T'storm possible Tuesday, August 13th, 2013

After a very gorgeous weekend with little to no humidity and cooler temperatures, a warm front will begin to slowly lift through the region during the day on Monday and with the passage of the warm front we will once again see the return of higher dewpoints/humidity along with the threat for showers on Monday.  The humidity won't be around for long though as a rather strong cold front is expected to cross the region sometime either late Tuesday or overnight Tuesday into Wednesday.  Right now there are differences in some of the computer model data on the exact timing of the cold front. 

An amplifying trough moving across the Great Lakes and southeastern Canada will allow for winds aloft to increase over southern New England.  Computer model guidance suggests that winds at 700mb will increase on order of 30-40 knots with winds at 500mb increasing to as much as 40-50 knots across the region.  While the low-level jet doesn't appear all that great, we still see winds at 850mb increase to 25 to perhaps 30 knots.  The region will also be in the right entrance region of a rather potent 100-110 knot upper-level jet streak.  

As the trough approaches the region, temperatures in the mid levels of the atmosphere will also begin to cool as 500mb temperatures are modeled to drop as low as -12C to perhaps -13C.  With 700mb temperatures around +5C to begin the day (although cooling as the afternoon goes on) this should yield to some much better mid-level lapse rates than we have been dealing with as they should be on order of around 6-6.5 C/KM.  

As far as instability goes, that picture is much more unclear right now as we may be dealing with a bit of cloudiness which could hold back temperatures.  Given how 925mb temperatures are modeled to be around +20C with 850mb temperatures closer to +14C, that would yield max temps with full mixing well into the mid to upper 80's.  However, with clouds expected to be around we probably won't realize this full potential so temperatures should be closer to 80F, although with more sun these temperatures could easily be boosted higher, especially locally where there happened to be more breaks.  Dewpoint temperatures are expected to climb into the mid-60s.  

The combination of surface temperatures near 80F, dewpoints in the mid-60's, and modest mid-level lapse rates, should yield to surfaced-based Cape values around 1500-2000 J/KG with mixed-layer cape values around 1000-1500 J/KG and surfaced-based lifted index values around -3C to -4C.

When taking into account the combination of strong winds aloft and potential for some modest instability, and a cold front/trough sliding east, we will be looking at the potential for some t'storms during the day on Tuesday.  

We will have to watch the exact timing of the front, however, as if the front comes through very late on Tuesday or during the night, then the threat for t'storms will be much more decreased as instability will be minimized.  However, some computer model guidance does suggest a pre-frontal trough will setup just east of the Hudson River Valley and that would be a focus for t'storms.

Given the strong shear/cold temperatures aloft, any t'storm would have the potential to produce strong winds and perhaps some hail.  

Severe Weather Potential Friday, August 9th, 2013

Friday has the possibility to be a fairly interesting and perhaps active weather day across southern New England as the possibility will exist for showers and t'storms.  While the overall threat may be torrential downpours leading to pockets of flash flooding, the possibility of some strong to severe t'storms will also be possible, in which strong winds and even the possibility of a few isolated tornadoes will be possible.

During the overnight hours of Wednesday into early Thursday, a warm front lifted northward through southern New England and this allowed for dewpoints/humidity to rapidly increase across the region as dewpoints have risen into the upper 60's to around 70F.

On Friday, a mid-level trough sliding east passed the Great Lakes will allow for an area of surface low pressure to slide east through southern Canada along with a cold front towards southern New England.  Some computer forecast models are also showing a "secondary" warm front development just to the west of the region early tomorrow along with another weak wave of low pressure.  It is these two features which is opening eyes with regards to the possibility of severe weather tomorrow.

Some computer forecast models are developing a fairly potent low-level jet later tonight, lasting through much of tomorrow with winds in the 925-850mb layer exceeding 30-40 knots which is quite strong for this time of year.  Not only are these models developing strong winds here but these models also are indicting strong directional wind shear with winds at the surface coming from the south with winds turning towards the SW/WSW aloft with height.  Below are two hodographs taken from today's 18z GFS run for Windsor Locks, CT tomorrow afternoon.  I have circled four areas of interest.  The first two areas of interest are showing the hodograph, which measures wind speeds/direction with height.  The "longer" the hodographs, the stronger the winds aloft are, the more curved they are, the stronger the turning of winds with height are.  The second two areas of interest are the models suggesting the potential for several hundred to perhaps 1000 J/KG of Cape, which with combined with those hodographs insists the potential for severe weather/tornadoes would be a possibility tomorrow:

When it comes to the low-level jet, however, often times computer models can overdo the strength of them as computer models can sometimes suffer from what is called convective feedback.  This is one factor that makes this forecast/potential rather unclear at this time.

As far as instability goes, weak mid-level lapse rates and extensive cloud cover are expected to hold back instability values, however, with surface dewpoints near 70F and rich low-level moisture, surfaced-based instability values could exceed 1000-1500 J/KG along with mixed-layer cape values and lifted index values in the -3C to -5C range.

If this low-level jet verifies, or comes close to verifying, and we are able to manage even a few hundred J/KG of Cape, any t'storm that develops will have to be closely watched as atmospheric conditions will be quite favorable for strong winds and perhaps even some tornadoes, especially with extremely low lifted condensation levels, the level at which the cloud base is located (to put it simply).