textproduct: Burlington

WHAT HAS CHANGED

As of 240 PM EDT Tuesday...

Downslope gusty wind probabilities up to 45 MPH on the western facing slopes of the central and northern Greens mountains, as well as the northwestern Adirondacks have increased. Have subsequently increased winds in the aforementioned areas.

KEY MESSAGES

As of 240 PM EDT Tuesday...

1. Isolated to scattered showers with a few thunderstorms possible this afternoon, and more likely Wednesday afternoon.

2. Widespread rain with embedded thunderstorms, with the potential for gusty winds, are expected Thursday as a strong area of low pressure passes to our northwest.

3. Cool and showery weather is expected over the weekend. Before temperatures return to seasonably warm next week, a low pressure system may bring widespread rain on Monday.

DISCUSSION

As of 240 PM EDT Tuesday...

KEY MESSAGE 1: Weak shortwaves out ahead of a more developed long wave trough will move from the southwest to the northeast this afternoon and tomorrow afternoon. Weaker forcing is expected today, but an isolated shower with a thunderstorm may be possible across the Adirondacks. Due to increasing clouds and some drier air, fog potential will be low tonight. Though chances will slightly increase if an area receives rain today and is in a sheltered location. Tomorrow, another shortwave with better forcing will move northeastward through northern New York into Vermont. Terrain driven showers will likely develop in the Adirondacks and then across the Greens with cooling air aloft and some weak surface based instability. Equilibrium levels will be quite low below -10C, which will keep most convection shallow. However, a few thunderstorms may be possible, with the best chances closer to the International Border and northern St. Lawrence Valley where longer daytime heating will be with better, albeit still weak, shear profiles will be located. Most showers or thunderstorms tomorrow will be more garden- variety with no significant weather expected. Highs tomorrow will be a few degrees warmer than today with winds turning to the south helping to increase some moisture. But we will lack the deep layer moisture needed for any moderate to heavy rain.

KEY MESSAGE 2: An unusually strong area of low pressure will track across eastern Ontario and pass just to our northwest on Thursday. Ensemble models depict this system to have a pressure between 980- 985mb, which is in the 0.1 percentile, or near record levels for overall surface pressure this time of year. This type of surface pressure would be more reminiscent of something found in our winter months. A precipitation shield associated with a warm front will begin to lift through the region early Thursday morning with strong ascent associated with waa and developing frontogenesis along the warm front at both 850 and 700mb. A solid swath of rainfall is expected within a 4-8 hour window around sunrise Thursday morning where the majority of the rainfall for the system will fall. Trends for the center of the system have jogged the center a bit north, which could cut down on the overall rainfall amounts. Furthermore, given the strong nature of the system, a strong LLJ around 70kts will likely lead to some shadowing in the central Champlain Valley with further rainfall reductions. As the warm front lifts, and we get into a pseudo warm sector, instability will begin to increase. However, soundings show a stout warm layer aloft, with likely convective debris and mid to high level clouds which will help to lower the equilibrium level and reduce the realized total CAPE. Regardless, some daytime heating may allow for a some thunderstorms to develop around noon and persist into the afternoon. Confidence in any severe weather is relatively low with a low CAPE, high shear type profile expected, but a few storms may become strong, mainly in southern Vermont. Pwats will be around 1.5 inches by the afternoon which could lead to some locally good downpours, but flooding risks will be low with the quick progression of the system as a whole. Right now current thinking is that widespread quarter to half an inch rainfall amounts are likely everywhere, with areas in the Northeast Kingdom and St. Lawrence Valley potentially seeing up to an inch. Areas that receive any thunderstorms, mainly in southern Vermont could see between 1-2 inches. Anything above 2 inches looks unlikely due to the lower confidence in strong convective elements and the quick progressive nature of the system.

The more concerning aspect to this system will be the potential for strong gusty winds during the day Thursday. As mentioned, there will be a strong LLJ to 70 knots around 4-5 kft moving into the region as the warm front lifts north Thursday morning. HRRR and NAM3 Model soundings denote the possibility of 40 knot winds at 900 ft in Cambridge, VT. Winds at the surface will be southeast to east Thursday morning towards noon, and with some cold air damming across the eastern Greens, and strong flow aloft, downsloping on the western facing slopes of the central and northern Greens is becoming increasingly likely. We have increased the forecast gusts to around 45 MPH for locations like Cambridge, Montgomery Center, and Middlebury (including other towns on the western facing slopes of the Greens) on Thursday. Channeled flow in the Champlain Valley will also lead to the potential for gusty winds around 40 MPH through Thursday late morning. We will continue to monitor the potential for any wind product issuances should trends continue. Winds will turn southwesterly by Thursday afternoon, weakening the downsloping winds in Vermont, but subsequently increasing or sustaining 30 to 40 MPH gusts across the St. Lawrence Valley. Winds do not totally calm Thursday night, as a pressure gradient will remain across the International Border, keeping winds relatively breezy into Friday. Northwest flow and cooling temperatures aloft will keep Friday relatively breezy with some chances for precipitation, mainly near the International Border.

KEY MESSAGE 3: As we move into the beginning of the astronomical summer, the weather pattern will be one more typical of winter with a deep trough over the northeastern US lingering through the weekend before finally retreating. There is increasing confidence in well below normal temperatures, particularly on Saturday when cool, west- northwesterly cyclonic flow will be present. The latest temperature guidance shows highs will certainly be cooler than normal and likely at least 5 to 10 degrees below normal; relatively large spread suggests forecast highs could further trend lower in the next couple of days, especially if the colder air aloft. There are subtle differences in the placement of the trough and shortwaves that traverse it from the northwest that lead to differing chances of rain showers, but the blend of this guidance seems reasonable at this time with numerous showers expected to pinwheel in from the north and west.

The takeaway would be that rain showers from Saturday into Saturday evening are likely to near certain, with chances peaking during the afternoon hours, but precipitation rates/amounts will not cause any impacts with very shallow instability expected. One camp of guidance, dominated by the ECMWF ensemble, is more amplified with the upstream heights and brings us more instability than the consensus forecast, such that a rumble of thunder can't be ruled out at this point. Shower chances will be substantially lower on Sunday as the trough and associated upper level low scoots well to our northeast, but lingering cyclonic flow will be present to support at least scattered afternoon showers, and perhaps a thunderstorm or two will develop as temperatures trend a bit more seasonable.

With regards to early next week, some deterministic guidance continues to show the potential for a potent low pressure system to pass to our south which would bring widespread rain and cool conditions to the North Country on Monday/Monday night. At this time, it is too early to know if the storm will come together and/or track northeastward to bring any substantial rain to our region. There are mixed signals across the ensembles right now to the point that a general chance of rain, increasing in likelihood Monday afternoon and decreasing Monday night, looks reasonable at this time. The threat of heavy rain is low but bears watching (the wettest solution, in 8% of the most recent full ensemble, does show 24 hour rainfall in the 1 to 2" range).

AVIATION /18Z TUESDAY THROUGH SUNDAY/

Through 18Z Wednesday...VFR conditions are expected along with mainly light winds. A spot shower is possible during peak heating this afternoon as a few cumulus grow overshooting tops in the western Adirondacks and a PROB30 was used at SLK from 19Z to 21Z given risk of convection; otherwise there will be no precipitation in the airspace today and tonight. While there is no mention of showers after 12Z, will note towards the end of the TAF period a few showers could develop with more coverage anticipated after 18Z. Winds generally look 10 knots or less with only modest southerly flow for much of the period.

Outlook...

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

BTV WATCHES/WARNINGS/ADVISORIES

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