textproduct: Burlington

WHAT HAS CHANGED

As of 319 AM EDT Saturday...

No significant changes have been made at this time. The threat for strong to severe thunderstorms is a little lower than before but the threat for localized flooding has gone up slightly.

KEY MESSAGES

As of 319 AM EDT Saturday...

1. Strong to severe thunderstorms and localized flooding possible on Sunday.

2. Big pattern change expected to occur Sunday night into Monday with temperatures returning to more seasonable values.

3. Expect widespread rain and unseasonably breezy conditions on Thursday. Near normal temperatures are likely for most of the midweek to late week period.

DISCUSSION

As of 319 AM EDT Saturday...

KEY MESSAGE 1: Much quieter weather is expected today with mostly sunny skies allowing temperatures to warm into the upper 70s to upper 80s. For Sunday, the threat for strong to severe thunderstorms looks to be on track with a strong cold front slated to move across the region Sunday afternoon and through the overnight hours. Forecast model soundings for Sunday afternoon are all over the place unfortunately with some models depicting a modest EML with a strong inversion below it. It's unlikely we would break the inversion the models are depicting if the event plans out as the NAM and NAM3 currently depicts. We should have ample instability around 1000 to 1500 J/kg coupled with deep layer shear in the 50 knot range which would be enough to support some supercell structure if we can get an surface based convection. RAOB sampling this morning will help give models a better handle on the EML and how the upper level pattern is developing which will give us a better handle on how Sunday may play out. The more concerning aspect at this time is the potential for some heavy rainfall. All guidance is showing a plume of deep layer moisture advecting from the Gulf and western Atlantic which will help push PWATs near 1.5" Sunday afternoon. Some models even depict PWATs up to 2.0". The soundings actually look favorable for heavy rain with tall skinny CAPE profiles with a deep warm cloud depth layer. It's possible we could see rainfall amounts of 1-2" in places with even localized higher amounts possible given the convective nature of the event. We have added heavy rainfall wording to the thunderstorms on Sunday as they could become problematic. We will watch this closely and have some better understanding on the potential impacts in subsequent forecasts.

KEY MESSAGE 2: A strong cold front will push through Sunday night into Monday morning ushering in a much colder airmass. Temperatures will return to more seasonable values for Monday and Tuesday with highs climbing only into the mid 60s to mid 70s. We will be underneath the influence of an upper level trough once again for much of next week which will keep some chances for showers each day with a pool of colder air aloft and several weak disturbances traversing the longwave pattern. None of these showers should be strong as we will be lacking significant moisture and instability.

KEY MESSAGE 3: Focus of the days 4 through 7 period (Tuesday night through Friday night) is on the expected Colorado low expected to develop and track northeastward through the Great Lakes region. This looks like an unseasonably strong system, with the ensemble MSLP through this period roughly 995 millibars, near the 1st climatologically percentile. The member storm tracks are fairly well clustered through Michigan and into central Ontario and Quebec Thursday night. That being said, some interesting differences in timing are noted with the GEPS being quite slow compared to the ENS & GEFS, and the GEFS for the 00Z initialization (latest data) shifted a bit northward closer to the Grand Ensemble track. Currently the probabilities of a 24 hour rainfall greater than 1 inch ending Friday at 8 am (rain is most likely from Thursday morning through Thursday evening) are about 25 to 50%, lowest in the northern Champlain Valley and greatest in much of northern New York, especially towards the St. Lawrence Valley and southern portions of the Adirondacks.

A more northward track which would reduce potential for heavier rainfall amounts but lead to greater risk of stronger wind gusts. While at this time range a broad slight chance of thunderstorms is also reasonable, looking at forecast soundings and the atmospheric environment it appears a low tropopause will limit cloud depths and the near surface temperatures will struggle to warm enough to produce much instability with moist adiabatic lapse rates present. So at this time, the primary possible hazard looks related to winds, mainly out of the south, during the day on Thursday. The average 24 hour maximum wind gust in the latest NBM provides a conservative 30 to 35 MPH range for much of northern New York and the Champlain Valley; reasonably worst case scenarios in the 90th percentile are in the 40 to locally 50 MPH range.

Behind this system, cyclonic flow will lead to continued chances of showers and as typical in a cold season pattern probabilities are greater in the Adirondacks and northern/northeastern higher terrain in Vermont.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/

Through 12Z Sunday...Prevailing VFR flight conditions will continue through the period with mainly SKC for most terminals. Winds will generally remain westerly today and shifting southerly tonight. Well mixed air through sunset supports modest gusts, most favorable with a southwesterly channeling scenario at MSS. As surface winds decrease after 00Z, a surge of southwesterly winds into northern New York is expected above the surface, supporting LLWS for at least a few hours. Farther east, while there will be enough winds aloft to preclude fog formation, the magnitude appears to be light enough to leave LLWS out of the TAF.

Precipitation remains unlikely, but there are indications especially after 06Z that isolated to scattered showers will slide across northern portions of the airspace. MSS currently is the only site that categorically looks likely to see some rain, although greatest chances are towards the very end of the TAF period. If trends continue, PROB30 and/or TEMPO groups will be needed at this site.

Outlook...

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

BTV WATCHES/WARNINGS/ADVISORIES

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