841
FXUS61 KBTV 200627
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
227 AM EDT Sat Jul 20 2024

.SYNOPSIS...
Seasonable temperatures will occur this weekend with a weak
front bringing some sprinkles or a very light shower Saturday
night. Mostly dry conditions will persist into next week before
the pattern shifts to daily potential for wetting rains and
afternoon thunderstorms as early as Tuesday. Some periods of
rainfall may be heavy in localized areas, but details are few
this far out.

&&

.NEAR TERM /THROUGH SUNDAY/...
As of 1022 PM EDT Friday...Forecast remains very much on track
so only made minor changes to bring it up to date with current
obs.

PREVIOUS DISCUSSION...High pressure will hold sway through
tonight into Saturday keeping temperatures right around seasonal
averages with another relatively cool night in the 50s to 60
degrees and highs tomorrow in the upper 70s to mid 80s. A weak
front will begin dropping southward out of Canada tomorrow
helping to tighten the pressure gradient and supporting area
southerly wind gusts 10-20 mph for Saturday. The front`s parent
low will be pulling well away into he Canadian maritimes causing
forcing along the boundary to fall apart with only sprinkles to
a few hundredths of an inch of rain possible. Tomorrow night`s
lows will be marginally warmer in the upper 50s/mid 60s due to
increased cloud cover.

&&

.SHORT TERM /SUNDAY NIGHT/...
As of 309 PM EDT Friday...A surface cold front will continue to drop
southward Sunday morning, with significant advection of cool, dry
air on its north side due to light northerly winds. The passage of
the front probably will be marked by a brief period of sprinkles
with some embedded light showers, mainly in northern areas. Model
guidance often has a wet bias with light precipitation, so the idea
of most locations seeing no measurable rain seems reasonable. In
addition, drying low level air will make it more difficult to have
sufficient moisture for rain as the day wears on. Highest chances of
seeing measurable rain Sunday morning will be in the northern
portions of the Adirondacks and high terrain more generally.

The axis of a strong upper level trough will be centered to our
north and east, which will ensure a beautiful summer day once the
clouds clear out. The very dry air will limit even fair weather
cumulus during the afternoon as dew points fall into the upper 40s
to low 50s. Temperatures will be a bit below normal. The more
anomalous 850 millibar air mass appears to stay just north of our
area, which will ensure that high temperatures still manage to be
comfortably warm in the 75 to 83 range for most locations. Weak high
pressure will be across the area with a front to our south; I do not
expect enough southerly flow to produce stratus, so areas of patchy
dense fog should develop late Sunday night in the typical river
valleys. Given how low afternoon dew points will be, it may be
difficult to see more widespread coverage of fog in the context of
crossover temperatures. In any event, it should be a cool night with
temperature dropping back into the 45 to 55 range for most spots.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
As of 309 PM EDT Friday...Monday will mark the fifth and final day of
lower humidity and primarily dry weather for our region. Then the
pattern will abruptly flip to a wet and humid one for the remainder
of the week. Through Friday, temperatures will stay seasonably warm
without any signals for hot weather. The best chance of seeing
uncomfortable heat is on Tuesday when temperatures are most likely
to be very warm with full sunshine and light winds; forecast WBGT
values creep into the extreme values during the afternoon in some
valley locations. Humidity will become high, with dew points almost
certainly running from the mid 60s into the mid 70s Tuesday onward,
with low spread in model guidance until late Friday. By the weekend
there are some indications that low level flow will become westerly
rather than southerly/easterly, which could end this string of humid
days.

With the latest NBM run, when there is the most spread in
temperatures is on Tuesday, especially in Vermont from the Lamoille
River Valley and points south. The potentially warmer conditions
southwards generally corresponds to a higher thunderstorm risk,
which is pretty high at this time range at about 40% to 50% from 2
PM Tuesday to 2 AM Wednesday. Unsurprisingly, higher probability of
showers exists as one goes southward as well during the period, with
6 hourly PoPs ranging roughly from 40 to 65% in this corridor
between 2 and 8 PM. Three of the four ensemble clusters show a
combination of at least modest instability coinciding with about 30
knots of 0-6 km shear. Given potential for a warm front in the
vicinity, Tuesday could be an active severe weather day if the
ingredients come together in our region.

Generally the highest likelihood of rain is on Wednesday
afternoon/evening, although rain will be possible at times late
Tuesday through at least Thursday night. NBM 6 hourly Precipitation
Potential Index suggests relatively high confidence of rain compared
to Tuesday in a similar timeframe despite similar PoPs. However, the
greatest magnitude of rain currently is indicated for the pre-dawn
hours on Wednesday, which falls well outside of the probabilistic
forecast data. What this tells us is that both the timing and
magnitude of rain is highly uncertain at this time. Outside of a
quasi-stationary front expected to be draped from west to east near
or over the North Country/northern Vermont, we don`t have an obvious
trigger for heavy rain. We will continue to monitor if the
ingredients may come together for a shorter period of intense rain
that would suggest a flood risk.

Looking at the ensemble clusters from the latest (12Z) guidance,
there is broadly good agreement in the upper air pattern with
southwesterly flow, but subtle differences lead to deeper moisture
with greater 700 millibar moisture in most scenarios where mid-level
winds are southwesterly rather than westerly. These wetter scenarios
currently show a pretty big divide, especially on Wednesday, between
most GEFS and most EPS/GEPS members. Therefore, the signal for heavy
rain and thunderstorms is rather mixed at the moment, with generally
a solution that has a stronger upstream trough more likely to lead
to more dynamic, impactful weather late Tuesday through Thursday.
Stay tuned!

&&

.AVIATION /07Z SATURDAY THROUGH WEDNESDAY/...
Through 12Z Sunday...Mainly VFR conditions are expected through
the period, though some intervals of FG/BR are possible thru 12Z
this morning at SLK, MPV, EFK. The daylight hrs will feature
passing high clouds (FEW-SCT200-250) with SW winds 5-10kts.
Locally at KMSS, valley channeled flow will yield SW winds 10-12kt
with gusts up to 20kts during the mid-day/afternoon hours. A
weak cold front approaching from southern Quebec and southeast
Ontario will bring increasing clouds Saturday night into Sunday
morning, generally BKN040-060. A few light rain showers or sprinkles may
(20-30% chance of measurable rainfall) affect the northern TAF locations
(MSS/SLK/PBG/BTV/EFK) during the overnight hours. Winds will shift light
N-NW after 04Z Sunday with the passage of the frontal bndry.

Outlook...

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

&&

.BTV WATCHES/WARNINGS/ADVISORIES...
VT...None.
NY...None.

&&

$$
SYNOPSIS...Boyd
NEAR TERM...Boyd/Chai
SHORT TERM...Kutikoff
LONG TERM...Kutikoff
AVIATION...Banacos