June 5, 2013: Rainy Friday From T.S. Andrea

Forecast Highlights:

A high pressure in place provided the region with another mostly sunny day, with high temperatures peaking in the low to mid 70s across most of the area. Similar temperatures will continue into Tuesday, but with rain returning for Friday and Friday night as Tropical Storm Andrea tracks up the East Coast and becomes an extratropical cyclone. Following the storm's departure, a drier but cooler than average weekend is expected, with rain returning for the beginning of next week as well.

---

Tonight - Friday Night: Dry, Then Rainy

The high pressure currently in place will continue to slide east, with a SE flow developing for Thursday with highs peaking in the low to mid 70s across the area. Mostly cloudy skies are expected, with isolated showers possible especially late in the day in western parts of the area as two systems approach the area from the south and west.

Expected Setup: Earlier this afternoon, Invest 91L in the Gulf of Mexico continued to organize and was upgraded to Tropical Storm Andrea, making it the first named storm of the 2013 Atlantic hurricane season. As of 11 PM, Andrea was located in the east central Gulf of Mexico with sustained winds of 40 mph, minimum pressure of 1002 millibars, and with a north movement at 6 mph. Andrea remains an overall disorganized tropical storm, struggling with 20-30 knots of shear, and is expected to continue struggling to significantly organize itself in the remainder of its short lifetime, peaking around 45 mph before making landfall in Florida on Thursday evening. A low pressure in the Ohio Valley will gradually move east, merging with Andrea on Friday as Andrea loses its tropical characteristics and becomes an extratropical cyclone, with the system remaining progressive as the coastal low pressure quickly tracks to the northeast off the coast, exiting the region on Saturday as an upper level trough moves overhead.

Forecast for NYC area: Both the Ohio Valley low pressure and Andrea's remnants are expected to produce heavy rain across the region, with continuing differences in the models regarding the axis of heavy rainfall and how much falls. The first system has higher confidence at this time, with widespread moderate to locally heavy rain spreading into Pennsylvania and New York on late Thursday into Thursday night. The heaviest rain will stay north and west of the area, but with light to moderate rain spreading in overnight into Friday morning. Steady moderate rain is expected to fall across the area throughout the day on Friday, followed by the coastal low pressure reaching the area overnight, with a track offshore. The precipitation shield is expected to be compact with the extratropical Andrea, with a relatively small area of heavy rain focusing north of the low pressure. Andrea will be fast moving, with rain ending before early Saturday morning.

The main area of uncertainty is regarding the location of the axis of heavy rain associated with Andrea, as most models show the area near the western end. Scenarios range from earlier GFS runs, which show the area in between the two heavy rain axes with less than 1", to the ECM with 2-3 inches of rain. For today's outlook, I sided closer to a compromise of the GFS and ECM, with 1.5 to 3 inches of rain in Long Island and southern CT, and 1 to 1.5 inch west of NYC, with the outlook subject to minor changes. Stay tuned for more information on tropical storm Andrea with Thursday's update.


Weekend - Next Week: Cooler Than Average, More Rain

As Andrea's remnants clear the region, a trough will move in, with partly to mostly cloudy skies expected for the weekend and highs peaking in the low to mid 70s further east and the mid to upper 70s further west. Mainly dry conditions are expected, although scattered showers cannot be ruled out on both days. On Monday, a relatively strong low pressure will track east through southern Canada, just north of the US/Canada border, producing additional showers and thunderstorms across the region. Behind the low pressure, cooler temperatures are expected for Tuesday and Wednesday, currently likely to peak in the lower half of the 70s, although there is some uncertainty regarding the exact setup, with the possibility that rain sticks around into the mid week, in which case temperatures would also be cooler than currently expected. Uncertainty increases going into late next week and next weekend, although the overall expectation is for moderating temperatures but with an absence of significant warmth, with temperatures generally near to below average. Stay tuned for more information on next week's outlook.