Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Burlington Vermont
757 PM EDT Thursday Aug 28 2014
high pressure building eastward into the region will provide the
north country with a cool night tonight and a sunny and dry
day on Friday. Changes are in store though for the weekend
with warming temperatures and increased chances for showers and
thunderstorms...especially on Sunday. Temperatures this weekend
will rise back into the upper 70s to lower 80s for afternoon
highs...and generally remain above seasonal averages for early
September during the early to middle part of next week.
Near term /until 8 am Friday morning/...
as of 619 PM EDT Thursday...inherited forecast is in great shape
with only a few tweaks needed to hourly grids for the next couple
hours to mesh with current observational trends.
Previous discussion...high pressure will build into the region
this evening and tonight with weakening northwest flow. This will
lead to a gradual dissipation of the stratocumulus clouds that
have been with US throughout the day...leaving US with mostly
clear skies for the overnight. However some clouds may linger in
the continued...albeit weak...north flow...especially over the
Northeast Kingdom. As skies clear and temperatures fall...some
patchy valley fog is likely in the favored locations. Looking at
lows in the 40s for most...except some 50s in the immediate
Short term /8 am Friday morning through Saturday night/...
as of 251 PM EDT Thursday...Friday is shaping up to be a great
late Summer day with mostly sunny skies as high pressure starts
the day directly overhead. It will slide off to our east gradually
throughout the day with some southerly return flow developing
late. Should see highs reach into the 70s pretty much area-wide.
The southwest flow really kicks in Friday night with warm air
advection ensuing as the upper level ridge off to our south
amplifies. Will see a steady increase in high clouds as well.
Temperatures will be much more seasonable than tonight with lows
from 47 to 57...warmest in the Champlain and Saint Lawrence valleys.
Deep southwest flow and warm air advection continues on Saturday
around the periphery of the Southeast Ridge. Looking at a mainly dry
day with limited instability and large capping inversion. Cannot
rule out an isolated shower or thunderstorm across northern and
western areas...but that should be the exception rather than the
rule. Should be a pretty nice day overall with sun filtered through
high clouds. Looking at highs from the middle 70s to low 80s. There
will be a noticeable increase in humidity...with dew points rising
back to near or above 60 by late in the day.
The southwest flow continues Saturday night with a frontal boundary
and trough approaching from the west. Precipitable water values also
markedly increase late Saturday and Saturday night to 1.5-1.8" and
some elevated instability also develops. Thus will be looking at
some increasing chances for showers and thunderstorms as Saturday
night GOES on. Temperatures will be quite mild with lows in the
60s for most.
Long term /Sunday through Thursday/...
as of 315 PM EDT Thursday...little change to the overall
expectation for the Sunday & Monday portion of the Holiday
weekend. Still looks like showers & T-storms will be around, most
numerous on Sunday. Still a few showers for Tuesday, then it
appears dry weather returns for Wednesday and Thursday.
For those that want more details, please continue to read on for
the daily specifics...
Sunday: deep southwest flow still in place, with precipitable
water values closing in on 2.0". That's about 2 Standard
deviations above normal, and that indicates some locally heavy
downpours are possible. Models have backed off a bit on overall
amount of dynamics that will be in place, and as a result, they
are not showing as much in the way of total precipitation. Should
be a good deal of clouds around, and that may also limit the
amount of surface instability we can generate, though models do
indicate a modest amount just based on a very humid boundary layer
(dewpoints way up in the 60s). Bottom line, still a bit uncertain
to exactly how the day will play out. Anticipate a decent amount
of shower/T-storm coverage at any time, so the 60% or so probability of precipitation seem
reasonable at this point. Shouldn't rain in any one location the
whole day, but still the idea of having the backup indoor plan is
a good one. 850mb temperatures will be up near 17c, which with
full sun would give US upper 80s for highs. Clouds will greatly
cut back on that potential, so model guidance blend of upper 70s
to lower 80s is fine. Short wave aloft traverses the region Sunday
night, so showers or perhaps a thunderstorm will continue much of
the night. Very muggy with those high dewpoints, so lows won't be
all that low -- middle to upper 60s.
Monday: still fairly warm/humid airmass, but as the shortwave
pushes off to the east, we'll have a little ridging develop. Thus
daytime heating will be the forcing mechanism for a few showers or
isolated T-storm, but coverage will be less than Sunday. Only
painting in 30-40% chance of showers, which for the optimists
means a 60-70% of no rain! 850mb temperatures around 15c, which
should give US another above normal day with highs in the lower
80s (could be a bit warmer if there is more sun). Still humid
Monday night, so another warm one with lows in the 60s. A spot
shower or two is possible as another shortwave approaches later at
Tuesday: a short wave aloft and weak front at the surface should
move through on Tuesday. 12z GFS and European model (ecmwf) are similar with the
scenario, though neither shows anything too strong. Went with a
model blend for pops, so have 40-50% chances. 850mb temperatures
still warm (15-16c), but again the amount of cloud cover is a bit
up in the air. Will again go with lower 80s for highs.
Wednesday & thursday: models greatly diverge at this point. GFS
is back on "it's gonna rain again wednesday", while the Euro is
dry. Looks like the GFS is having some convective feedback issues
over the Ohio Valley on Monday/Tuesday and this turns into the
blob of precipitation for US. The European model (ecmwf) has been much more steady
with previous runs, so went much more toward that model. In
addition to the lack of rain, the Euro has more west-northwest
flow, meaning cooler/drier airmass in place. For example at 850mb
the Euro has 12c vs 16c on the GFS. That's the difference between
upper 70s and upper 80s for highs. GFS also has dewpoints in the
60s versus 50s for the European model (ecmwf). Thursday is a similar situation, as
the GFS has residual showers, while the European model (ecmwf) is dry. So stayed
dry. Gets more uncertain with temperatures as now the GFS is
finally bringing in cooler air, while the European model (ecmwf) is showing that
much warmer air from continuing heat wave in the central US starts
to get in here. The Euro now has the 16c at 850mb (or upper 80s at
the surface) compared to 14c at 850mb (lower 80s) for the GFS. I
kept with the lower 80s at this point.
Aviation /00z Friday through Tuesday/...
through 00z Saturday...mainly VFR conditions expected through the period
as a ridge of high pressure builds east from the Great Lakes. Low
and middle level clouds across the region at 23z Thursday will
gradually dissipate by 06z Friday with skies becoming mostly clear.
BUFKIT forecast soundings show winds just above the surface will
be a bit stronger and will likely inhibit the formation of fog
obvernight...except have gone with a mention of dense fog at kslk where
winds will diminish earlier and skies clearing out faster there.
Outlook 00z Saturday through Tuesday...
00z Sat-12z Sat...primarily VFR as high pressure continues over
12z Sat Onward...mainly VFR...with MVFR/brief IFR in scattered
showers and thunderstorms.