319
FXUS61 KBTV 211948
AFDBTV
Area Forecast Discussion
National Weather Service Burlington VT
348 PM EDT Sun Jul 21 2024
.SYNOPSIS...
High pressure crests overnight supporting formation of patchy fog,
especially for sheltered valleys that received rain Sunday morning
where fog may become dense. Monday afternoon, flow switches out of
the southwest with increasing clouds and precipitation chances
becoming increasingly likely overnight into Tuesday through
Thursday. Rain could be heavy at times with periods of
thunderstorms, especially Thursday as a front swings into the North
Country.
&&
.NEAR TERM /THROUGH MONDAY NIGHT/...
As of 347 PM EDT Sunday...With dry surface conditions today,
temperatures will cool quickly after sunset. Patchy fog is likely to
form, especially towards the Canadian border and northern Vermont
where rain amounts ranged from 0.10 to 0.50 inches. Other sheltered
valleys and typically favored locations will likely have fog as
well. Some localized spots may be dense.
Flow turns southwesterly Monday as an extension of the Bermuda ridge
builds northward ahead of an approaching mid level trough. This
morning`s front will push northward with precipitation chances and
clouds increasing over Monday evening into early Tuesday. Some
elevated energy will accompany the boundary as it moves northward
supporting at least a slight chance of thunderstorms. Model trends
have been supportive of increasing precipitation amounts especially
from central Vermont/Adirondacks to locations south in the vicinity
of a quasi-stationary boundary with indications of an approaching
open trough. Recent dry conditions will largely preclude any chances
of flooding Monday night into early Tuesday, but this rain could end
up being a priming rainfall for later issues should multiple rounds
of thunderstorms occur.
&&
.SHORT TERM /TUESDAY THROUGH TUESDAY NIGHT/...
As of 347 PM EDT Sunday...Active weather likely continues on Tuesday
with chances for isolated to scattered thunderstorms in a weakly
unstable air mass, especially near the higher terrain and over
northern New York. These thunderstorms, given a low shear
environment, would be of the pulse variety with a low risk of severe
weather. We also do not see heavy rain indicators, so while
localized heavy rain is always possible with thunderstorms,
torrential rain is unlikely. That being said, while cloud layer flow
looks sufficient to keep storms moving along, we will have to watch
for repeated rounds with westerly shear vectors parallel to the
front.
Coverage of storms will be limited by modest mid level height rises
and dry air following the wave that passed to our east in the early
morning hours. However, a weak frontal boundary is expected to be
just north of the International Border attached to broad area of low
pressure centered in southern Ontario and the North Country/Vermont
topography should help generate lift when combined with modest
heating. The richest moisture and greatest instability in our region
is most likely to be focused in northern New York where thunderstorm
chances are slightly higher; mean surface-based CAPE from the NBM
shows mid-afternoon values above 1000 J/kg in the Route 11 corridor
from Potsdam southward. At the same time, is a very difficult
forecast with large spread in forecast instability, in part owing to
differences in cloud cover and boundary layer moisture. In general,
it is hard to rule out diurnally driven showers impacting just about
all locations, so PoPs in the 40 to 60% range seem reasonable.
Tuesday night showers should gradually wind down. A fairly weak low
level jet will develop, which should cause mountain summits and Lake
Champlain to see some 20 MPH winds as opposed to the light winds
otherwise seen during the period. The southerly flow aloft will
support a mild night. Have not added fog yet to the forecast, but at
the surface light east/southeast winds for most locations will allow
temperatures to drop back to the dew points in the upper 50s to mid
60s resulting in widespread near 100% relative humidity moderately
humid conditions with dew points in the upper 50s to mid 60s.
&&
.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
As of 347 PM EDT Sunday...Wednesday continues to look rainy, although
chances of impactful rainfall remains low and consistent with the
WPC excessive rainfall outlook keeping the risk out of our area.
Both lack of surface heating and warm upper level air look to
minimize instability as upper heights rise during the afternoon
hours. The large scale pattern featuring high pressure near the
Maine coast will contribute to limited instability as marine
influenced air is injected into our region. Potential for higher
amounts of rain than currently forecast would be associated with any
wave of low pressure that could ride along the frontal boundaries to
enhance surface convergence, but again, rainfall rates with limited
instability would preclude flash flooding.
More and more Thursday is in focus as the day to watch for impactful
weather given much better dynamics at play for organized convection.
The NBM isn`t particularly bullish on instability, but the multi-
model consensus and model clusters all show good signals for overlap
of at least moderate CAPE and shear Thursday afternoon. Greatest
potential for strong thunderstorms at this point would be in the
Champlain Valley, but there is plenty of time to hone in on the
details. Given potentially less soil capacity following rain on
Tuesday and Wednesday, the thunderstorm risk will be accompanied by
flood risk, with a marginal WPC excessive rainfall outlook remaining
in effect for Thursday.
Following the passage of a cold front from the west, the remaining
of the period is looking dry and with high confidence of no rainfall
especially on Saturday and Sunday when skies should be mainly sunny.
Temperatures will trend from seasonable to a bit above normal on
Sunday with moderately low humidity.
&&
.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
Through 18Z Monday...VFR conditions are prevailing with dry air
moving into the North Country under light northerly flow.
Overnight, favorable conditions for fog development will occur
as winds drop to calm before becoming terrain driven. Best
chances of fog will be 05-13Z at SLK/MPV/EFK/MSS, but cannot
rule out some fog at BTV given recent rainfall. If light
easterly drainage flow does develop like some models suggest,
fog off of the Winooski River could advect over the BTV
terminal. Light southerly flow and VFR conditions return by 14Z.
Outlook...
Monday Night: VFR. Chance SHRA, Slight chance TSRA.
Tuesday: VFR. Chance SHRA, Slight chance TSRA.
Tuesday Night: Mainly VFR, with local MVFR possible. Chance SHRA,
Slight chance TSRA.
Wednesday: Mainly VFR, with local IFR possible. Chance SHRA,
Slight chance TSRA.
Wednesday Night: Mainly VFR, with local IFR possible. Chance
SHRA.
Thursday: Mainly MVFR, with areas IFR possible. Likely SHRA,
Slight chance TSRA.
Thursday Night: Mainly VFR, with areas MVFR possible. Chance
SHRA, Slight chance TSRA.
Friday: Mainly VFR, with local MVFR possible. Chance SHRA, Slight
chance TSRA.
&&
.BTV WATCHES/WARNINGS/ADVISORIES...
VT...None.
NY...None.
&&
$$
SYNOPSIS...Boyd
NEAR TERM...Boyd
SHORT TERM...Kutikoff
LONG TERM...Kutikoff
AVIATION...Boyd
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