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Posted at 02:03 PM ET, 02/14/2012

Another iffy snow threat in the offing

Yesterday Jason posted that several of the international models were forecasting a storm to be located off the North Carolina coast Sunday raising the potential for snow. But last night only the European model and its ensembles held on to the idea of a coastal low tracking close enough to the coast Sunday to threaten the area with precipitation. And now the latest European model is backing away from the idea.

The European model solution from last night offers what would normally be an almost perfect track for producing a snowstorm in Washington. And the model does forecast some snow across the area. However, despite its forecast for some snow, it is also is forecasting surface temperatures to be above freezing through the event.

Most of the other models keep the storm track far enough south that our area would get little or no precipitation. So right now, probably the most that you could say about this weekend is there is a chance of rain or snow. But, it’s even more likely the entire system stays far enough to our south that we remain dry.

DETAILED MODEL DISCUSSION

The GFS models’ suppressed surface low track (well south of D.C.) gets support from the last night’s UKMET, Canadian model and its own ensemble forecast system. However, the European model’s own ensemble mean (from last night) was in pretty good agreement with the operational run.


120 hr GFS and European model 500 mb height and vorticity (spin) forecasts valid 7 p.m. Saturday evening (Penn State)

Subtle differences in the pattern end up making big differences on how much the low develops and where it tracks. Let’s look at the 500 mb height (around 18,000 ft) and vorticity (spin) forecasts from last night’s GFS and European models (above). Notice first that toward the northeast portion (right upper) of each map there are big differences in the configuration of the vortex over Eastern Canada with the GFS version (left image) being much more expansive and consolidated than the European version (right side image). The strength of the GFS vortex does not allow the flow to develop a southerly component and instead the upper level winds (Blue arrows) are almost due westerly at our latitude. Such a strong vortex also tends to damp out and suppress the approaching system from the west.

By contrast, the less consolidated European vortex pattern to the northeast of the approaching trough (dip in the jet stream) allows the winds (above right) to become southwesterly across our region ahead of the wave (dip in the jet stream and contours).

Essentially, the European allows some phasing between a upper level trough coming eastward across the Plains with another upper level system coming across the deep south. The southwesterly flow ahead of the system helps pull moisture northward towards our area.

All the x’s and shortwaves (red areas) on the two maps are troubling for snow lovers. When phasing of two upper level impulses is needed to bring a storm northward up the coast, it is easier to get the phasing right when there are less moving parts (upper level impulses). For that reason, despite the European models slightly better verification statistics, it probably should be viewed with a little skepticism. Also, when there are so many fast moving upper level impulses in play the models often have poor run-to-run consistency. This pattern has lower than normal predictability. The multiple impulses literally can cause chaos and play havoc on the models.

Complicating the forecast is the GFS tendency to suppress storms making it impossible at this time range to definitively choose between the two camps. This morning’s GFS and its ensemble still keep the storm suppressed which I think is the slightly more likely solution but with little confidence.


Now let’s look at the last night’s GFS and European model surface maps from the same run (above) and look how different they are in their timing of the low and how far north it comes. The GFS, including this morning’s run, keeps the low far enough south when it tracks off the coast to keep any precipitation away from us. The European model is much stronger and farther north with the low leading to the much wetter solution. Both have the freezing line (zero Celsius line) well south of us at 850 mb (5000 ft - colder enough for snow at that level) but the wetter European model solution has the surface freezing line well north. It’s a little early in the game to take the European model surface temperature forecast too seriously as dynamics can lead to a cooler solution on occasion. However, it does argue that even if the European solution is correct, this will be another one of those messy events where areas north and west have a better chance of getting accumulation compared to areas near and southeast of the city. And it argues there is potential for a mostly rain event despite the rather favorable track of the surface low. .


Last night’s 3 hour European snow accumulations ending Sunday afternoon. Snowfall legend is shown but it does not take into account warm surface temperatures in its accumulations.
For those snow lovers who like to fantasize and need a snow fix, the European snow map from last night is a fine Valentine’s day present. Despite the relatively warm surface temperatures, it forecast snow over D.C. (the black dot). However, the product assumes a 10-1 snow to liquid ratios which is unlikely if the temperatures end up above freezing. Still, the product suggests there is some hope for snow lovers - although the latest European run puts a bit of a damper on that.

Now that I’ve got all the snow lovers salivating, it’s time for the bad news. In my view, the European solution from last night is the less likely one as it would need perfect timing of all the various players. I don’t think that is likely. Sure. we may thread the needle but the eye of it right now still looks pretty narrow. Still, there is lots of uncertainty and what I think means little. The European model from last night could be right and it certainly suggests the system is worth monitoring during the next couple of days. And even though the latest European model takes away the storm, small shifts in future runs could bring it back.

By  |  02:03 PM ET, 02/14/2012

Categories:  Latest, Winter Storms

 
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