textproduct: Burlington

WHAT HAS CHANGED

As of 301 PM EDT Monday...

Have added in enhanced wording to zone forecasts in northern portions of Vermont and northeastern New York for damaging winds.

KEY MESSAGES

As of 301 PM EDT Monday...

1. A Heat Advisory is in effect for most of Vermont and northern New York on Tuesday. Some localized fire weather concerns are possible.

2. An enhanced risk of severe thunderstorms remains for northeastern New York and northern Vermont Tuesday night when significant severe weather and isolated flash flooding is possible. A more typical potential for showers and perhaps some thunderstorms returns for Thursday.

3. Near to slightly below average temperatures and dry conditions are expected Friday before temperatures moderate back to around average over the weekend as chances for showers and a few thunderstorms return.

DISCUSSION

As of 301 PM EDT Monday...

KEY MESSAGE 1: Following a stretch of warm days with low humidity, significantly hotter and more humid air remains on track for tomorrow. The 925 millibar mean temperatures across guidance continues to suggest extreme heat building through the day (values in the range of 26 to 28 Celsius at 5 PM translates to temperatures in the mid to upper 90s at near ground level in our deeper valleys). Statistical guidance continues to trend slightly higher for surface temperatures. The most unusual heat relative to climatology still looks to develop over the southern Champlain Valley due to westerly downsloping, but not far off will be the entire Champlain Valley and St. Lawrence Valley as hot air surges in from the southwest. Looking at greatest potential for near record high temperatures in this area with an opportunity to approach 100, while low to mid 90s are common elsewhere between 2 and 7 PM. The air mass does look humid but not quite oppressive, which keeps us largely thinking higher end Heat Advisory and not Extreme Heat Warning level for this event, especially due to the gustiness and deep mixing we'll see through the day given the increasing pressure gradient as temperatures soar. Note that the Adirondack region which is currently not in the Advisory is borderline for our criteria with slightly higher heat and humidity compared to the higher terrain areas in Vermont, and may need to be added if forecasted temperatures tick up any further. Depending on later development of convection/rain cooled air, unusually breezy conditions and humid air will slow the cooling curve into the evening, supporting uncomfortable heat lingering into the night, especially in more urban areas.

As sharply drier air returns behind a strong cold front on Wednesday, low relative humidities in combination with gusty west- northwest to near 25 MPH will pose a fire weather risk in southern portions of our region. This area will have had an extended dry period if they miss out on rain Tuesday night. Think the Upper Valley and lower elevations in Addison, Rutland and Windsor counties will be vulnerable to fire starts and spread, so this is something to keep in mind following the active weather on Tuesday.

KEY MESSAGE 2: A regionally higher end, unusual severe thunderstorm setup continues to be on target based on the latest forecast data. Both global synoptic scale and nested, convection allowing models, with high CAPE and high shear along with a clear trigger with a strong shortwave energy colocated with a southward moving cold front. The overlap of CAPE and shear of this magnitude per the ECMWF Extreme Forecast Index is near the upper end of climatology (very extreme), especially towards the International Border. While there is a lot of typical spread in location/timing of the deep convection based on different initiation and storm motions. The latest full run of the HRRR shows a reasonable depiction of how things may play out, with the targeted area primarily from northeastern Vermont and points east. The NCAR hazard guidance using this model and machine learning techniques shows a very high probability of severe thunderstorms, again centered to our east in southern Maine, but extending westward into northern New York considering the inherent uncertainty in the location of these types of systems. Risk timing is greatest between 7 PM and midnight. All hazards are possible, including low probabilities of wind gusts greater than 70 MPH and a tornado risk, although we emphasize straight line winds are much more likely to be the hazard in this environment given fairly straight hodographs in the low levels. The risk dwindles somewhat as one goes south and west as dwindling instability with time suggests a weakening trend with these storms; however, a long tracked, well- developed line or supercell poses some risk across our region, consistent with the Slight (level 2) risk of severe thunderstorms south of the Enhanced area (level 3).

To summarize the convective threat: the pre-storm environment Tuesday evening will likely have effective shear of near 60 knots, associated with upper level winds near 70 knots and 700 millibar winds near 40 knots, and MUCAPE near 2000 J/kg; local research shows these parameters are conducive to a combination of supercells and quasi-linear convective systems, which produce widespread damaging wind along with localized, brief tornadoes. The chances for significant impacts (wind damage and/or isolated flooding) in northeastern Vermont continue to be favored, with somewhat lower chances towards the south and west. One failure mode for this event (severe weather does not occur) is if the initial deep convection misses to our east; in this scenario, showers ahead of the cold front only come through our area later in the night when instability has waned. As with all deep convection, isolated flash flooding will be possible, especially if thunderstorms redevelop on the western periphery of linear segments while the mesoscale system shifts southward. Guidance in the latest forecast cycle has backed off a little bit on this potential, but bears watching.

Following this activity and any additional showers ahead of the cold front early Wednesday morning, the next chance for showers and thunderstorms comes on Thursday ahead of a weaker variety cold front. This front will also come from the north, but with much less heat and humidity to work with and more limited wind shear, expect much less impact with perhaps a couple of afternoon/evening thunderstorms depending on the timing of convection.

KEY MESSAGE 3: Model consensus is strong for dry conditions Friday with temperatures in the mid/upper 70s to around 80 degrees as a northern stream continental airmass tracks over the region. Spread in models solutions increase over the weekend especially in timing of the next precipitation features. Blended and ensemble guidance are leaning towards periods of showers moisture lifting southwest to northeast Saturday while mid-range deterministic models have a more defined gap of drier conditions with precipitation shunted south of Vermont due to a trough digging more sharply southward. The forecast for the weekend is tracking more closely to blended guidance and persistence at this time with widespread 30% chances of showers and potentially some afternoon thunderstorms, but will reserve some judgment and watch trends closely over the next couple of days given the drier trend in deterministic runs. Shower chances later Sunday into Monday continue to line up with another passing trough that is nearly universally present across model guidances suggesting an increase in statistical confidence that forecast 30-50% chances of showers remain valid.

AVIATION /00Z TUESDAY THROUGH SATURDAY/

Through 00Z Wednesday...VFR conditions will continue with the primary threat to aviators being some LLWS overnight into Tuesday 05-14Z. Winds will decrease overnight under nocturnal stability. As westerly flow aloft increases, some LLWS will develop and continue through daybreak. Coupling south to southwest surface winds 12-15Z will remove the threat of LLWS, but will result in some SFC gusts 20-25kts in general and up to 30kts at MSS where flow aligns with the St Lawrence Valley. Weather will mainly hold off until after 00z Wednesday.

Outlook...

Tuesday Night: VFR. Chance SHRA, Slight chance TSRA. Wednesday: VFR. NO SIG WX. Wednesday Night: VFR. NO SIG WX. Thursday: VFR. Slight chance SHRA, Slight chance TSRA. Thursday Night: VFR. NO SIG WX. Friday: VFR. NO SIG WX. Friday Night: VFR. NO SIG WX. Saturday: VFR. Chance SHRA, Chance TSRA.

CLIMATE

High temperatures on Tuesday will be hot, but we are at the climatological warmest part of the year. Therefore, it is far from a certainty that any records will occur.

MSS looks most likely to set a new record. PBG also is currently forecast to set a new record. MPV, and especially BTV and SLK, are unlikely to do so. See below for current records:

Max Temp Records Date BTV MPV MSS PBG SLK 07-14 100|1995 94|1952 92|2012 95|1952 95|1934

High Min Temp Records DatePBG 07-14 70|1974

BTV WATCHES/WARNINGS/ADVISORIES

VT...Heat Advisory from 11 AM to 8 PM EDT Tuesday for VTZ001>011- 016>018-020-021. NY...Heat Advisory from 11 AM to 8 PM EDT Tuesday for NYZ026>029- 031-035-087.