Area Forecast Discussion
Issued by NWS Burlington, VT

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FXUS61 KBTV 262343
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
743 PM EDT Thu Apr 26 2018

.SYNOPSIS...
The unsettled weather pattern is expected to continue into the
weekend with periods of showers late Friday and then again late
Saturday through Sunday. Temperatures will trend below normal
into the weekend, but much warmer temperatures are expected to
return for the middle of next week.

&&

.NEAR TERM /THROUGH FRIDAY NIGHT/...
As of 733 PM EDT Thursday...Water vapor shows closed circulation
has moved east of our cwa, while some mid level moisture has
returned on northwest flow on backside of system as drier air
develops aloft. This moisture and weak surface boundary helped
to produce a light rain shower here at BTV around 23z...but this
has since moved into central/eastern vt and is dissipating.
Expecting a dry night with the potential for some br/fog to
develop in the deeper sheltered valleys. Some uncertainty on how
quickly we clear and if winds decouple in the boundary layer
enough to support the temps to reach cross over values. Have
continued to mention some br with temps in the m/u 30s to l/m
40s valleys. Have adjusted hourly temps/dwpts to match crnt
trends and updated pops based on radar, which was to mention chc
pops further south thru 01z. Rest of fcst in good shape.


previous discussion below:
the upper low we`ve been tracking all day is now just about
over the Champlain Valley as I type this discussion. Tried to
see if I could see a big "L" in the sky to mark the center, but
no such luck. Still just a few showers scattered about,
primarily with the upper low. Visible satellite imagery still
shows a good amount of lower clouds, evidence of low level
moisture, across the region. Breaks in the clouds are out there
as well. The upper low seems to be behaving itself, so by late
evening it will be east of the region, and that should mean any
chances of showers will also be long gone. Have used combo of
the 12z BTV 4km WRF and 18z HRRR hi-res models to drive the
hourly evolution of where those showers are expected (primarily
the Champlain Valley and across northern VT). Expect the low
clouds to slowly clear as well. They won`t totally go away, but
we`ll have periods of mostly clear. Given all the recent rain,
I`d expect at least some patchy fog to form after midnight,
especially where the winds lighten up. Not the best fog
situation, so not over the top confidence in such development.

Lows tonight will drop into the upper 30s to mid 40s, which will
be near normal.

Friday starts dry. 12z guidance not showing any big changes to
what we had been looking at, so although I did blend some of the
12z guidance into the forecast, the net result was minimal
changes. Generally looking for clouds to increase and thicken up
by mid-day, with rain showers spreading from south to north in
the afternoon. This will be from the system that is currently
down across the southern US which will lift rapidly
northeastward and across our region thanks to the deepening of a
upper level longwave trough well to our west. With the southerly
flow, and at least a little sun to start the day and 925mb
temperatures in the 7-10C range, we should top out well into the
50s to lower 60s in most of the region -- which is about 2-4
degrees above normal.

Rain showers will be most concentrated across Vermont, with
lesser chances across northern NY. Not looking for a lot of
precipitation, perhaps 0.2-0.4" across VT and less for northern
NY. Showers end late Friday evening, with temperatures Friday
night mainly in the 40s under plenty of clouds.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
As of 355 PM EDT Thursday...The short term forecast will consist
of a continuation of our showery pattern driven by a persistent
upper low overhead. Going into the weekend, shortwave energy
diving down the backside of the trough will further amplify the
pattern and form a closed low at the base of the trough. The
closed low will then linger over the northeastern US for the
remainder of the weekend, providing a cold pool of air aloft
that will increase instability in the low/mid levels. In terms
of sensible weather, this setup will bring us continued
convective precipitation, and a trend towards cooler/breezier
weather to finish out the weekend.

Timing/positioning the individual features that will provide the
forcing for the showers remains the main forecast challenge, however
the bigger picture for how the weekend will playout is becoming
clearer.  Since the shortwaves responsible for this weekend`s
weather have moved inland over western Canada overnight, they are
now able to be better sampled by our observational network and thus
better handled by the forecast models. Q-vector convergence progs
the best synoptic forcing within the upper low to setup over the
North Country/southern Quebec/southern Ontario border Saturday
afternoon/evening, so continued the idea of a drier Saturday
morning, ramping up PoP to chance/likely through the
afternoon/evening. In addition to the more widespread precip under
the dynamic upper low, higher resolution models are indicating the
formation of a north/south oriented zone of enhanced low level
frontogenesis that will be the focus for some more localized banded
heavier rain Saturday evening through Saturday night.  Given the
amount of cold air aloft, there will be enough instability in the
low/mid levels to support some convective heavier rain along this
front, particularly during the Saturday evening time frame. The NAM
is being particularly aggressive with pivoting a zone of strong
surface to 800 mb fgen forcing and low level instability over
the forecast area, suggesting the potential for some locally
heavier banded rain moving through Saturday night/Sunday. The
dynamic upper- level support is definitely there to support this
scenario, however the amount of mesoscale forcing than will pan
out along with the amount of moisture flux into the area could
be limiting factors. We will be monitoring this closely as the
forecast area has already received quite substantial rainfall
from the mid-week system, so the amount of rain that falls this
weekend will further impact area rivers and streams. See the
Hydro Section of the AFD for further details.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
As of 355 PM EDT Thursday...Showers from the upper low will taper off
through the day Monday before ridging finally builds in from
the west and brings a pattern change. Southwesterly return flow
around a strengthening high will advect in warmer, moist air
that will support temperatures warming into the 60s Tuesday and
into the 70s towards the middle of the week. The amount of
sunshine we see during this time will depend on the
amplification of the ridge and the amount of Gulf moisture that
will advect into our area. The core of the high pressure and the
best subsidence will be centered our south over the Mid-
Atlantic/Southeast Coast. This will leave those of us further
north still potentially vulnerable to any shortwaves that move
through that may bring some clouds or showers. Regardless of
whether any disturbances transit our area of not during this
time frame, the amount of warm air advection within the broader
flow is impressive and will likely support temperatures 10+
degrees above normal. Chances for more widespread unsettled
weather return during the second half of the week as the ridge
begins to weaken.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
Through 00z Saturday...Another challenging aviation forecast
with regards to clouds/winds and potential fog/br development
aft 06z. Crnt obs show a mix of vfr to mvfr to ifr with general
improving trends expected at all taf sites by 03z to mvfr/vfr.
If we see more clearing and winds decouple expect areas of
fog/br to develop, given plenty of boundary layer moisture from
recent rains. Have mention tempo between 07z-11z for ifr vis
btwn 1-2sm at mpv/slk/mss. Winds should limit fog potential at
rut/btv and pbg. Any fog/br will quikly lift btwn 10-12z Friday
as mid/upper level clouds develop from south to north ahead of
our next system. Expect rain to arrive aft 18z with mvfr cigs
possible at mpv toward mid afternoon. Elsewhere...vfr vis/cigs
should prevail.

Outlook...

Friday Night: Mainly VFR, with local IFR possible. Slight chance
SHRA.
Saturday: Mainly VFR, with local MVFR possible. Chance SHRA.
Saturday Night: Mainly VFR, with local IFR possible. Chance SHRA.
Sunday: Mainly MVFR, with areas VFR possible. Chance SHRA.
Sunday Night: Mainly MVFR, with local VFR possible. Chance SHRA,
Chance SHSN.
Monday: Mainly MVFR, with areas VFR possible. Chance SHRA.
Monday Night: Mainly VFR, with local MVFR possible. NO SIG WX.
Tuesday: VFR. Slight chance SHRA.

&&

.HYDROLOGY...
As of 400 PM Thursday...the widespread 1/2 to 1" of rain, along
with continued steady snowmelt from the remaining snow at the
highest elevations has resulted in most area rivers, especially
in the northern sections, running fairly high. Additional
precipitation for late Friday/Friday night will keep river
levels from dropping much. No flooding is expected the next
couple of days, but there is some concern for what could happen
later in the weekend. There will be another round of rain late
Saturday/Saturday night. Some guidance indicates some
significant rainfall amounts, though there is quite a bit of
uncertainty in those details. If the heavier rain scenario does
pan out, with rivers already running somewhat high, this would
suggest we may be looking at some flooding issues for Sunday. At
this point it`s a low confidence scenario, but worthy of at
least mention.

&&

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

&&

$$
SYNOPSIS...Nash
NEAR TERM...Nash/Taber
SHORT TERM...RSD
LONG TERM...RSD
AVIATION...Taber
HYDROLOGY...Nash



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