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WHAT HAS CHANGED

As of 943 AM EDT Sunday...

A Flood Watch has been issued for most of northern and central Vermont, outside of Grand Isle County for heavy rainfall between 1 to 2 inches, locally up to 3 inches, through 8 AM tomorrow morning. Heavy rain over repeated areas could lead to flash flooding. Additionally, the Storm Prediction Center has brought the rest of the region into a Slight Risk (level 2 out out 5) for severe weather through this evening.

KEY MESSAGES

As of 215 AM EDT Sunday...

1. Strong to severe thunderstorms and localized flooding possible today and tonight.

2. A return to more seasonal weather early next week with daily chances for showers.

3. Another storm system is expected to bring widespread rainfall to the region on Thursday.

DISCUSSION

As of 215 AM EDT Sunday...

KEY MESSAGE 1: A Flood Watch has been issued for most of northern and central Vermont, outside of Grand Isle County, until 8 AM tomorrow morning for heavy rainfall amounts between 1 to 2 inches, possibly up to 3 inches, in areas that receive multiple rounds of showers and thunderstorms. We've got a bit of a messy 24 hours ahead of us with multiple rounds of showers and thunderstorms possible ahead of a strong cold front late this afternoon into this evening. Currently, we have a bit of a deformation axis setting up across the St. Lawrence Valley which is allowing for a few showers with embedded rumbles of thunder to track across the Valley. Minimal rainfall has been reported thus far but the main concern is how the longwave pattern is beginning to shift. Moisture will continue to advect eastward throughout the day today with a little boost from the Great Lakes to help bump PWATs up near 1.75 inches across northern New York and 1.25 to 1.5 inches elsewhere. These plume of greatest moisture is likely to be trapped under the developing deformation axis which could lead to a longer period of rainfall; some of which is expected to be very heavy at times. Model soundings across New York show between 1000 J/kg of CAPE (GFS) to 2000 J/kg of CAPE (NAM) this afternoon with an increasing warm cloud depth layer. These soundings support very heavy rainfall and with a synoptic cold front with strong frontogenesis/convergence, it's feasible we could see some localized 2.5+ inch rainfall totals in some locations from northern New York across northern Vermont (best chances near the International border). Those with outdoor plans this afternoon, and especially tonight, are advised to keep an eye on the weather as we could be in store for some heavy rain and isolated flash flooding.

In addition to the hydrological concerns, we're also keeping an eye on the severe potential this afternoon into the evening. The forcing between the upper level trough and surface cold front are slightly displaced but the cold front is expected to move across the region right around peak heating. With temperatures in the mid to upper 80s and dewpoints in the lower 60s, plenty of instability will be present. We are looking at "inverted v" soundings in the low level with model forecasted DCAPE in the 600-1000 J/kg range. This should support some damaging wind gusts with even a few wet microbursts possible in stronger storms this afternoon. Deep layer shear in the 50 to 60 knot range will help organize thunderstorms with a few supercells possible. The main thinking is that the cold front would quickly catch up to any supercells with the main storm mode being multicellular/QLCS. Damaging winds will be the main concern for sure this afternoon with a brief spin-up tornado unlikely but possible. We will be hard pressed to see large hail greater than 1 inch in diameter due to a high melting layer but we could still see smaller hail within thunderstorms this afternoon. The main window for severe storms will be between 4 PM and 10 PM.

KEY MESSAGE 2: Following the passage of the cold front, we will be under the influence of an upper level trough once again. A return to more seasonal temperatures with highs in the mid 70s to lower 80s is forecast through Wednesday with lows in the 50s and 60s. Broad cyclonic flow could yield some isolated to scattered diurnal shower chances through the first half of the upcoming week but no significant rainfall is expected at this time.

KEY MESSAGE 3: Models continue to be in very good agreement that another potent storm system will track near the St. Lawrence Valley on Thursday and bring widespread rainfall. There is enough confidence in the guidance to include 80+ PoPs at this time. There is the possibility for some heavy rainfall again on Thursday and we have already begun coordinating with the Weather Prediction Center about the potential for at least a marginal risk for flash flooding. Some of the ensemble guidance is already hinting at 1-2 inches of rain on Thursday so it'll be something to watch closely, especially depending on how much rainfall is received from the storms today and tonight.

AVIATION /12Z SUNDAY THROUGH THURSDAY/

Through 12Z Monday...Showers are approaching from the west, but VFR conditions are expected to continue through about 16-17z before more widespread rainfall overspreads the area. MVFR visibilities are expected as rain begins with ceilings lowering to MVFR values a few hours behind the onset of rainfall. Some thunderstorms are expected this afternoon as a strong cold front moves across the region. A few PROB30s are noted where better instability lies over KEFK/KBTV/KPBG/KSLK, with the greatest chance for thunder between 18z-00z. Winds will increase from the south this afternoon with some gusty winds up to 20 knots expected in the Champlain Valley. Winds will likely become erratic near thunderstorms this afternoon but should settle into a southerly to southwesterly flow behind the thunderstorms and ultimately veer to the west/northwest in the wake of the cold front.

Outlook...

Monday: VFR. NO SIG WX. Monday Night: VFR. NO SIG WX. Tuesday: VFR. NO SIG WX. Tuesday Night: VFR. NO SIG WX. Wednesday: VFR. Chance SHRA, Slight chance TSRA. Wednesday Night: Mainly VFR, with local IFR possible. Chance SHRA. Thursday: MVFR/IFR conditions possible. Windy with gusts to 30 kt. Definite SHRA, Slight chance TSRA.

MARINE

A Lake Wind Advisory is in effect for the broad and southern waters of Lake Champlain through this midnight. South winds ahead an approaching cold front will lead to increasing winds on Lake Champlain enhancing gusts and wave conditions. Thunderstorms will also be possible today with sudden increases in gusts. Winds on the lake will increase to 10 to 20 knots with gusts up 25 knots, becoming strongest between 1 and 8 PM. Waves will be generally 1 to 2 ft. Gusts will weaken back to around 10 knots between after sunset. Boaters today should stay weather aware, and have a way to receive warning alerts.

CLIMATE

Record Daily Precipitation may be possible today:

June 14: KMSS: 0.63/2005 K1V4: 0.66/2005

BTV WATCHES/WARNINGS/ADVISORIES

VT...Flood Watch through Monday morning for VTZ002>010-016>018. NY...None.