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  Monday August 21, 2017

 

NWS Area Forecast Discussion


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FXUS61 KBTV 210202
AFDBTV

Area Forecast Discussion
National Weather Service Burlington VT
1002 PM EDT Sun Aug 20 2017

.SYNOPSIS...
High pressure will be the main influence on the region`s
weather through Monday, with clear to partly cloudy skies and
seasonable temperatures. An approaching low pressure system will
increases chances for showers and thunderstorms Tuesday, with
showers lingering into Wednesday. The remainder of the week is
shaping up to be dry, with plentiful sunshine and below normal
temperatures.

&&

.NEAR TERM /UNTIL 6 PM MONDAY EVENING/...
As of 957 PM EDT Sunday...late evening refresh again just
synching up current obs with the forecast. Pretty much clear
across the region. Few patches of strato cu across the northeast
kingdom and thin cirrus approaching from the west, but thats
about it. Temperatures have fallen a little bit faster than
forecast, but expect them to begin to steady off shortly, so
overnight low temperature forecast for 50s to lower 60s still is
on track.

Otherwise to rehash what I typed up earlier this evening...

still expecting just the outside chance of patchy fog for those
few areas (primarily eastern VT and `Dacks) where locally
cooler areas drop below crossover temperature (roughly 54-57F)
for a few hours. Quick learning lesson, "crossover temperature"
refers to when the air temperature drops below the lowest
dewpoint reading during the prior afternoon. It`s a good fog
forecasting technique under fairly quiet weather regimes where
radiational cooling is expected. Tonight comes close to that
type of scenario, except that we`ll be having 15-20 knots of
wind about 500-1000 feet above the surface. That wind may keep
just enough mixing going on in the boundary layer to keep fog at
bay, well, at least thicker fog. In any case, if I`m spending
this much time writing about a little patchy fog, you know it`s
a very quiet weather night :)


Rest of discussion from dayshift forecaster...

Upper low retreating east and high pressure building tonight
with clearing and subsidence. Surface high centered over the mid
Atlantic states will move east during the period, and surface
ridge extending north into the forecast area will shift east by
Monday. Light northwest to west wind tonight will become
southerly by Monday morning. Warm air advection on southerly
flow, abundant sunshine and rising heights result in the
potential for max temps on Monday to reach well into the 80s.
925mb temp of 22- 23C around BTV supports max temp near 89.

Excellent prospects to see the partial solar eclipse on Monday
with little in the way of cloud cover.

&&

.SHORT TERM /6 PM MONDAY EVENING THROUGH TUESDAY NIGHT/...
As of 332 PM EDT Sunday...Active weather during the short term
time period. Deepening 500 mb trough to the west sweeping
through the great lakes strengthening low level flow from the
southwest, especially in the St. Lawrence valley. Precipitable
water begins to climb Monday night, reaching nearly 2 inches by
Tuesday. Surface dewpoints rebounding into the mid to upper 60s
areawide, and around 70 not out of the question in the St.
Lawrence valley. Low level jet of 30-40 knots at 925 through 850
mb increases to 50 knots at 500 mb mainly over the St. Lawrence
Valley, provides for increasing 0-6km shear, however upper
level jet dynamics don`t quite line up, with right rear/left
front couplet lining up further west and north over southern
Ontario.

Surface boundary, mainly evidenced by wind shift and lowering
dewpoints, will reach St. Lawrence valley Tuesday evening and
sweep eastward through the forecast area by Wednesday morning.
Storms to fire ahead of the boundary Tuesday afternoon in the
St. Lawrence valley and move east through evening. Overall POPs
increase Tuesday morning west to east, and locally run BTV4
based pops from model reflectivity were blended in to show
categorical probabilities with a forecast line of storms.

Storms to be strongest northern New York where low level jet
dynamics and shear line up with surface based CAPE generally
1500- 2000 j/kg. As storms move east they will move out of best
shear environment but continue to feed on moisture rich air and
afternoon temperatures again into the 80s. Heavy rainfall rates
possible with efficient warm rainfall processes, however storms
should be moving quickly enough to preclude a flood threat.
Added gusty wind/heavy rainfall wording to likely and higher POP
Tuesday afternoon into evening.

Overnight lows will not cool off much especially Monday night
in high dewpoint air, and cooling/drying will not arrive in time
Tuesday night to be of much help. Max temps Tuesday well into
the 80s once again with warm air advection, and will be quite a
muggy day.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
As of 253 PM EDT Sunday...The extended forecast will feature
large scale pattern change as deep full latitude trof develops
across the eastern Conus and ridge builds out west. This will
result in temps returning to at or below normal values with much
less humidity. The forecast challenge will be timing individual
short waves in the southwest to eventually northwest flow aloft
and potential areal coverage of showers. Overall...the
atmosphere will have much less available moisture as pw values
drop below 1.0 and instability is very limited.

On Wednesday...first embedded 5h vort arrives around 18z... along
with secondary surface convergence line with some additional weak
low level cold air advection behind boundary. Instability is driven
off sfc heating and developing cool pool aloft with deepening
trof...so 30 to 40% chance of showers with maybe a rumble or two of
thunder...but cape values are weak. Soundings show high cloud bases
with deep dry layer so areal coverage of showers will be
limited...but some localized gusty winds possible based on sounding
profiles. Progged 850mb temps around 10c support highs upper 60s
mountains to upper 70s warmest values. Low level cold air advection
continues overnight into Thurs with progged 850mb temps dropping
btwn 5-7c by 12z Thurs. The gradient flow will limit fog development
and keep temps from completely bottoming out with lows mainly 40s
mountains to mid/upper 50s valleys.

Thursday...another embedded 5h vort in the west/northwest flow aloft
and associated pocket of -20c air moves across our cwa. This energy
will limited moisture will help to produce isolated/scattered
showers during on Thurs aftn. Once again areal coverage and amount
of showers will be limited...because of pws around 0.50 to 0.70.
Progged 850mb temps near 6c support highs upper 50s/lower 60s
mountains to lower 70s warmest valleys.

Friday into next weekend...will feature building 1025mb surface high
pres while aloft remains active with embedded 5h vorts in the
northwest flow. Each piece of energy will have less impact with
limited moisture...so will keep Friday dry at this time... with temps
mainly 60s mountains and lower/mid 70s warmer valleys. Lows mainly
in the mid 30s to upper 40s depending upon locations...with coldest
values at SLK/NEK...especially on Saturday morning. Next weekend is
looking mainly dry with slightly below normal temps and low humidity
values. Highs in the 60s mountains and 70s valleys with lows ranging
from the lower 40s to mid 50s.

&&

.AVIATION /02Z MONDAY THROUGH FRIDAY/...
Through 18Z Monday...Aviation challenge tonight will be fog/br
potential at slk/mpv...followed by increasing surface winds on
Monday afternoon. Crnt surface map shows 1020mb high pres over
central PA with ridge axis across northern NY. This has created
clear skies and light winds with vfr conditions.
Tonight...expecting vfr to prevail with exception of periods of
ifr in fog/br possible at mpv/slk btwn 07-11z. Couple of factors
limiting widespread fog include 950mb winds of 15 to 20
knots...two days removed from rainfall... and surface temps only
briefly going below cross over values from earlier this aftn.
Have mention tempo group at both mpv/slk for potential ifr in
shallow fog/br. Otherwise...surface gradient increases on Monday
with localized gusts to 20 knots possible at slk/mss/btv aft 16z
with vfr prevailing at all sites.

Outlook...

Monday Night: VFR. Slight Chance SHRA...Slight Chance TSRA.
Tuesday: VFR/MVFR. Breezy. Likely SHRA...Slight Chance TSRA.
Tuesday Night: VFR. Breezy. Chance SHRA...Chance TSRA.
Wednesday: VFR. Chance SHRA.
Wednesday Night: VFR. NO SIG WX.
Thursday: VFR. Slight Chance SHRA.
Thursday Night: VFR. NO SIG WX.
Friday: VFR. NO SIG WX.

&&

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

&&

$$
SYNOPSIS...Hanson
NEAR TERM...Hanson/Nash
SHORT TERM...Hanson
LONG TERM...Taber
AVIATION...Taber


 
 
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