First True Taste of Late Fall to Arrive This Weekend.

We will be experiencing a taste of late fall weather this weekend as an Arctic cold front pushes through the region and ushers in the coldest airmass of the fall season thus far.  The coldest of the airmass will move in place during the day on Saturday and we can expect nighttime lows across many locations reaching as low as the mid to upper 20's both Saturday night into Sunday and Sunday night into Monday.  Daytime highs on Sunday and Monday may also fail to reach 50F across many spots.  If you're not a fan of this cold, the good news is it's stay will be rather brief as by Tuesday a warm front swings north and temperatures return to more seasonal levels.

So what is the cause for the incoming cold?  Well a substantial ridge of high pressure has been building across the western-tier of the United States and extending into western Canada.  This ridge is in response to an increasing state of the Pacific-North American teleconnection pattern.  The response here across the northeastern United States will be for a very sharp trough and associated Arctic cold front to push through.  On the backside of this front, our airmass will be deriving straight from the Arctic, hence the early season, albeit brief, cold.  We can see below:

What does this translate to in the lower levels of the atmosphere and at the surface?  You get it...cold.  We are looking at temperatures between about 2500ft and 5000ft ASL as much as -3 to -4 standard deviations below-normal come Sunday morning!

Here is one computer model's forecast projection (The GFS model) for 2-meter temps both around 5 AM Sunday morning and 5 AM Monday morning.  The GFS shows temperatures about 2M off the ground well in the mid to upper 20's, which actually means some of the typically colder locations may even see temperatures each the lower 20's!

Fall is upon us and we will certainly get our true taste of it this weekend!  However, more seasonal temps are just right around the corner.

Low-topped thunderstorms Friday 10/0/2015?

A cold front will be approaching southern New England during the day on Friday and out ahead of this cold front scattered showers and perhaps even a few thunderstorms are expected to develop.  In fact, some potential may even exist for a few strong to severe t'storms, but the probability/confidence is on the lower side.  If the probability/confidence is on the low side than why mention it?  Well the setup that will be in place on Friday is termed a low cape/high shear setup.  This means that while appreciable surface instability will be lacking, winds as you advance upwards in the troposphere will be quite strong.  When dealing with the potential for strong to severe t'storms, one thing we like to see is higher amounts of instability, but that can be compensated for with stronger wind energy aloft.  While these low cape/high shear events don't always produce strong to severe t'storms, it is important to follow the setup closely because when they do produce, sometimes the storms can pack a punch!

A strengthening trough to our west associated with a vigorous piece of energy at 500mb will push a cold front towards southern New England.  Out ahead of the cold front, a warm front will push through southern New England and usher in juicer air.  This is evident by forecast surface dewpoint temperatures well into the lower 60's and Precipitable Water Values (PWATs) increasing towards 1.5'' to 1.8''.  In fact, these values indicate we could see some heavier downpours around on Friday.



While clouds will be a limiting factor as to how high temperatures will become, temperatures in the warm sector are expected to climb into the upper 60's to even lower 70's.  The combination of a moist low-level airmass, surface temps around or just north of 70F, and the presence of steepening 700-500mb lapse rates upwards of 6.5 C/KM (meaning for every 6.5 KM you ascend in the 700-500mb level, the temperature will decrease by as much as 6.5 C), will lead to the development of some weak surface-based instability across the region.  In fact, some computer forecast guidance develops anywhere between 250-700 J/KG of surface-based instability with lifted index values around -1C to -2C.  While these values aren't particularly high and certainly nothing that would make you think strong to severe thunderstorms, its when you combine these values with stronger wind shear that can lead to the potential.

Computer forecast models develop a 500mb mid-level jet streak of 50-60 knots pushing through New England between 18z-0z (2:00 PM to 8:00 PM EDT) Friday.  This is something that can really help to enhance the lifting process of air parcels and could enhance the potential for some t'storms
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(500mb wind graphic from the GFS...notice the tan-ish color traversing through New England...this is an area of enhanced winds within the main jet stream and this is referred to as a jet streak)


With a sharpening trough and vigorous piece of shortwave energy moving through southeastern Canada, this will increase the pressure gradient across the region and in response we will see increasing winds throughout the troposphere.

The combination of strong winds aloft and weak surface-based instability could in fact yield to the potential of some low-topped thunderstorms forming ahead of the cold front on Friday.  The term :low-topped" just indicates the height of the cloud tops...with these setups the cloud heights will not very often exceed 20,000ft in height.  This is different than your typical summer t'storms where cloud tops can exceed 40,000 to even 50,000ft.  However, given how winds aloft will be very strong, if clouds can develop strong enough updrafts (this is where the presence of instability comes in) and tap into the strong shear, we would see a few strong to even severe t'storms around.

Outside of thunder potential, expect a few bands of heavier showers to move through as well which could lead to some localized spots of flooding, especially in the flood prone areas.