Weather with Bill Evans

Good Day On This Thursday September 23, 2021 A cold front will be slowly moving eastward from central PA and central NY state to eastern PA near the NJ border by the start of the evening. During this time period, breezy SE flow continues across the local forecast region with widely scattered showers through the morning into early afternoon. By mid to late afternoon, numerous showers are expected towards NYC and locations north and west, while to the east, showers remain widely scattered. Also by mid to late afternoon, instability will be increasing in the so a few thunderstorms will be possible and will have in forecast as a slight chance. Any thunderstorms will be capable of producing heavy rain as airmass will be warm and humid. The heaviest of the showers will still be west of the forecast region by the start of the evening. Regarding the warm and humid airmass, precipitable waters will be increasing to around 2 inches for NYC, Nassau, SW CT and to the west through NE NJ and the Lower Hudson Valley. This precipitable water value will be well above the OKX sounding climatology`s 90^th percentile for this time of year, which is just over 1.5 inches. Abundant clouds, breezy SE winds, and higher shower coverage will limit the high temperatures. Went with a consensus of raw model data and NBM, with highs ranging from the mid 70s to near 80. For tonight through Friday, the cold front approaches near and eventually moves across the region. Bulk of the rain expected within this time period with heavy rain likely at times. The precipitable water remains near 2 inches for the western half of the region and decreases slightly to near 1.8 inches for the eastern half of the region. There will be scattered embedded thunderstorms from time to time with elevated instability expected. Bulk shear from 0-6km above the ground increases to 25-35 kt so some thunderstorms could be severe with strong wind gusts. Also, there is indication within the HRRR for some updraft helicity within certain areas, particularly north and west of NYC so an isolated tornado will be possible. This is from the directional shear from southeast to more south within the low levels. Both the factors for heavy rain and severe thunderstorms are higher for western half of the region compared to the eastern half of the region. For NE NJ, Lower Hudson Valley, NYC, Nassau, SW CT, heavy showers and embedded thunderstorms expected from the evening into the first half of the overnight (00Z-07Z). For Suffolk and the rest of Southern CT, heavy showers and embedded thunderstorms expected (07Z- 14Z), although the intensity is forecast to be a little less with precipitable waters slightly decreasing as well as less bulk shear 0- 6km above the ground (20-25 kt). Specifically, looking at mesoscale models such as the HRRR, indicates potential for up to 1 inch per hour rates at times with shower developing and moving south to north across the region as the cold front slowly moves east. About 2 inches of rain is indicated to fall during a 3-4 hour period. Locally higher amounts of rain will be possible. Flash flood watch currently covers NE NJ, Lower Hudson Valley, NYC, Nassau and Southern Fairfield. The same heavy rain potential and quick accumulations seem also probable for Northern Fairfield so flash flood watch was expanded to include that additional zone. New Haven CT and Western Suffolk NY may also get some flash flooding but line of showers and embedded thunderstorms will be weakening as it moves across and felt any flash flooding would be more isolated in these zones so at this time left these zones out of the flash flood watch. In addition, HREF also indicates potential for one inch per hour rain rates and conveys ensemble max 3 hour precipitation that reaches 2 to 3 inches in locations within the western half of the forecast region. To the east, closer to 2 inches in 3 hours. HREF indicates very low chance (20 percent) for rain rates to exceed 1 inch per hour. For mid to late Friday afternoon, instability will have shift east of the region as will the higher precipitable waters of 1.7 to 1.8 inches. This will be coincident with the eastward movement across the region of the cold front. Shower intensity will be trending lower and thunderstorm chances will diminish. Outside of Eastern Long Island and Southeast Connecticut where showers will linger into the late afternoon, mainly dry conditions are expected to the west mid to late Friday afternoon. For Friday night, high pressure builds in from the west. Surface winds become more westerly in the evening, then more northerly overnight. Dry conditions are expected with negative vorticity advection in the mid levels. Clouds are expected to decrease. For Saturday, a longwave trough approaches from the west with high pressure at the surface weakening across the region. Some height falls at 500mb with some positive vorticity advection will allow for some more cloud development. Light northerly surface flow in the morning will become more variable in the afternoon with sea breeze circulations likely to develop. Mainly dry conditions are expected. Lows tonight consensus of raw model data from upper 50s north and west of NYC to lower 70s for Eastern Long Island and Southeast Connecticut where cold front will still be ahead of by daybreak Friday. For temperatures, lows tonight are a consensus of raw model data. They range from upper 50s north and west of NYC to lower 70s for Eastern Long Island and Southeast Connecticut where cold front will still be ahead of by daybreak Friday. Highs Friday are a blend of NBM and consensus of MOS with highs in the low to mid 70s. NBM and CONSMOS blend was used for lows Friday night and highs Saturday which will be near normal. Showers may linger across eastern LI and SE CT for the first half of Saturday night as the cold front will be slow to shift east. High pressure meanwhile slowly builds in from the west. An upper trough axis approaches Saturday night and shifts through during Sunday. The cyclonic flow aloft will promote some cumulus development, but that appears to be the extent of it as moisture looks too limited for precipitation. Another weak upper level trough moves through, but this looks to be mainly south of the area, and not associated with any precipitation. The ECMWF and Canadian models show a cut off low that becomes semi- permanent Monday night over southeast Canada which is due to a weak low developing along the old frontal boundary, while the GFS pushes this low east of the area. This is leading to disagreement among the global models through the mid week period. Timing and placement differences are shown regarding longwave-troughing and shortwave energy reaching the Northeast and Mid-Atlantic and the resulting surface features. It currently appears that a cold front, surface trough, or even a developing low pressure system moves through during Tuesday night or Wednesday. Despite the spread in solution, all models are showing some precipitation in some portion of the area. Temperatures will be within a few degrees of normal through the long term period, with relatively low humidity levels. Tight pressure gradient sets up between offshore high and low pressure approaching from the west with associated cold front. SE winds increase to 15 to 20 kt today with gusts in the 25 to 30 kt range for all waters. For Friday, SCA remains just for the ocean with lingering higher seas as pressure gradient weakens with cold front moving across and weakening. Sub-SCA conditions on all waters is expected Friday night through Saturday. Winds remain below 25 kt from Saturday night onward, though occasional gusts to 25 kt are possible on the eastern ocean zone Sunday night. Waves will remain below 5 ft through Saturday night. Waves build to 5 ft again on the ocean Sunday into Sunday night as a west-southwest to west flow increases. Waves diminish Monday. Flash flooding possible this afternoon for locations north and west of NYC and for NYC, NE NJ, Lower Hudson Valley and SW CT tonight. Minor flooding possible farther east across Suffolk County on Long Island and New Haven through New London CT, particularly in low lying and poor drainage areas for overnight tonight into Friday morning. No hydrologic concerns Friday night through next Tuesday. A high risk for the development of rip currents continues today and likely through Friday as well for the ocean beaches. Long period swells from the southeast will allow for ocean seas to remain in the range of 4 to 6 ft during this time.