The official first day of summer is now less than 24-hours away and it won't take very long at all for the weather to feel like summer as a very hot and humid airmass is currently on it's way to southern New England!
Currently a warm front is located just to the west of southern New England and this warm front will continue working eastward towards the region and eventually clearing the region by late tomorrow morning. Once the warm front passes through a blast of hot and humid air will invade the region lasting through the next few days.
The airmass working into the region is something more commonly seen during those hottest periods of July/August. The degree of the heat that will be seen across the region tomorrow is something that isn't typically seen in these parts this early in the summer.
The combination of strong sunshine, temperatures at 925mb and 850mb (~25000ft ASL and ~5000ft ASL respectively) ranging from around +25C to +28C (~80F) at 925mb to +19C to +21C (~70F) at 850mb, and strong atmospheric mixing will all lead to surface temperatures ranging from about the mid to upper 90's across much of the region...a few locations may even tough 100F! The only areas which will be cooler will be the immediate coastline and elevations above 1000', but even here it will be tough to escape the heat as many of these locations will range from the upper 80's to the lower 90's!
While Wednesday will be incredibly hot with these temperatures it's possible Thursday is even a few degrees higher across some locations. In fact, Thursday maybe the best chance for a few locations to hit 100F. The areas which have the best shot at 100F are Windsor Locks/Hartford, CT, Taunton, MA, Ashford, NH, and Chicopee, MA. The reason being some computer models are hinting at surface winds on Thursday to be mainly from the west to even slightly northwest. A wind direction from here causes down slopping. This is when winds moving down the edges of mountains/very large hills warms the air as it works down the slope of the mountains/very large hills. This down slopping can often tack on as much as 2-3F on surface temperatures.
Besides the high heat over the next few days we will also have to contend with some fairly high humidity levels and high dewpoints. Normally in these cases with extreme heat we don't normally have to contend with high dewpoints throughout the course of the entire day. Normally it is the morning and evening hours which have the higher dewpoints as during the afternoon hours strong mixing brings drier air down form higher up in the atmosphere and this helps to mix out the low level moisture resulting in lower dewpoints. This case will be a little different, however. With the passage of this warm front there is expected to be an abundance of low-level moisture that will work in place. Computer models are showing 925mb dewpoints to be around +20C (~68F) and 850mb dewpoints to be around +15C (59F). With a strong cap in place at 700mb (~10,000ft ASL) it will be very difficult to mix down the much drier air that exists about 10,000ft ASL and higher up. With all this said dewpoints on both says should be into the mid and upper 60's and even a 70F reading or two is possible, especially in the morning hours and then again during the evening hours. Now it's possible dewpoints on Thursday end up being a few ticks lower across the areas affected by down slopping as down slopping will help to dry the air out a little more.
The combination of the hot temperatures and dewpoints over the next few days will make it feel more like 105F at times, especially during the hottest hours of the day.
As we move into Friday things become a bit more interesting. In order for a "heat wave" to be official one of the several major reporting climo stations must see temperatures hit at least 90F for at least 3-consecutive days. Friday does have the potential to hit 90F, however, there will be a cold front slowly moving towards the region. With the approach of the cold front along with height falls in the mid-levels of the atmosphere this should allow for clouds to develop along with showers and t'storms. While computer models have trended a little slower with the cold front it does appear that we may start clouding over as early as 1-2 PM. This would be just before peak heating which would mean a limit on how high temperatures get.
With a cold front working into such a hot and humid airmass the potential will exist for t'storms and some could become strong to severe given how the atmosphere will be quite unstable. There are several factors though to consider when dealing with the potential for strong to severe t'storms;
1) Timing of the cold front. While computer models have trended slower with the cold front it appears the front will be close enough to at least western sections of southern New England to where clouds/showers/t'storms may develop too early on in the afternoon. This would limit the level of heating we would see and would also cut back on the amount of instability that would be in place.
2) Low-level airmass drying out too quickly. Some computer models have winds in the lower level of the atmosphere coming from a more westerly/northwesterly direction. As mentioned above this is something that could really help to dry out the low level airmass much quicker thus decreasing the surface dewpoints and also decreasing the level of instability that would be in place. These westerly/northwesterly low-level winds would also tend to limit the level of convergence along the cold front which would decrease the degree of upward motion along the front meaning storms may not be as widespread.
3) Wind Shear aloft. Winds aloft aren't exactly very strong. In fact, the strongest winds aloft are well behind the cold front are are located along the leading edge of an eastward progressing mid/upper level trough which won't arrive until Saturday AM. Vertical wind shear values are rather modest at around 25-30 knots and there region just appears to be in the right exit region of a very modest 60-70 knot secondary upper-level jet max. Given the lack of shear this would reduce the level of lift along/ahead of the cold front which would also lead to less in the way of t'storm development.
4) Mid-level lapse rates. Mid-level lapse rates could be a concern although computer models do increase them as mid-level heights continue to fall and some cooler air works in aloft. Computer models do show mid-level lapse rates to be right around 6.5 C/KM which is decent. Any less, however, and that would be another negative factor in updraft strength.
Given the above Friday has the potential to generate a cluster of t'storms, which would likely evolve into a squall line or perhaps even multiple lines given how the wind alignment will be unidirectional. The main threat would be strong winds which could knock down small trees, limbs, and power lines. Given the presence of high dewpoints any storms would likely produce very heavy rainfall, possibly leading to minor street flooding/poor drainage flooding. It's still a bit uncertain though as to how widespread the t'storm threat will be given the above factors mentioned.
It's possible a threat for strong t'storms could exist on Saturday as well, especially across central/eastern sections of southern New England if the cold front has not cleared these areas. By Saturday that mid/upper level trough will have worked into the region and this is when the stronger winds aloft would be present. Computer models are also indicating some instability present early AM Saturday so this will have to be watched.
Once the front/trough clear the region we will return to much more seasonable weather temperature wise along with lower dewpoints and humidity levels.
No comments:
Post a Comment