textproduct: Burlington

WHAT HAS CHANGED

As of 243 AM EDT Monday...

Confidence has increased in the potential for heavier downpours and localized flash flooding on both Tuesday and Wednesday. Area of highest concern is northern New York, but any spot that sees multiple days of heavy rain will need to be monitored.

Based on latest model trends, have incorporated a sharper gradient of precipitation chances and amounts Tuesday and Wednesday due to the main axis of moisture and instability over New York on Tuesday as well as southeasterly and easterly downsloping on Wednesday.

KEY MESSAGES

As of 243 AM EDT Monday...

1. Scattered showers and thunderstorms are expected this afternoon, mainly across the Adirondacks, featuring occasional heavy downpours.

2. More widespread showers and thunderstorms are expected Tuesday through Thursday as a slow moving system tracks through the region through the midweek, bringing the potential for localized flash flooding.

3. An unsettled weather pattern prevails late week into next weekend as departing system is replaced by another developing mid/upper level trof.

DISCUSSION

As of 243 AM EDT Monday...

KEY MESSAGE 1: Surface high pressure is currently stationed across the Ohio Valley early this morning with upper level troughing directly overhead northern New York and Vermont. Throughout the night, temperatures are expected to fall into the mid 50s to mid 60s. This will also bring the potential for patchy valley fog, especially in Adirondack locations that received rainfall yesterday (portions of western Adacks reported 0.50-1.50 inches). Throughout the day today, troughing will slide east of the region, but we should have enough shortwave energy, moisture, and warmth to allow for some showers and thunderstorms, about 25-65% chance in northern New York, highest in the northern Adirondacks. For Vermont, chances are lower in general, around 5-40%, and mainly in the Greens or from any precip that forms in the Adirondacks and holds together as it drifts eastward. Should the New York locations that received sufficient rainfall yesterday (these areas may include Hogansburg, Brasher, Tupper Lake, Parishville, Horseshoe Lake, South Bombay, Stark, and eastern Potsdam) receive another round or two of showers or thunderstorms today we'll need to monitor these areas for potential flash flooding, but this threat is highly conditional. The Weather Prediction Center currently has most of northern New York, outside of the Champlain Valley, under a Marginal Risk (Risk level 1 out of 4; 5% chance for rainfall exceeding flash flood guidance) for today. Northern New York rivers could see some small rises, but nothing near bankfull with the recent dryness we have had. Temperatures today will be in the upper 70s to mid 80s, slightly above seasonal normals. Tonight, shower activity should dwindle again like yesterday, and lows will be relatively mild in the mid 50s to mid 60s. Total precipitation amounts will be around 0.10-0.25" in the Adirondacks and parts of the Greens, locally higher in heavy, slow moving storms.

KEY MESSAGE 2: A slow moving upper low is expected to track through the region midweek, bringing chances of persistent showery weather Tuesday through Thursday. On Tuesday, models place the upper low somewhere from the southern Ontario-Quebec line to central New York, and its associated surface low pressure around eastern Lake Ontario and central New York. This multi-layer low pressure is expected to move very little Tuesday and Tuesday night, spinning cyclonically over the Northeast and drifting slightly eastward. As it does so, it will send plumes of moisture into our area, resulting in widespread showers and thunderstorms likely for northern New York and southern Vermont. Northern Vermont is a little less certain, with several models keeping the main slug of rain from reaching the Vermont- Canada border and Northeast Kingdom, largely due to downsloping off the Green Mountains with a southeasterly flow. The main moisture and instability looks to be stationed over New York, with ridging anchored over Maine. Tuesday will also be one more chance for temperatures to reach the upper 70s and 80s before the system wicks away our warmth, which will help fuel any thunderstorms. Models are suggesting Pwats could reach as high as 1.75-2.00 inches in some locations, and surface instability runs around 1500 J/kg, producing tall, skinny CAPE soundings. At times, storm motions look to run under 10 knots, indicating any showers or storms could be slow moving, heavy rainers. Forecast additional precipitation amounts from early Tuesday morning into late Tuesday night will be as high as 0.50-1.00" Adirondacks and west as well as portions of southern Vermont. For the northern Champlain Valley as well as northern Vermont and southeastern Vermont, we're looking at closer to 0.05- 0.50". At the moment, the most likely 24 hour period for the St. Lawrence Valley to get 1 inch is Tuesday and Tuesday night. Any areas that see heavier downpours today will need to be especially watched on Tuesday. The WPC has a Marginal (at least 5%) Risk of excessive rainfall over the entire forecast area Tuesday.

On Wednesday, the multi-layer low pressure swings southeast into the mid-Atlantic, continuing southeasterly to even easterly flow at times across the forecast area. Precipitation continues to look more likely in southern portions of the forecast area with downsloping in the wider valleys, and we lose the majority of instability. Our highs only reaching the upper 60s to mid 70s, we won't see the same convective showers and storms that we're expecting Tuesday. Still, steadier rain could produce as much as 0.75-1.25" of additional rain in southern Vermont, with 0.30-0.75" elsewhere. Wednesday is another period that the WPC has our entire area under Marginal Risk of excessive rainfall. River rises will be expected by Wednesday night following the bulk of the precipitation. At the moment, the most likely period of 1 inch within 24 hours for most of the area falls from Tuesday night to Wednesday, though southeastern Vermont could make out especially well Wednesday-Wednesday night in southeasterly flow. Once again, locations that receive heavy rain today or Tuesday will need to be closely monitored on Wednesday. Lingering showers and thunderstorms are also likely on Thursday as the upper level trough slowly exits the region and the occluded surface low remains nearby while temperatures rebound slightly into the 70s. Most likely location of any lingering showers or thunderstorms on Thursday will be eastern Vermont, closest to forcing.

KEY MESSAGE 3: An unsettled weather pattern continues for late week into next weekend as first system slowly departs by Friday, before another one arrives late Sat into Sunday. Still a large spread in timing of each system, especially toward next weekend, but cutoff and closed 7-5h circulation, feel the potential for showers wl linger into Friday associated with initial trof and weakening area of low pres. ECMWF shows weak differential heating boundary with some instability progged acrs northern NY/Champlain Valley, while deeper moisture/clouds linger over VT on Friday. This convergence and instability wl support a the threat for showers and embedded storms on Friday.

Weak ridge tries to build into our cwa for Saturday, before next s/w and associated energy arrives late Saturday into Sunday. The evolution and track of 700-500mb trof, along with associated sfc low pres wl determine placement and areal coverage of showers/storms for this upcoming weekend. Latest guidances shows Saturday highest potential over northern NY, while Sunday activity wl be more widespread impacting both VT and NY. Limited run to run continuity and large spread in potential solutions results in low forecaster confidence and associated predictability on probability and timing of precip, especially Sunday into early next week. Temps wl be driven by clouds/precip, but mostly above normal values anticipated, especially at night with lows in the upper 50s to upper 60s, while highs range from the mid 70s to mid 80s. Some levels of humidity are expected, but the probability of much above normal temps or significant heat is low given anticipated pattern.

AVIATION /12Z MONDAY THROUGH FRIDAY/

Through 12Z Tuesday...Any lingering fog and mist at SLK/MPV will lift by 13 with VFR prevailing. A 20 to 30% probability of showers with embedded storms are possible this afternoon, with greatest potential over the northern TAF sites. Fog/mist will be conditional on if any rain falls at a taf site this aftn/evening. The highest potential will be at SLK, given rainfall there yesterday. Light terrain driven winds will become south-southwest at 4 to 8 knots by 15Z.

Outlook...

Tuesday: VFR. Chance SHRA, Chance TSRA. Tuesday Night: Mainly VFR, with local IFR possible. Chance SHRA, Slight chance TSRA. Wednesday: MVFR/IFR conditions possible. Definite SHRA, Chance TSRA. Wednesday Night: Mainly VFR, with local IFR possible. Chance SHRA. Thursday: MVFR/IFR conditions possible. Likely SHRA, Slight chance TSRA. Thursday Night: Mainly MVFR, with local IFR possible. Slight chance SHRA, Slight chance TSRA. Friday: Mainly VFR, with local MVFR possible. Slight chance SHRA, Slight chance TSRA.

BTV WATCHES/WARNINGS/ADVISORIES

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