Just one more day of high humidity to go as a cold front is slowly creeping towards the region. Once the cold front pushes through the region sometime tomorrow, much more seasonable temperatures will arrive along with much lower humidity values. In fact, a few nights later this week will give us temperatures feeling more like fall. Before the cold front moves through though we may have to deal with more downpours and t'storms and some of these storms may be quite strong.
The million dollar question right now regarding tomorrow is exactly when the cold front will slide through the region. Currently, the projected timing of the cold front appears to occur at the time of day when conditions could be rather ripe for strong/severe t'storms, however, this window is very critical right now. If the front slides through 3-4 hours later that what is currently modeled than the threat for strong/severe t'storms could be higher. If the front moves through around the time the models are suggesting, the better threat may exist across RI and eastern MA. If the front moves through even 2 hours quicker than what is modeled, than the threat for strong to severe storms would be extremely low region wide.
If past history is any indication, typically computer models are a bit too quick to move cold fronts through the region, especially when you have an amplifying trough and/or strong ridging out in the western Atlantic. When looking at the overall synoptic pattern and the models handling of the evolution of the synoptic pattern over the next day, we see some strong ridging in the western Atlantic along with an amplifying trough allowing from winds in the mid to upper-levels of the atmosphere to work parallel with the cold front. Typically this scenario would actually allow for a slower eastward progression of the cold front but this does not guarantee it. Sometimes, short wave energy associated with the cold front and surface low will de-amplify and this could cause a disconnect between the trough and the cold front. i.e. the cold front begins progressing east quicker than the trough. This will have to be closely monitored throughout the remainder of the night and into the morning hours tomorrow.
Another issue we have to contend with will be cloud cover and instability, which always seems to be a cause for concern here. Today, much of the region was socked in with overcast, although there were several breaks of sun across many locations. Upstream there was a quite a bit of convection today and cloud debris continues to move into the region. It's quite likely tomorrow there will be lots of cloud cover which will ultimately hold back instability values. What does seem highly possible though is there are many breaks within the clouds allowing for pockets of stronger destabilization. The more sun and stronger the heating, especially when coincident with a slower frontal passage, the better chance for strong to severe t'storms. This will also have to be monitored in the morning hours.
Winds aloft will also be fairly strong tomorrow with vertical shear values favorable for storm organization and longevity. There also appears to be a decent amount of winds turning with height in the 0-3km levels with 0-3km helicity values perhaps exceeding 150-200 m2s2. 0-1km helicity values though are quite low, however, if surface winds back ahead of the cold front, these values will be much higher. Temperatures in the mid--levels of the atmosphere will also be cooling, allowing for steepening mid-level lapse rates.
Tomorrow, besides the threat for torrential downpours and isolted flash flooding, the potential will also exist for numerous t'storms with several becoming strong to severe. However, the extent of the threat will all be determined by the timing of the front and if we can destabilize the atmosphere enough. If we are able to get strong/severe t'storms the potential would be large hail and damaging winds.
The potential will be monitored through the night and into the morning as the details start to become much more clear.
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