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FXUS61 KBTV 081122 AFDBTV

Area Forecast Discussion National Weather Service Burlington VT 722 AM EDT Wed May 8 2024

SYNOPSIS
Showers with embedded thunderstorms will move across the region this morning. Although there will be a brief break in the precipitation, another round of isolated to scattered showers and thunderstorms are expected later this afternoon into the evening. The rest of the week will be cool and damp with rounds of showers.

NEAR TERM /THROUGH THURSDAY/
As of 712 AM EDT Wednesday...The forecast remains on track this morning. Our first batch of showers is crossing the region at this hour. The heaviest rainfall rates have been across central NY, where some NY mesonet sites have picked up in excess of half an inch in the past hour. This heavier rain is rapidly approaching south central VT, and should exit into NH by mid morning. Lightning activity has been on a downward trend, but there could still be a rumble of thunder or two this morning.
Further north, over a vast majority of northern NY and northern and central VT, rain should be light to occasionally moderate this morning. Latest model guidance still indicates the potential for isolated to widely scattered showers/thunderstorms this afternoon, perhaps a few on the strong to locally severe side, particularly over portions of the southern Champlain Valley/south central VT. Forecast has this all covered, so no changes were needed with this update.

Previous discussion...No significant changes in thinking for today's forecast, particularly in regards to thunderstorm potential.
Showers with embedded thunderstorms are ongoing across the eastern Great Lakes early this morning. This activity is associated with a warm front which stretches from its parent low located over Lake Huron eastward through northern PA/NJ. This whole system will push eastward today into tonight, making for a potentially active day for the North Country. Long story short, the potential for strong to locally severe thunderstorms is still very conditional and could really go either way as ingredients will need to come together just right. Read on for details.

The first round of precipitation will spread over the area this morning. SPC mesoanalysis shows elevated instability of 400-700 J/kg along with a 100+ kt 300 mb jet, which have served to keep thunderstorms going overnight. Have noted some observations of moderate to even briefly heavy rain with these thunderstorms, and that trend will continue as the system moves into our region. Showers will move into the St Lawrence Valley around daybreak, quickly spreading eastward through NY and across VT by mid morning. A dry slot, wrapping around the southern side of the low and readily evident on satellite imagery, will lift into our area behind this initial batch of showers/thunderstorms, giving us a brief break in the precipitation. What happens thereafter remains somewhat uncertain as much depends on just how dry the incoming airmass is. Even at this time, the hi-res CAMs are having a hard time agreeing on how much clearing we may see with this dry slot, which in turn would determine how much warming we can get. Model soundings still indicate a subtle warm nose/cap will exist aloft, and more dry air/warming would help to overcome this convective obstacle. In this scenario, isolated to widely scattered showers and thunderstorms would develop mid-late afternoon as SB CAPE values approach 800+ J/kg.
Should the airmass hold more moisture, warming would be less pronounced and instability would remain elevated, generally remaining in the 400-750 J/kg range. This would make for more showers, but less in the way of strong convection. Shear still looks ample at 40-50 kt, so any thunderstorms that are able to become a bit more robust will have the potential to become strong to perhaps locally severe. Main threats would be strong/damaging winds and small hail. The central/southern Adirondacks eastward through southern Champlain Valley/southern VT, particularly central Addison southward through western Rutland Counties, have the best chances of seeing more clearing, more warming, and thereby more instability and better chances for stronger convection. Although it has varied some from run to run, soundings off the HRRR and to a lesser extent the NAM3 both indicate that SLK and RUT break the cap this afternoon, and at the least show gusty surface winds, with upwards of 45-50 kt at the top of the mixed layer. All that being said, SPC's latest Day 1 Convective Outlook still has our southern border included in a Marginal Risk, which seems reasonable at this point.

Low pressure will drift over the region tonight, and any showers will come to an end from west to east. Moisture will become confined to mainly the lower levels, so showers may actually transition to patchy drizzle or fog, keeping the overnight cloudy and damp. This moisture will lift a bit on Thursday, keeping clouds around, but should remain shallow enough to preclude widespread showers.
However, a few spotty showers could develop east of the Adirondacks and Greens as flow turns more easterly through the day. After lows in the 40s tonight, tomorrow will be cool, with highs remaining in the 50s in most spots, with perhaps a few locations hitting 60F.

SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY/
As of 353 AM EDT Wednesday...The main story is a marked trend towards less rain. That being said, it remains a low predictability forecast with regards to rainfall amounts and timing. Generally, the latest model guidance shows a later arrival of rain with low pressure tracking farther south, which results in potentially a sharp north/northeastern gradient. Portions of northeastern Vermont may see only a short period of light rain while southwestern areas get a soaking rain for the entire period. The latest GFS ensemble probabilities of 0.01" in 24 hours continue to support the idea that it will rain across the region. However, changing the threshold to 0.1" tells a different story with much higher chances just south and west of our region, and sharply lower chances in northeastern Vermont. The NBM 6 hour precipitation QMD technique shows large spread in rainfall amounts for this entire period, especially in western Vermont into the Adirondacks, suggesting that the fairly steady chances of rain throughout this period will need to be fine tuned. Given the trends towards a later and lesser rain event, have cut back precipitation chances further to indicate more of a 30-50% chance of rain in hourly PoPs. Again, on a 24 hour basis it looks like it will rain areawide, but the extent of the rain remains uncertain. The type of rain will be a stratiform/stable rain with deep marine influence, so rainfall intensity will be limited even if duration is long. Temperatures will be highly dependent on how far north/east the shield of precipitation and thick clouds makes it; a drier day would support partial sunshine such that temperatures could end up in the 60s in northern areas and several degrees cooler to the south. It could be quite uncomfortable in the mountains if the steady rain materializes with low freezing levels; a cold rain with temperatures in the upper 30s and lower 40s is possible.

LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/
As of 353 AM EDT Wednesday...Chances of showers then continue through Sunday night. Nothing really to write home about with the weekend conditions as an open wave crosses the region without a surface feature to organize precipitation around. Like last weekend, Sunday may be the wetter day of the weekend as the upper level low moves overhead. However, the details are pretty murky with how different pieces of vorticity rotate around the longwave trough over the northeastern CONUS. Most of the latest global model clusters do show the cold pool aloft sufficient to eke out an average of around 100 J/kg of surface based CAPE Sunday afternoon, with greater instability values in northern New York into the Champlain Valley.
With light cloud layer winds, this environment would support numerous slow moving showers but unlikely to produce any downpours let alone thunder.

The upper level pattern looks to evolve such that the cool and unsettled pattern ends on Monday, although chances of showers do continue. Upper level data in the global model clusters show good agreement on northwesterly flow aloft and west/southwest low level flow, indicative of warmer conditions. Would not be surprised if high temperatures on Monday and Tuesday trend warmer. At this point seasonable highs and lows are reasonable, as temperatures could be tempered by cloudier weather associated with any surface boundaries that may move through the region.

AVIATION /11Z WEDNESDAY THROUGH SUNDAY/
Through 12Z Thursday...VFR conditions will prevail through much of the first half of the TAF, with a 2-4 hour window of MVFR ceilings/visibility in widespread showers this morning. After 18Z, showers end and conditions should return to VFR through about 00z, with the exception of KMSS where northeasterly flow will maintain MVFR ceilings. There will also be scattered showers/thunderstorms 20z-00z, with MVFR or even IFR possible at any affected terminal. Ceilings start to lower back to MVFR after 00z, and while showers will end, patchy fog and drizzle is expected. IFR is possible overnight, though have left out of the TAFs due to uncertainty. Winds will be light and variable through this morning before trending SSE at 5-10 kt except locally northeast at KMSS. Gusts up to 25 kt possible at KRUT this afternoon. Winds will turn to the north/northwest this evening and overnight behind a cold frontal passage.

Outlook...

Thursday: Mainly VFR, with areas MVFR possible. Chance SHRA.
Thursday Night: Mainly VFR, with local MVFR possible. Chance RA.
Friday: VFR. Chance RA, Chance SHRA.
Friday Night: VFR. Slight chance SHRA.
Saturday: VFR. Chance SHRA.
Saturday Night: Mainly VFR, with local MVFR possible. Chance SHRA.
Sunday: Mainly MVFR, with areas VFR possible. Chance SHRA.

BTV WATCHES/WARNINGS/ADVISORIES
VT...None.
NY...None.

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