Radar — KCXX
Surface Analysis — Eastern U.S. (North)
Satellite
Nearest monitored road: US-7 (Primary)
As of 300 AM EDT wednesday...
Forecast rainfall totals have been increased slightly. The isolated to scattered flash flooding risk remains on track, and a flood watch remains in effect for portions of the area through this evening.
As of 300 AM EDT wednesday...
1. Waves of locally heavy rain will continue to move through the forecast area through tonight. Isolated to scattered flash flooding is possible and a flood watch remains in effect for portions of the area through tonight.
2. Briefly dry conditions are expected on saturday, and then chances for showers and thunderstorms will increase again.
As of 300 AM EDT wednesday...
Key message 1: a slow-moving low pressure system to our south with continue to send waves of rain through our forecast area through tonight. A flood watch remains in effect for portions of southern and central vermont, as well as essex county new york, where isolated to scattered flash flooding is possible.
Observed rainfall amounts so far have been highly variable across the cwa as expected, with localized areas that have multiple heavy showers and thunderstorms move through having picked up 1-2 inches so far. These areas include higher elevations of the southern greens in eastern rutland county, a narrow swath of northern chittenden county to southern franklin county near milton towards st. Albans, and an area between hammond and gouverner in southern st. Lawrence county. Outside of these areas, reported storm total rainfall amounts as of early wednesday highly variable but generally under an inch. Overall, the area has been able to absorb these precip amounts with no reported issues thus far in our cwa.
After a brief break in precipitation overnight, the next wave of light to moderate rainfall is moving northward over western vt and eastern ny early this morning. Rainfall rates within this wave are expected to generally stay between a tenth and a quarter inch per hour, but some isolated higher rates are possible.
The heavier rainfall threat increases later this morning, when the center of the low will shift closer to our forecast area. As the low moves north, southerly flow will become easterly. This will direct moisture upslope over the eastern slopes of the southern and central greens. As this happens, expect a window of steady precipitation over these upslope areas with rainfall rates between 0.25 to 0.5 inch per hour, with the potential for briefly higher rates. While the heaviest precipitation of the event is forecast to stay to the south of our cwa boundary, there will be a window this morning through mid- afternoon where the low will pivot to our south and we have the potential to see some impressive rainfall accumulations in portions of our area. More specifically, an additional 1-3 inches of rain through tonight is forecast over portions of southern and central vermont, along with the eastern slopes of the northern adirondacks. Higher additional amounts, with localized 3-4 inches of additional precipitation today, cannot be ruled out over the higher elevations of the southern green mountains.
Current one hour flash flood guidance from the nerfc ranges from 1.5 to 2.5 inch/hour across most of our cwa...lowest in the previously mentioned areas that have received the most rain out of the event so far. Six hour flash flood guidance is 2-3.5 inch/6 hour. The anticipated additional QPF in comparison with the flash flood guidance does suggest that some isolated to scattered flash flooding is possible today, but flash flooding is not expected to be widespread. The area of highest concern in our cwa remains higher elevations of southern rutland and windsor counties. Much of the area included in our flood watch is included within the slight (at least 15% risk) of flash flooding today as per the latest excessive rainfall outlook issued by the weather prediction center. Given antecedent dry conditions leading up to the event, rivers are generally forecast to remain within banks, though some flashier streams will run high.
The threat for flash flooding will decrease tonight as the low begins to exit to the east. Showers will wane in intensity and coverage through the day thursday, eventually fully coming to an end by thursday night.
Key message 2: an upper ridge will settle over the area friday night into saturday. Could be a foggy saturday morning, especially after our recent wet period. A digging trough along the corn belt will allow warm, humid air to return. Temperatures will climb into the 80S across the forecast area. A few afternoon showers may try to develop. A range of forecast soundings suggest a mid-level warm layer around 600mb should prevent convection, but that it erodes saturday evening. So a late showing of afternoon convection seems possible saturday, mainly along the mountain spines with limited forcing and light flow in place.
On sunday, a northern stream trough will provide some focus for precipitation sunday afternoon-monday while the trough over the corn belt gets dragged farther south. The northern stream system will usher in somewhat dry and stable air into the region. As the southern trough lingers across the southern US and a bermuda high develops, we'll be just north of the tropical plume of moisture this combo will produce. So tuesday appears to be somewhat drier, but then that plume appears likely to shift north some on wednesday. With this regime, we'll likely be somewhat warmer than average, with upper 70S to mid 80S for daytime highs and mid 50S to mid 60S for nighttime lows.
Through 00Z thursday...the initial portion of the forecast has been faring well enough, but did make some modest changes based on shifts to the south and east. The first six hours are of relatively good confidence, with periods of moderate rain producing 2-6sm visibility. Ceilings are gradually falling towards 2500-5000 ft agl, albeit somewhat slower than expected. The worst surface conditions around 09Z-15Z with most terminals MVFR or IFR at that time frame. This will be timed with heavy rain associated with a surface low expected to pass north. Winds will go calm or variable on its approach. Southeast winds will transition to northeast, and then north approaching 06Z. Wind speeds will be strongest about 12Z-22Z at 6-11 knots, and a few gusts up to 20 knots is possible at krut. Eventually, secondary low pressure will develop more strongly off the new england coast. Surface winds will begin to diminish, but the flow aloft will remain easterly to southeasterly at 30-40 knots at 2000 ft agl across much of vermont. Some llws is noted. As the surface low decays, conditions are expected to improve some, and then trend towards steady state. There will likely be some redeveloping convection 18Z wed to about 03Z thurs, which could produce locally heavy rain reducing visibilities, and it is possible another swath of precipitation arcs northwest, but confidence is lower at this time.
Outlook...
Thursday: mainly VFR, with areas MVFR possible. Chance shra, slight chance tsra. Thursday night: mainly VFR, with areas MVFR possible. No sig wx. Friday: mainly VFR, with local MVFR possible. Slight chance shra. Friday night: VFR. No sig wx. Saturday: mainly VFR, with local MVFR possible. No sig wx. Saturday night: VFR. Slight chance shra. Sunday: MVFR/IFR conditions possible. Chance shra, chance tsra.