textproduct: Burlington

WHAT HAS CHANGED

As of 217 PM EDT Friday...

Chances for showers early Sunday morning in southern portions of the region have increased. Confidence in isolated to scattered showers and a few thunderstorms for late Sunday has increased slightly. Temperatures for Monday have decreased. Winds and precipitation chances for Tuesday/Tuesday night have decreased.

KEY MESSAGES

As of 217 PM EDT Friday...

1. Dry, clear, and seasonably cool weather will continue through Saturday, supporting our coolest morning of the season so far.

2. Rounds of showers and breezy periods will follow early Sunday and Monday mornings, with isolated to scattered showers and a chance of thunderstorms in between. Warm and humid conditions on Sunday will trend much cooler and drier on Monday.

3. A progressive pattern will support periods of rain showers with a trough favored to move through Tuesday night into Wednesday.

DISCUSSION

As of 217 PM EDT Friday...

KEY MESSAGE 1: High pressure will dominate the first part of the weekend. Aside from diurnally driven cumulus, an expansive area of cloud-free skies is noted on satellite imagery today across the Northeast and Great Lakes regions, which increases confidence in quiet weather continuing through tomorrow as the center of high pressure drifts to our east. This scenario will lead to a large swing in temperature from sunrise to midday. Morning temperatures in our region are on track to be the coolest we have seen since June 3rd. Here in Burlington, a low temperature below 50 is expected. That would end a string of exactly 100 days with a minimum temperature of 50 or higher, which is the second longest streak on record (longest is 106 days three years ago). With increasing southerly flow during the day and abundant sunshine, temperatures should surge following the dissipation of river valley fog, topping out in the mid to upper 70s for most locations. These temperatures are a few degrees above normal.

KEY MESSAGE 2: While no significant impacts are expected, an active period of weather appears to be headed our way from Saturday night through Monday morning. Three distinct chances for showers in portions of our region are anticipated associated with different fronts.

The first one will be tied to a wave of low pressure passing well to our south. There have been indications of this system for a few days and the latest guidance seems to be in excellent agreement on a slug of moisture and some elevated instability tracking into southern Vermont by midnight. An upper level shortwave passing to its north may aid in more widespread coverage of showers, which leads to uncertainty in how far north higher PoPs should be. Generally risk of heavier showers and perhaps a rumble of thunder looks relegated to our southern areas in Vermont, but at least some shower activity is increasingly likely in much if not all of Vermont and northern New York at some point in the overnight hours. Measurable rain is greatly favored for central/southern Vermont and the southern Champlain Valley. Total rainfall during this period will be variable, with mainly light amounts under 0.25" along with localized pockets of near or a bit above 0.5". Alongside the showers, a fairly potent low level jet will translate northeastward through the region, with strongest 850 millibar flow likely staying in the southern Vermont areas producing brisk mountain summit winds. However, localized valley channeling of southerly (or southwesterly in the St. Lawrence Valley) winds are favored areawide. There is a strong consensus of 30+ knot winds over the eastern portion of Lake Champlain through much of the Sunday morning hours.

The second round of showers will be tied to the lead cold front approaching from the west midday Sunday. The timing of this boundary along with modest height falls will result in marginal instability to support slight chances for late afternoon and evening thunderstorms. The risk of thunderstorms should generally shift eastward through this period from northeastern New York to northeastern Vermont; probabilistic hazard guidance is in fairly good consensus that suggests thunderstorms are likely in the region but with a low chance for any particular location given the limited instability. The isolated risk of a stronger variety thunderstorm also is present, driven by a severe wind risk amidst 40-50 knots of deep layer shear with veering profiles and greatly increasing speeds with height. The Day 3 Severe Weather Outlook currently is in the 1 out 5 category (isolated severe storms possible) for most of our region coinciding with this potential event; given the progged instability being modest (mixed layer CAPE around 500 J/kg) and limited coverage of thunderstorms amidst dry air aloft, would expect the outlook risk level will not increase as we get closer.

The final round of showers will be tied to a vigorous shortwave trough that helps bring in a surge of colder/drier air into our region early Monday morning. This forcing is stronger that the surface cold front, so while the potential for heavy rainfall is low, the rain during this period may be relatively widespread and shallow instability should lead to brief downpours. Have noted precipitation chances have come up, but the likely scenario supports only upwards of an hour or two of rain occurring at any one location. With uncertainty in timing of these showers, the broad-brushed PoPs will need to be fine tuned as it becomes more clear how the showers will progress through the region.

Monday then will trend dry as high pressure with origins in northwestern Canada will build in. Have noted that the latest guidance has 850 millibar temperatures only in the 2 to 3 Celsius range during the day on Monday, or a bit lower than what we are seeing today. If this verifies, high temperatures will be even lower than the current forecast with valleys only reaching the low to mid 60s even with deep mixing.

KEY MESSAGE 3: 500 mb flow in the mid-high latitudes are favored to be mainly zonal with Arctic air trapped well north of the region. Fast- moving troughs will be moving through this flow with model trends maintaining a weaker wave passing Tuesday night into Wednesday. The biggest question is whether or not a quasi-stationary boundary with linger over portions of northern New York and Vermont through the end of the week. Latest trends point to a more southerly push over what was presented in guidance yesterday, so this would lead to lower chances of prolonged precipitation. This is reflected in WPC probabilities of rain chances mid/late next week generally in the 20- 40% range outside of the next wave moving through. The next wave approaches Thursday and may result in increasing precipitation chances once models consolidate around wave timing. With the ambiguity on how the surface boundaries will resolve, highs in the 70s remain reasonable with lows generally ranging in the mid 40s to mid 50s.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/

Through 18Z Saturday...VFR conditions will prevail at our taf sites through around 06Z Saturday as high pressure and dry air continue to build across our region. Northwest winds 4 to 8 knots with localized gusts at MPV of 12 to 18 knots. Overnight the aviation challenge becomes timing and areal coverage of fog and associated LIFR/VLIFR. Highest probability (80-90%) of IFR or lower conditions is at SLK/MPV with likely (55-65%) at EFK and chance (30-40%) at BTV/RUT and MSS. Have used prevailing group at SLK/MPV for fog, tempo at EFK and just vcfg at RUT/BTV for now, as confidence is too low to mention IFR attm with light and variable winds. VFR conditions return 12-13Z with light surface flow out of the south developing; PBG could see some gusts 15-20kts with winds coming from off of Lake Champlain.

Outlook...

Saturday Night: Mainly VFR, with local MVFR possible. Chance SHRA. Sunday: Mainly MVFR, with local IFR possible. Chance SHRA, Slight chance TSRA. Sunday Night: Mainly VFR, with local MVFR possible. Chance SHRA. Monday: Mainly VFR, with local MVFR possible. NO SIG WX. Monday Night: VFR. NO SIG WX. Tuesday: VFR. Chance SHRA. Tuesday Night: VFR. Chance SHRA, Slight chance TSRA. Wednesday: Mainly VFR, with local MVFR possible. NO SIG WX.

BTV WATCHES/WARNINGS/ADVISORIES

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