Area Forecast Discussion

WHAT HAS CHANGED

As of 249 AM EDT Tuesday...

Thunderstorm chances for Wednesday have been lowered as an earlier timing of the frontal passage allows for less destabilization.

KEY MESSAGES

As of 209 PM EDT Tuesday...

1. Showers with some embedded thunderstorms this afternoon and evening, diminishing overnight.

2. Some lingering showers Wednesday as a front moves through, then becoming drier through Friday.

3. Quiet weather and increasing temperatures are expected this weekend before another storm system brings showers and thunderstorm chances to the region early next week.

DISCUSSION

As of 209 PM EDT Tuesday...

KEY MESSAGE 1: Scattered showers and thunderstorms will move into the area this afternoon from the west, driven by shortwave energy rotating around the base of an upper level trough. A longwave trough axis will gradually shift eastward today through eastern Ontario, placing our forecast area in deep southwesterly flow ahead of the trough. This setup will briefly drive a higher dewpoint airmass northward into our area this afternoon and evening, when PWats are forecast to rise to 1.5+ inches in valley locations. Satellite imagery and observations show ample cloud cover over our area, which is limiting heating. Have continued to cap temperatures slightly lower than suggested by the NBM, with highs this afternoon expected to remain in the mid 70s to low 80s. These temperatures, along with dewpoints rising to the 60s, will allow for some marginal destabilization to develop. The latest model guidance suggests MLCAPE values will rise to 250-750 J/kg later this afternoon. With no strong boundary moving through and a lack of low-level convergence, this afternoon's showers and thunderstorms will be generally be tied to more disorganized individual shortwaves moving through the broader flow. This leads to the best shower and thunderstorm chances this afternoon into this evening, when the individual shortwaves currently visible on water vapor satellite imagery over and north of NY will move into our area. 0-6 km bulk shear values between 20 and 40 knots could support the development of some more organized multicell clusters...though overall lack of strong instability will remain the limiting factor. Some storms today may produce gusty winds, small hail, and locally heavy rainfall. Given the strong background flow with gusts 20 to 30 mph common in the Champlain Valley right now, it won't be difficult to get some gusts to at least 40 mph or higher within showers or thunderstorms. Coverage of showers and thunderstorms will diminish after midnight tonight with the loss of instability and deeper moisture.

Storm total QPF amounts for this afternoon through tonight will generally range from 0.25 inch near the International Border to 0.75+ inch over the Adirondacks. Noting some swaths of 1.0+ inch QPF progged in some of the higher res guidance in areas where heavier showers or thunderstorms track through. This does look like a reasonable scenario for some isolated locations based on available moisture profiles. While 1.0+ inch rainfall totals will be the exception rather than the norm, the most favorable locations for heavier rainfall tracks will be over the southern St. Lawrence Valley, the Adirondacks, and potentially into portions of southwestern VT. Some ponding in poor drainage areas is possible, but overall flood risk is low.

KEY MESSAGE 2: A cold front will move through on Wednesday from west to east, with latest guidance favoring a slightly faster progression of the boundary. Scattered showers and thunderstorms can be expected morning through early afternoon as the front moves through, though cloud cover and the earlier timing of the frontal passage will limit destabilization. Though the overall severe threat is low, can't rule out an isolated stronger thunderstorm capable of briefly heavy rainfall and gusty winds. The trend towards a lower severe threat tomorrow due to the earlier timing of the frontal passage is reflected in the latest Convective Outlook update from the Storm Prediction Center. This outlook has much of Vermont outlined in the General Thunder risk, and shifts the Marginal severe risk further east and out of our forecast area.

High temperatures for Wednesday will be dependent on frontal timing, with the earlier onset of cold air advection in our western counties keeping highs in the 60s over the Saint Lawrence Valley and Adirondacks. Further east, The Champlain Valley along with eastern Vermont will see highs in the 70s.

Late Wednesday through Friday will be dry with high pressure in control. Humidity levels will be comfortable with highs in the 70s Thursday and mid 70s to mid 80s Friday.

KEY MESSAGE 3: High pressure will begin to drift east this weekend with more southerly to southwesterly flow, helping to advect warmer air into the region. Temperatures will warm into the low to mid 80s in general, with perhaps a few spot upper 80s in the Champlain Valley. Given an unfavorable moisture advection pattern, dewpoints should remain on the drier side for the weekend, remaining in the 50s. Warm air advection will keep overnight temperatures on the mild side in the low to mid 60s. Subsidence under the high should keep any precipitation at bay through most of the weekend with mainly clear skies. Our next system is expected to swing through the area late Sunday into early next week. A cyclonic gyre over northern Quebec will bring troughiness to the East Coast, with a few shortwaves riding along the southern periphery of the gyre through the Great Lakes into our region. There is still quite the spread in the timing and phasing of any of these shortwaves, but trends have denoted best chances for precipitation to be Monday and Tuesday afternoons, which is in line with the current WPC forecast. As these waves pass through they will take with them small chunks of heat and moisture, gradually knocking our temperatures and humidity levels down into mid next week.

AVIATION /00Z WEDNESDAY THROUGH SUNDAY/

Through 00Z Thursday...Changeable and variable conditions will continue through the next 12 hours for most sites as a series of boundaries moves through the entire area. Current radar shows two areas of thunderstorms around both MSS and PBG. Have a PROB30 with VCTS for both terminals for an hour at PBG, and until 03Z at MSS with training thunderstorms along the US 11 corridor (Best chance of a terminal impacting thunderstorm is at MSS). The MSS boundary will continue to drift east across SLK and into PBG/BTV between 04-07Z. Thunderstorm chances will decrease in time, but a few strikes in the vicinity of the terminals cannot be ruled out; thunderstorm chances will be about 20-40%. As the showers and thunderstorms move through between 04-07Z, LLWS associated with a 40knot jet at 2000ft agl will be present at RUT/MPV/EFK. The difficult part of this forecast will be how low ceilings get. With a combination of ongoing rain, and a developing inversion tonight, ceilings will likely bounce between IFR and low MVFR beyond 04-06Z, particularly at SLK/MPV/EFK. BTV/PBG should have enough flow under the inversion to keep cigs above IFR. With added moisture, some low lying fog will be possible due to more sheltering at SLK/MPV with IFR vsbys possible, mainly after 07Z. GLAMP probabilities show 40-60% chance of IFR ceilings between 06-11Z at SLK/MPV. Towards sunrise, conditions should gradually improve towards VFR. A few light showers will be possible at MSS/MPV/EFK, but terminal impacts are not expected. A boundary will cross the region tomorrow morning turning winds to the west southwest.

Outlook...

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. Saturday Night: VFR. NO SIG WX. Sunday: VFR. Chance SHRA, Slight chance TSRA.

MARINE

As of 209 PM EDT Tuesday...

A Lake Wind Advisory remains in effect with sustained south/southeast winds 20 to 35 knots and waves in the 2-4 foot range. Winds will peak late this afternoon/early this evening, then diminish to 15 to 25 knots overnight with waves 1 to 3 feet. Wednesday, winds will decrease to under 20 knots and the Lake Wind Advisory will likely be able to be dropped early in the day.

In addition to the strong background winds, showers with some embedded thunderstorms are expected late this afternoon into this evening. Within these showers and thunderstorms, gusty winds may briefly change directions and intensify. Some cloud to ground lightning is also possible.

BTV WATCHES / WARNINGS / ADVISORIES

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