Area Forecast Discussion

WHAT HAS CHANGED

As of 214 AM EDT Saturday...

The potential for a few strong to locally severe storms remains on track from the previous forecast with chances for isolated embedded gusty winds and small hail. Have slightly increased total rainfall amounts through Sunday as well.

KEY MESSAGES

As of 214 AM EDT Saturday...

1. A few strong to locally severe storms are possible this afternoon and evening with isolated gusty winds, hail, and frequent lighting as the primary threats.

2. Increasingly humid weather is expected beyond the latter half of next week, with the potential for heat risk concerns.

DISCUSSION

As of 214 AM EDT Saturday...

KEY MESSAGE 1: The potential for strong to locally severe storms today remains on track with no major changes to the forecast. The Storm Prediction Center has placed portions of southern Vermont, including southern Rutland and southern Windsor Counties into a Slight Risk (level 2 of 5) for potential severe weather this afternoon. Upper level troughing centered over the Great Lakes this morning continues to rotate eastward. Multiple embedded shortwaves on the latest RAP analysis are evident, with the first of which beginning to enter the St. Lawrence Valley. Associated shower chances are beginning to spread east, though dry air from today and from the retreating ridge are leading to mainly virga in this initial round of radar returns. Strong moisture advection via a jet stream connection to the Gulf with help saturate the column this morning into the early afternoon as Pwats increase to 1.5 inches across the entire region. Given the high precipitable water associated with this system, localized heavy rain may be possible with any showers that do develop, though torrential downpours are not favored due to a lack of true convergence and strong shear, at least this morning. Into this afternoon the rain will favor rounds of showers and embedded thunderstorms which will be transient enough to keep our flood risk low.

The initial shortwave shifts east by late this morning with some clearing potentially in its wake via subsidence. This dry slot is already evident on GOES-19 water vapor imagery across eastern Michigan. Some peaks of sunshine will allow for an increase in instability, mainly in the southern regions where distance from the core of the trough will be greater. Surface instability is progged to climb towards 1200-1800 J/kg on the NAM3 and HRRR model soundings with slightly steepening lapse rates to 6.5 to 7C. The issue with any convective initiation with the well saturated columns associated with the high Pwats leading to likely a mess of convective debris throughout the late morning to early afternoon hours. As the boundary layer evolves in time with some efficient mixing from an increasing LLJ around 20Z to scour out some low level moisture, instability will increase across southern Vermont. Bulk 0-6km shear is progged to increase at the same time to around 30-40 kts per the latest HRRR soundings supporting some thunderstorm activity. Furthermore, DCAPE in model soundings ranges from 650-850 J/kg which would support locally gusty winds, especially when combined with the locally higher pwats. The main impacts from these potential storms will be localized heavy rainfall, frequent lighting, locally gusty winds, and some small hail. Hail growth zone thickness to around 10,000 would support some small hail formation. Timing of the stronger storms is likely between 2 and 8 PM. Primary modes look to be isolated pulse cells which may grow upscale with unidirectional shear into a few mini bowing segments. The convective threat today should end by 02-03Z with the loss of daytime instability and weakening shear profiles.

A secondary shortwave on the back of the departing long wave trough Saturday night into Sunday will bring another chance for showers and embedded thunderstorms, mainly across southern Vermont. Northerly flow will be advecting cooler air from the north with increasing waa flow to the south leading to an area of enhanced convergence in the Champlain Valley Saturday night. Models show a good reinforcement of moisture along the northern Greens with potentially a few hours of 0.1 inch an hour rain rates with a quarter to a third of an inch possible between midnight and daybreak Sunday. On Sunday, from central Vermont to the southeast, additional shower activity will be possible, though instability will be lower, only to around 500 J/kg on model soundings for VSF. Continued chances for small hail will be possible, though the chances of heavy rain will be less likely as the jet streak slides south will a drying of the mid to upper levels. Temperatures will trend cooler on Sunday with highs only in the upper 60s to low 70s. As rain ends Sunday afternoon and temperatures cool overnight, some patchy fog may be possible, especially in the cold hollows of the higher terrain Sunday night.

KEY MESSAGE 2: The long range pattern will feature warm to hot weather with increasingly muggy dewpoints. As surface high pressure off the Eastern Seaboard ambles farther into the Atlantic, deep southwesterly flow will increase. 925mb temperatures will be around 21-22 C, which usually corresponds with temperatures in the mid 80s to around 90 F on Wednesday. Different forecast scenarios have come into better alignment with a slow moving trough in a manner that's fairly consistent with our recent pattern. It shows a weakening upper trough shifting in around Wednesday night, and then ridging amplifying over top across Canada which shunts it southeastwards. Within the warm, moist air mass, this will likely support greater coverage of showers and thunderstorms on Thursday and Friday.

Following that, the high amplitude ridging that has persistently developed from the Great Lakes into James Bay will settle more east while a broad longwave trough becomes established across James Bay. This will result in continued deeper southwesterly flow, meaning warm-to-hot and muggier conditions will be here to stay. The probabilities of Heat Indices climbing at or above 100 F are beginning to rise across the Champlain Valley towards 30-40%. The new probabilistic format of HeatRisk, which is specially formulated to consider potential heat related illnesses indicates 50-70% chances of reaching Major levels, which indicate the potential for heat-health risks to anyone within our area.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Through 12Z Sunday...Light rain has been lifting northeast across Vermont and northern New York. Despite most precipitation being light with clouds generally above 7000 ft agl, there have been some visibility reductions mainly 4-8 SM, but locally down to 2-3 SM. This first wave of rain will shift east over the next few hours while south to southwesterly flow begins to accelerate with sustained winds around 7 to 12 knots and intermittent gusts of 16 to 22 knots.

The precipitation outlook beyond 16z-17z is challenging. There's little agreement on convective initiation and how much activity we will receive. Thus, the forecast still uses mostly PROB30s to indicate chances for thunderstorms between about 18z-01z. Expect changeable weather. A sharper trough will begin to approach 21z-02z, and this will begin turning winds southwest to west with a slight increase in wind speeds and gusts. Behind the trough, additional showers are expected with lowering ceilings, likely 800-2500 ft agl beyond 04z. Pockets of LLWS are also expected with northwest winds of 35-40 knots at 2000 ft agl. This will move east about 08z, but additional rain will move north to south from the international border following this.

Outlook...

Sunday: Mainly MVFR, with local IFR possible. Chance SHRA. Sunday Night: VFR. Patchy BR. Monday: VFR. NO SIG WX. Monday Night: VFR. NO SIG WX. Tuesday: VFR. NO SIG WX. Tuesday Night: VFR. NO SIG WX. Wednesday: VFR. Chance SHRA.

BTV WATCHES / WARNINGS / ADVISORIES

VT...None. NY...None.