textproduct: Burlington

WHAT HAS CHANGED

As of 302 PM EDT Monday...

Thunderstorm risk for Wednesday in Vermont has continued to decrease.

KEY MESSAGES

As of 302 PM EDT Monday...

1. Scattered showers and thunderstorms Tuesday afternoon and evening, followed by a lower chance on Wednesday primarily in northern and eastern Vermont. Strong wind gusts are possible with a few storms, especially in southern St. Lawrence County.

2. Blue sky conditions for Thursday with cool and crisp air.

3. A warming trend at the end of the week followed by a chance of thunderstorms Sunday.

DISCUSSION

As of 302 PM EDT Monday...

KEY MESSAGE 1: Tranquil weather today will become dynamic on Tuesday as an unseasonably strong low pressure system (roughly 992 millibars over northeastern Ontario) drags multiple fronts through the area through Wednesday night. Strong southerly flow between this low pressure area and high pressure to our east will transport increasing humidity following a couple of dry days, with dew points climbing back into the 60s by afternoon. Generally with large scale divergent flow ahead of a potent upper level trough, scattered showers will develop Tuesday, while thunder risk is limited by instability struggling to reach 500 J/kg. Greater instability is favored in northern New York where a severe thunderstorm risk remains.

Damaging wind gusts are driving the severe weather outlook for tomorrow. The risk timing appears to begin roughly 2 to 3 PM, peak around 4 to 6 PM, and diminish by 8 PM. Thunderstorms are strongly favored downwind of Lake Ontario, then trend southward into central New York and points south and east with time. Therefore, in our forecast area the chances of these stronger thunderstorms is much higher in St. Lawrence County and perhaps Franklin County. These areas may have just enough favorable ingredients for a localized wet microburst to occur; CAPE should reach around 1000 J/kg, some mid-level dry air will be present, and steep 0 to 3 kilometer lapse rates are favored given the time of day and deep mixing promoted by the background wind. Cell motions will tend to be from southwest to northeast given the cloud layer flow, and there could be multiple rounds of thunderstorms in this area. The latest HREF local probability matched mean shows 6 hour precipitation approaching 3 inches, and convection allowing models depict hourly rainfall rates between 1 and 2 inches. Altogether, an isolated instance of urban/flash flooding (affecting part of a town or two) is possible along with the severe wind threat.

Farther east, peak instability is expected to diminish greatly, in part due to some stable marine influence from south- southeasterly flow and also due to potential daytime cloudiness from scattered showers ongoing during the morning to our south. As such, risk of thunderstorms of any variety is fairly low from the Adirondacks eastward, especially towards northeastern Vermont. Even in the most unstable air in our western areas, it appears CAPE will be insufficient for supercells. So the possibility of severe storms making it farther east is low, contingent on cells organizing in a quasi-linear fashion and sweeping eastward. That being said, a few rumbles of thunder will be possible amongst widespread showers Tuesday evening driven by cooling temperatures aloft, but the overlap of sufficient moisture and instability is limited and would not expect much activity overnight besides widespread showers. Rainfall amounts will tend to average from 0.3" to 0.7", with the typical locally higher amounts within thunderstorms anticipated.

Based on the latest forecast data, the Wednesday convective threat for Vermont has trended downward. Most guidance shows isolated coverage of showers and maybe a thunderstorm or two developing ahead of the lagging surface cold front during the midday or late afternoon. If, and it's a big if, there is enough instability to overcome copious dry air aloft and associated poor lapse rates in the mid-levels, ample deep layer ahead of the front will support organized thunderstorms that would be capable of strong winds, with probabilities increasing in eastern Vermont. We'll have to watch trends, but for now it looks like a mainly quiet weather day with only slight chances for thunderstorms.

KEY MESSAGE 2: An unseasonably cool air mass, with minimal high altitude smoke, should make for outstanding weather for outdoor activities on Thursday as an upper level trough pushes to our northeast. Westerly flow in the morning will probably support some rough conditions on Lake Champlain that should improve during the day as anomalous low level temperatures moderate along with decreasing wind speeds. Expect high temperatures not too far off of seasonable norms due to abundant sunshine, currently projected to range through the 70s.

KEY MESSAGE 3: A surface high pressure will dominate Friday and Saturday, bringing a warming trend with it. Daytime temperatures in valley locations will slowly rise from the upper 70s on Thursday to the upper 80s by Sunday. We are tracking a low pressure system that gets pushed north of our CWA by the upper level ridge. If the current forecast holds this system will not impact our CWA. The most impactful system in the extended will be an upper level trough and associated surface front that comes through on Sunday. This system is forecasted to bring a chance of rain showers and a slight chance of thunderstorms Sunday into Monday.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

Through 00Z Wednesday...VFR conditions will prevail at all our taf sites overnight as southerly flow ahead of our next system and dry air at the surface prevents fog development overnight. As low pressure approaches a warm front may produce a few showers on Tuesday morning, but conditions should stay VFR. Additional showers and storms are expected after 18z Tues with some localized MVFR cigs/vis possible in the heavier convective elements. In addition, gusty southerly winds develop on Tuesday ahead of approaching cold front with localized gusts 15 to 25 knots possible, along with areas of low level wind shear and turbulence, especially near terrain features.

Outlook...

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

BTV WATCHES/WARNINGS/ADVISORIES

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