Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Burlington Vermont
342 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 345 am EDT Wednesday...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
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 Friday. 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 /08z Wednesday through Sunday/...
through 06z Thursday...expect generally VFR conditions except for
possible fog/mist with IFR or lower visibility at mpv, slk and
mss through the overnight. Potential for scattered showers/storms
Wednesday during the daylight hours.
Infrared satellite and radar mosaic shows broken cloudiness with
very light showers from Champlain into St. Lawrence valleys.
Showers should continue east in short order, but there's some
question on if middle-level cloud shield can clear out enough to
promote mist/fog formation. A pocket of clearing back into the
Ottawa Valley should help to clear out mss and perhaps slk. Though
its currently clear at mpv now, it's less certain that clearing
could happen once cloud deck moves in. In each case, opted for
tempo IFR visibilities. Winds generally light north under 5 kts.
Threat of showers/storms develops once again, tied primarily to
the diurnal heating period. Hi-res guidance points shows potential
for all taf sites except rut. Roughly followed this by indicating
6sm rain showers for this taf forecast. Stronger storms capable of small
hail and gusty outflow capable of turbulence. West to northwest
winds around 10 kts (gusts under 20 kts) become light west by
Outlook 06z Thursday through Sunday...
06z Thursday - 12z Thursday...VFR with periods IFR/LIFR fog at mpv
and slk terminals.
12z Thursday - 00z Friday...mainly VFR with spotty diurnal-driven
00z Friday Onward...daytime VFR with evening IFR/LIFR fog at mpv
and slk terminals.