Heavy Rains and Some Flooding Followed by a Flash Freeze

The end of the week and beginning of the weekend appears to be rather interesting weather wise across CT as a surge of unseasonably mild and moist air pushes into the state along with some very heavy rains. Temperatures on Friday and overnight Friday are expected to push well into the 50's with dewpoint temperatures expected to climb into the 50's as well ahead of an approaching cold front associated with a rather deep trough. The presence of a warm and moist airmass along with lift associated with the approaching front and trough will set the stage for some periods of heavy rainfall. The combination of heavy rainfall, temperatures into the 50's, and dewpoints into the 50's will contribute to rapid snow melt (the dewpoints into the 50's will erode the snow very quickly). The combination of rapid snow melt and heavy rainfall will yield pockets of flooding across the state, especially the typical flood prone areas. When the front moves through mid-to-late morning on Saturday temperatures will drop very quickly and this will lead to flash freezing of any water as temperatures fall below 32. There is even a small window for some freezing rain across parts of the state Saturday morning as well, however, with ground temperatures likely still very warm this potential should be rather small.

Currently, a trough has been digging into the western-tier of the United States and as this trough enters the western United States the response has been for ridging to develop across the central and eastern United States:

As this ridge continues to build northward and build in higher geopotential heights this will work to push the colder temperatures northward into Canada. Also, an area of high pressure which is moving off the New England coast will help promote a southerly flow in CT (remember the flow around high pressure systems in the northern hemisphere is clockwise) and this will begin transporting milder and more moist air into the state:

As the trough progresses eastward across the country and approaches we will see strong lift thanks to the advancing cold front,/trough and strong jet dynamics aloft. This combined with the high moisture content in the atmosphere will contribute to periods of heavy rainfall during the day on Friday and then again overnight Friday through Saturday morning. On Friday computer forecast models are suggesting precipitable water values (PWATS) exceeding 1.50'' which is very impressive for the month of January:

During the day Friday, the state will be in the right entrance region of a 120+ knot upper-level jet (not the upper-level jet streak which is north of the border into Canada) which is a favorable quadrant for enhanced lower-level divergence and upper-level convergence. This will only enhance the potential for blossoming and heavy rainfall. This will also enhance the likelihood for multiple waves of low pressure to develop along the cold front which further enhances the degree of upward vertical motion in the atmosphere:

Computer forecast models also suggest high vertical velocities at 700mb (~10,000' off the ground) and 850mb (~5,000' off the ground) which will enhance the potential for periods of heavy rainfall across the state during the day on Friday and then again overnight Friday into Saturday morning:

The heavy rainfall potential may also be further enhanced if there are any convective elements involved with the rainfall, especially Friday night into Saturday morning as some computer forecast models indicate a pocket of a few hundred J/KG of most-unstable CAPE perhaps moving over the state. This could yield the potential for an embedded thunderstorm which would locally enhance rainfall rates and rainfall totals:

Just how much rainfall are we expecting? Given everything described above, along with what computer forecast models are indicating, we are looking at the potential for 1-3'' of rainfall statewide between Friday and Saturday with some areas perhaps seeing amounts ranging between 3-4'' of rain. This, combined with snow melt will surely lead to flooding in spots. If travelling and you encounter a road with standing water, DO NOT attempt to drive though it. It only takes a few inches of moving water to lift up your vehicle:

As we move into Saturday morning and through the morning things become quite intriguing as the cold front begins to push through. As the cold front does move through, winds will quickly shift from a southerly direction more towards the northwest and this will usher in much colder air and quickly too. Temperatures will quickly crash back from the 50's to the 40's and into the 30's and upper 20's. As this happens, the ground will begin to cooler very quickly and as ground temperatures begin to fall below freezing any liquid water will quickly begin to freeze and this will allow for walkways and roads to ice up. Any untreated surfaces will become rather icy and extra caution should be given when out driving and walking. As the front passes the winds will also begin to become a bit more gusty as well so temperatures will feel even colder than they actually are. While a small probability, some computer forecast models so linger some moisture around as temperatures drop to below freezing. If this does happen, there could be a risk for some freezing rain. Initially, with ground temperatures too warm it may not freeze to surfaces right away, however, it adds to the water on the ground which will eventually freeze. Temperatures into the 20's for high and lows in the teens will once again return for the end of the weekend as we move into the new work week.