Weather with Bill Evans

National Weather Service New York NY Friday May 9, 2025 A frontal system and associated low will slowly track across the area today. A weak cold front will cross the region Saturday night, with high pressure building in for early next week. Another low pressure system approaches for the middle of next week. * A slow-moving low pressure system will bring soaking rainfall to the area today into early tonight. * 1-2" total additional rainfall expected through tonight. Isolated higher totals possible. * A Flood Watch is in effect for southern Connecticut, and Long Island. An upper-level low will drop towards the area this morning from the Great Lakes region as a low at the surface approaches from the southwest this morning. This will lead to increasing precipitation across the region, overspreading west to east. Some of the more persistent rain with higher rain totals this morning into early afternoon is expected to be along the northern tier of the low approaching from our southwest, leading to more soaking rain in areas north and northwest of NYC. By late afternoon, the low is expected to near or over NYC. This will lead to most rain tapering at this time over NE NJ and NYC with mainly isolated to spotty showers left over. More light showers may continue at this time north of the low, north of NYC. As the low positions itself over NYC this afternoon, a strong 30-40 kt LLJ will develop on the east side of the low, fueling strong warm air advection and frontogenesis. This will lead to banded heavier rainfall on the eastern side of the low with some elevated thunderstorms into the early evening. Unfortunately, the location of this band varies among the 00Z HREF CAMS, MPAS and RRFS due to slightly different positions of the low. The consensus seems to be a heavier band of rainfall may set up across Suffolk county and extend into SE CT late afternoon to early evening. While most guidance agrees setting this up in Suffolk county and SE CT, some still bring this band a little farther west into Nassau and parts of New Haven or eastern Fairfield counties. It seems as the low spins over NYC, east of the low, this band may move slower for a 2-4 hour period before it begins to quicken its exit, progressing faster to the east as the low eventually moves north and east of NYC later in the evening. This happens when the surface low essentially gets captured and becomes stacked with the upper low. Overall, expected rainfall this morning into tonight will be an additional 1-2 inches on top of what we have already seen. Highest totals are expected to be on Long Island, S CT and northern portions of the Lower Hudson Valley. Some isolated higher totals of 3-4" is not out of the realm of possibility. Rainfall rates mainly look to range 0.5"/hr to 1.0"/hr this afternoon and evening. However, peak rates in some stronger banded rainfall or convection east of the low this afternoon and evening could be between 1.5-2"/hr in isolated spots. The 00Z HREF 40-km neighborhood probabilities of rain rates greater than 1"/hr peak at 70% across most of Nassau and Suffolk county, while 10-km neighborhood probabilities peak around 50-60% mainly for Suffolk county. A Flood Watch has been continued for Bergen & Passaic counties, the Lower Hudson Valley, and for S CT. Suffolk and Nassau counties on Long Island have been added to the Flood Watch, based on trends in the latest guidance. Hydrologic river and stream issues are possible in areas north and northwest of NYC and into S CT given already wet soils and streams running higher than normal. There is a chance some streams could reach minor flood stage. Areas of southern Connecticut and Long Island may see isolated instances of flash flooding. Wherever the heaviest rain bands set up could see flash flood guidance met in these spots. Flash flooding will mainly be limited to urban, poor-drainage, and low-lying trouble spots, should it occur. Significant flash flooding is not currently expected. Rainfall will taper into tonight as low pressure moves north and west of the area. Only isolated or spotty light showers are expected on the backside of the low, with the best chances in northern interior portions of NY & CT in our CWA. Dry weather is expected by midday Saturday and into the weekend with the low exiting to the northeast and high pressure gradually beginning to take over. Another shortwave and associated surface cold front moves across the area Saturday night into early Sunday morning with Canadian high pressure building into its wake and gradually rising heights. Breezy NW/WNW winds are expected through the day Saturday due to an increased pressure gradient at the surface from low pressure departing northeast. Highs will remain at or slightly above normal in the upper-60s to low-70s on Saturday and Sunday. High pressure will keep conditions dry through at least much of Tuesday. The next chance of precip comes by later Tuesday or Wednesday with the approach of the next low pressure system. Temperatures through the long term will remain at or slightly above normal with highs in the 70s. Seas build on Fri as low pres develops S of the waters and then tracks across LI. A SCA has been issued on the ocean for this. Winds increase all areas late Fri ngt. Speeds are mrgnl so did not issue a sca on any of the protected waters for this. SCA conditions could continue on the ocean waters through Saturday for marginal 25kt gusts and 5 ft seas. Thereafter, conditions look to remain below SCA. 1-2 inches of rainfall is expected this morning into tonight with isolated higher totals possible. A Flood Watch has been continued for Bergen & Passaic counties, the Lower Hudson Valley, and for S CT. Suffolk and Nassau counties on Long Island have been added to the Flood Watch, based on trends in the latest guidance. Hydrologic river and stream issues are possible in areas north and northwest of NYC and into S CT given already wet soils and streams running higher than normal. There is a chance some streams could reach minor flood stage. Areas of southern Connecticut and Long Island may see isolated instances of flash flooding from heavy rainfall. Wherever the heaviest rain bands set up could see flash flood guidance met in these spots. Flash flooding will mainly be limited to urban, poor- drainage, and low-lying trouble spots, should it occur. Significant flash flooding is not currently expected.