...when they start showing 0C from 925-950mb during steady precip, its usually only a matter of itme before it snows regardless of what they try to show at the surface unless you have a wind off the ocean which this event did not...and a wind off the ocean usually will struggle to have 0C as low as 925-950mb anyway, so those cases are rare.

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Weatherbug's are not always accurate...33 seems low to me based on surrounding obs, but maybe you were close....point is you were going to go to snow (you were already mixing) assuming we actually had more precip....I don't ever recall an event in mid winter where the column from 875-900 mb and up was below freezing and it was a prolonged synoptic rain event with temps in the mid 30s....and this event was no different.....nobody was going to get hour after hour of rain and 34 with this setup...I'm not saying anyone advanced that idea, but when it looked like more of a prolonged event into the overnight hours, anyone suggesting an all rain event with temps in the low to mid 30s would have busted

I want a 1a precip type report, :P

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I think so...this stuff looks to head through DC as well...maybe the 0z NAM will be right...goodness you would think it could get a 12 hour forecast right.....

the 0z nam spit out an underwhelming 0.02" qpf after 6z at DCA. so if it's right, we'll get some flurries/sprinkles inside the beltway...

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I just use 36-37 as general idea of going to snow as I don't think this setup supported a prolonged rain event if one could maintain that temp or lower based on my experience....If one looks at old metars which I do all the time we see that over and over in rain to snow events....obviously the 925-950mb layer is the one that actually matters, but 36-37 or lower usually suggests that that layer is there or getting there soon

Yes...as long as there is no inversion (which you have stated many times) then its usually a pretty good temp to base a mix/changeover on. 36 seems to be a favorite number here when we are changing over in a a situation with no warm layers aloft.

Realistically you want that 0C height getting below 950mb as the flakes have a hard time surviving more than 400 meters of above freezing air....but seeing 925-950mb on the models knowing mod/heavy precip will be around ensures that the 0C will continue to lower without aid of warm air advection which this type of event clearly lacked. So that's why its typically good to look for those wetbulb zero heights on the soundings when steady precip is around...it can kind of give a threshold for changeovers. Models generally don't lower those wetbulb heights fast enough in those setups. N and central MD saw this play out with more snow in the afternoon than models gave originally.

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