Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Burlington Vermont
721 am EDT Wednesday Aug 5 2015
showers with embedded thunderstorms will be possible
again today with temperatures mainly in the 70s. A slow drying trend
will occur on Thursday and continue into the weekend as surface high
pressure builds into the north country. Temperatures will feature
cool nights and mild days with values very close to normal for this
time of year.
Near term /until 6 PM this evening/...
as of 719 am EDT Wednesday...minor update to reduce probability of precipitation across
most of Vermont and northern New York this morning. Current radar shows
only a few very light rain showers across the Northeast
Kingdom...which will exit our County Warning Area by 12z. Otherwise...expecting a
mainly dry forecast through this morning...before additional showers
with embedded storms develop this afternoon from surface heating/weak
instability. Skies will be partly to mostly cloudy with highs
mainly in the 60s mountains to 70s valleys today.
Large scale shows deep middle/upper level trough across the NE Continental U.S.
With numerous embedded 500 mb vorts. Water vapor shows best deep layer
moisture has shifted into the Middle Atlantic States with trough axis
directly overhead. This will result in cool pool aloft with 500
temperatures near -18c today...which will help to steepen lapse rates and
create some instability. However...soundings show less instability
than yesterday from cooler surface temperatures and drier dewpoints...in addition
precipitable water values are less today. These factors all support less areal
coverage of convection today and weaker updrafts with less
organization. We will mention chance probability of precipitation with greatest coverage from
the northern dacks into central/northern Vermont this afternoon...along with
isolated thunder. Given low wet bulb freezing levels between 7000 and
8000 feet some pea size hail will be possible in the strongest
cells...but not widespread in coverage. Quantitative precipitation forecast values will general be
less 0.10" but localized higher in the strongest storms. Prognosticated
850 mb temperatures between 8-10c support highs mainly in the u60s to lower 70s
mountain towns to M/u 70s warmer valley locations.
Short term /6 PM this evening through Friday/...
as of 345 am EDT Wednesday...little overall change to large
scale synoptic pattern is expected through Friday with general
middle/upper level trough across the NE Continental U.S.. however...best deep layer
moisture will be suppressed to our south during the period with much
less areal coverage of shower activity expect on Thursday. Surface high
pressure located over Hudson Bay is prognosticated to nose into northern New York
during this period...resulting in drier weather with large daily swings
in temperatures. Latest trends in guidance has been for coastal system to
pass well south of our County Warning Area with greatest impacts across the southeast Continental U.S.
Into the Middle Atlantic States. Northwest flow aloft combined with weak 500 mb
vorticity and cool pool aloft will produce spotty mainly daytime heat
driven showers across the dacks into the mountains of central and
northern Vermont on Thursday. Will mention schc in the cpv and chance probability of precipitation in the
mountains with quantitative precipitation forecast values generally < 0.10.
Prognosticated 850 mb temperatures cool another degree or two and support
middle/upper 60s mountains to nek and middle 70s warmer valleys...several
degrees below normal for early August. Little change anticipated
on Friday with general northwest flow aloft and building surface high pressure.
Will mention schc probability of precipitation in the mountains and keep rest of forecast dry at this time.
Prognosticated 850 mb temperatures between 8-10c support highs mainly in the 60s mountains
to 70s valleys. Lows will generally be in the 40s colder mountain
valleys to 50s in the wider Champlain and Saint Lawrence valleys
through the period.
Long term /Friday night through Tuesday/...
as of 331 am EDT Wednesday...mid-level cyclonic flow/longwave
troughing prevails in the mean across the north country during the
extended. All-in-all, a generally uneventful period of weather
into the early part of the weekend, with better chances for
precipitation holding off until Sunday and early next week.
Forecast follows blended guidance unless noted otherwise.
Earlier cycles of global guidance suite had indicated a potential
coastal low developing off the middle-Atlantic coast later Friday and
its precipitation shield expanding into eastern sections of
Vermont on Saturday. While recent guidance still shows this
system, a noted trend has been to a position further off the coast
resulting in a more optimistic early-weekend forecast. So the
biggest change to the inherited forecast was to lower probability of precipitation for the
Saturday/Saturday night period for Vermont. I didn't want to
completely go dry on the chance that a westward recurve reveals
itself in subsequent guidance runs, but i've lowered down to
00z GFS/ec solutions start to differ after Sunday on sensible
weather details. While both global solutions depict a 1000-500 mb
thickness gradient/warm front across the north country and rather
weak lobes of vorticity maxima acting on the boundary, the GFS shows
hardly any precipitation at all. That's a strange/unrealistic
result. I've therefore hedged closer to the wetter European model (ecmwf) in
keeping chance probability of precipitation going Sunday. Chance probability of precipitation continue into early
next week as the baroclinic zone makes little eastward or
northward inroads and may serve as a conduit for continued rain
chances. There is some limited instability (lifted indices of -1
to -2), so opted for showers/thunderstorms each day.
Temperatures through the period are close to near early-/mid-August
normals with highs in the 70s and lows in the 50s/near 60.
Aviation /11z Wednesday through Sunday/...
through 12z Thursday...VFR outside of developing scattered
showers/storms with briefly reduced visbilities as low as IFR.
Potential for IFR/LIFR fog at mpv and slk into the overnight.
Infrared satellite shows scattered-broken clouds extending from northeast
corner of Vermont back through the Adirondacks ahead of surface trough over
southern Quebec. These should thin out into the morning hours.
However with a cold pool aloft associated with deep upper low,
scattered showers/storms will develop with daytime heating. Any strong
storm that develops will be capable of brief IFR-visibility
downpours, small hail and gusty outflow producing localized
turbulence. Overall coverage of strong storms will be limited.
Based on hrrr and recent btv WRF guidance, showers/storms most
favored for all terminals except rut and starting around 17z with
6sm rain showers. Daytime winds west to west/southwest 6-10 kts gusts
under 20 kts.
Any showers/storms quickly dissipate near/after sunset. Risk of
IFR/LIFR fog at slk and mpv is conditional on clearing, which
isn't certain. I've opted to leave out for now. Winds should
become light northwest (variable at times) this evening.
Outlook 12z Thursday through Sunday...
12z Thursday - 00z Friday...mainly VFR with spotty diurnally-driven
00z Friday Onward...daytime VFR with evening IFR/LIFR fog at mpv
and slk terminals.