Radar — KENX
Surface Analysis — Eastern U.S. (North)
Satellite
Nearest monitored road: US-7 (Primary)
Axis of heaviest QPF has shifted east into the high terrain of western new england. Accordingly, the flood watch has been expanded to include all of southern vt. WPC has also placed this area into the moderate risk ero today, and has added a moderate risk ero tomorrow for western ma and southern vt. Finally, will mention the potential for a few stronger storms with damaging wind gusts this afternoon and evening.
1) multiple rounds of showers/thunderstorms will produce heavy rain this afternoon through wednesday, especially areas south of i-90 and in western new england where a flood watch is in effect for flash flooding through wednesday afternoon.
2) despite areas of heavy rainfall, current river forecasts suggest threat for river flooding is low at this time. However, if any of the higher-end rainfall totals are realized then isolated minor river flooding would be possible.
Key message 1... As of 2:55 PM EDT...current sfc analysis shows an elongated sfc low located over western ny and pa. This low has developed in response to an anomalous upper trough currently seen on goes 16 wv imagery over the great lakes. As this upper low closes off this evening and an upper shortwave tracks around its southern periphery, a coastal low will form off the mid atlantic coast and eventually track near or over long island tonight into tomorrow, bringing periods of heavy rain and thunderstorms to the region. The sfc low and ull both sit and spin overhead through thursday, bringing continued showers to the region under the upper-level deformation axis.
Current cira alpw satellite imagery shows deep moisture surging into the region this afternoon ahead of the approaching sfc low aided by a strengthening southerly llj that will eventually turn to the southeast tonight and the east tomorrow. Pwats increase to 1.75 to 2.0 inches this afternoon into tomorrow, which is anomalous but not exceptionally so for late july. Warm cloud depths of around 12 kft will help to promote efficient warm rain processes, while unidirectional flow will promote backbuilding and training of storms. Mean cloud layer winds of only around 15 KT with slower mbe vectors will also support slow storm motions. The strength of the forcing will be quite unusual, with MSLP and upper-level height anomalies as low as 3-4 std dev below normal overlapping this very moist atmosphere. Model forecast soundings continue to show a tall, skinny CAPE profile, with at least some elevated instability through wednesday evening that will support the potential for heavy downpours. So, all signs are there for very heavy rain.
The location of the heaviest rainfall amounts will depend on exactly where training and backbuilding of convection sets up, which is still hard to determine several hours out from the heaviest rain. That said, we have seen a fairly large eastward shift with numerical guidance in the axis of heaviest QPF with this morning's 12Z runs. Over the peaks and east-facing slopes of the berkshires, E/SE upslope enhancement will likely lead to some rainfall totals of up to 3-5", with some higher amounts locally up to 7" not out of the question. Rainfall amounts have trended down slightly for the mid hudson valley, and there may actually be some downsloping here off of the high terrain, but the catskills are still expected to see up to 3-5" of rain with locally higher amounts up to 6-7" as well. Rainfall rates this afternoon and tonight are likely to exceed 1-2" per hour at times and may even briefly reach 3" per hour for a few areas in the flood watch. Rainfall rates tomorrow don't look quite as high as today, but could still exceed 1" per hour especially in southern vt and the berkshires. With this rain tomorrow falling on already saturated soils, there will be a continued flash flood threat into tomorrow.
Given the changes in expected QPF, we have expanded the flood watch into southern vt in coordination with wfo btv. WPC has also expanded the day 1 moderate risk ero into this area, and has introduced a day 2 moderate risk ero for much of our western new england areas while maintaining the marginal to slight for eastern ny. If some of these high-end rainfall amounts are realized, then numerous instances of flash flooding and some significant flash flooding will be possible. Local research has shown that many of our summer days featuring widespread flash flooding are associated with an upper lows like the one that will be overhead tonight and wednesday, so this has added to our confidence in the potential for flash flooding. While many areas have been quite dry recently and could use some rain, this looks like a classic case of too much rain too quickly.
Some lingering showers are expected thursday, but are not expected to result in additional flooding. A dry period is then expected for friday and saturday, with increasing chances for rain again sunday or monday ahead of the next approaching upper trough and cold frontal system.
Finally, will mention a low threat for some stronger storms this afternoon. Low-level and deep-layer wind SHEAR are weak, but latest SPC mesoanalysis shows 1000-1500 j/kg of mlcape across the region, with the highest values south and east of the capital district where there have been more breaks of sun this morning. Therefore, we can't rule out a few instances of wet microbursts leading to damaging winds with any taller storms this afternoon. However, threat for severe weather is much less of a concern than that for heavy rain and flash flooding.
Key message 2...latest hefs (ensemble river forecasts) continue to show a low (generally <10-15%) chance for main stem river flooding with this event, likely due to antecedent dry conditions and rainfall deficits. That said, within-bank rises on most rivers, creeks, and streams are expected. If some of the higher end rainfall totals are realized (5-7"), then some minor river flooding would be possible. The best chance of seeing these rainfall amounts is in the eastern catskills and higher terrain of western new england.
Through 00Z thursday... Widespread rain and scattered thunderstorms continue across all terminals throughout this evening, yielding MVFR and IFR vis and cigs to start off this taf period. Periods of heavy rainfall could bring down vis to 1 sm and lower throughout the overnight period. Kgfl is expected to see the greatest reduction in vis overnight and into the early morning, with the potential of fog formation on thursday. Rain will start to become more scattered on thursday morning and afternoon and conditions will gradually improve throughout that time. However, cigs and vis are expected to remain MVFR and IFR through the end of the taf period.
Southerly to southeasterly winds with moderate gusts prevail at the start of this period at all terminals except for kalb. Winds will gradually weaken throughout the overnight period and begin to veer more easterly, with winds prevailing out of the east to northeast throughout thursday at all terminals.
Outlook...
Wednesday night: high operational impact. Definite shra...tsra. Thursday: high operational impact. Likely shra...tsra. Thursday night: no operational impact. No sig wx. Friday: no operational impact. No sig wx. Friday night: no operational impact. No sig wx. Saturday: no operational impact. No sig wx. Saturday night: no operational impact. No sig wx. Sunday: moderate operational impact. Chance of shra...tsra.