Thursday 5/9/13 convective threat!

[weatherwiz]

After being stuck in a pattern in which a strong ridge has dominated the eastern 1/3rd of the country, there are signals that the pattern is finally set to change and we will get our first taste of these changes, possibly by as early as Wednesday, as a cut-off low slowly drifts it's way up the east coast.  

 

As the area of low pressure slowly works overhead, the cold pool associated with the low pressure will also begin to work overhead and this will give us some pretty cold mid-level temperatures as 500mb temperatures will approach -15C to -20C with 700mb temps falling <0C.  This will yield to some pretty low freezing levels.

 

With such cold temperatures aloft, coupled by surface temperatures potentially getting through the 60's to near 70F (especially with any peaks/breaks of sun) and dewpoints creeping through the 50's and the presence of some steep mid-level lapse rates, this will create some weak surface-based instability!  As of now, some models are developing as much as 500-750 J/KG of SBcape along with LI values getting as low as -2C to -3C.  We could also see localized higher values in any areas which receive more surface heating.  

 

While wind fields aloft will be very weak lift from the area of low pressure should yield to the development of showers as the strength of the lift increases as the area of low pressure moves closer.  Given the potential for some weak surface-based instability, we also could be looking at the possibility of low-topped thunderstorms, and with low freezing levels, some of the stronger cores could end up producing some small hail.

 

Given the lack of stronger instability, and very weak wind fields, all the updraft strength will come from heating and the lift associated with the low so we would be looking at pulse-type storms.  These factors should also preclude any real threat for severe hail, although you can never rule out one cell becoming strong enough to do so.

 

Besides the threat for convection/small hail, we could also be looking at torrential downpours, although we are not looking at a widespread soaker.  What to watch for is upslope induced showers, especially if the wind trajectory is mainly from the SE.  Again, not everyone will see t'storms or heavy rainfall but we certainly could see localized areas of heavier rainfall along with convection.  

[forkyfork]

weak cape with a nearly saturated profile, cutoff low with disturbances, blocking high to the northeast... that's a recipe for heavy rain

[Quincy]

weak cape with a nearly saturated profile, cutoff low with disturbances, blocking high to the northeast... that's a recipe for heavy rain

A quick NAM sounding forecast for the Wiz metropolitan area:

[forkyfork]

bump. band of convection pressing NE today

[ineedsnow]

would be nice to move this northeast a bit

 

http://www.spc.noaa.gov/products/outlook/day3otlk.html

[ct_yankee]

would be nice to move this northeast a bit

 

http://www.spc.noaa.gov/products/outlook/day3otlk.html

 

 

 

 

Actually the GFS had this into parts of CT yesterday, it was hinting at the potential for at least low-end severe consistently over 3 different runs... Then of course this morning it all seems to have moved west and south, well out of our area.  So it goes. (sigh)  I have to say that I expected no less.