Scientific Forecaster Discussion
fxus61 kbtv 260203
Area forecast discussion
National Weather Service Burlington Vermont
1003 PM EDT Tue Jul 25 2017
upper level low pressure will slowly pull east of the area
this afternoon, allowing skies to clear. After high pressure
brings a short window of fair and dry weather on Wednesday,
shower and thunder chances return by Thursday into Thursday
evening as a weak front crosses the area. Dry conditions then
return by next weekend as surface high pressure builds across
Near term /through Wednesday night/...
as of 1001 PM EDT Tuesday...just adding to below...made a small
tweak to kslk for 41 vs. 39 for a low with low clouds
precariously close to them and expected fog development. Hrly
obs match with this change I made 3 hours ago, thus no changes
to current fcst. Trends are occurring as expected, gradual
drying and diurnal clouds dissipating. Right now...just a
wait/see on if these current/expected trends continue which we
believe will and then its waiting for fog to develop overnight.
Afternoon discussion...the sunshine is coming, I Promise. The
thick deck of strato- cumulus clouds are slowly pushing
southeast as well as eroding due to drier air from the northwest
and subsidence associated with a building ridge. Latest GOES-16
satellite visible imagery shows a lot of breaks in the clouds
across northern sections of Vermont and New York. These trends will
continue such that the clearing skies move from north to south.
Most of the region should be clear before sunset, the exception
perhaps in south-central Vermont where some clouds will linger
latest. As the sun comes out, temperatures will pop up a few
degrees rather quickly, thanks to strong Summer sunshine. Radar
not showing any precipitation of consequence, so have taken out
any mention of showers for the remainder of the day.
Still looking at a high pressure, light wind and mostly clear
skies type of night. Given recent rains and wet soil, that
strongly suggests the development of locally dense fog in
normally fog prone valleys, and even a light fog in many other
areas. Even the hi-res models, such as the 3km NAM, seems to
have quite a realistic projection of the fog areas in it's
visibility fields. With a radiational cooling night,
temperatures will vary quite a bit from location to location,
with 40s in the cooler locations to 50s in the lower elevations.
Still an argument going on between the NAM met MOS guidance and
GFS mav MOS guidance with respect to how cold it will get in
Saranac Lake. NAM says 37. GFS says 47. I'm confident the final
result will be in between those two values, and think perhaps a
little closer to the NAM than GFS. It will ultimately depend on
when the fog forms there. The earlier the fog, the slower the
temperature drop will be.
Wednesday still looking to be a great outdoor activity type of
day. Plenty of sunshine, though the next weather maker will be
starting to spread some clouds in late in the day across
northern New York. With 850mb temperatures about +12c, that should
support high temperatures topping out around +27c (about 80f)
in the lower elevations.
Wednesday night will start dry, but as trough aloft approaches
the region, we'll see showers develop from west to east --
especially across the northern half of the region. Not very
strong dynamics, nor any instability to speak of, so at this
point it looks like run of the mill showers. Rainfall totals
should be mostly less than 1/4" prior to daybreak Thursday.
Have pops ramp up into the 55-65% chance range after midnight
across the north, and 35-50% for the southern half of the area.
The clouds and precipitation will keep temperatures much more
mild, ranging from the upper 50s to lower 60s.
Short term /Thursday through Friday/...
as of 324 PM EDT Tuesday...a digging mid/upper-level trough with
associated pre-frontal energy will enter the St. Lawrence
Valley early Thursday morning. This leading energy will move
west to east across the County Warning Area Thursday morning helping to initiate
some showers across northern New York and northern Vermont. Instability
chances will be low in the morning increasing slightly during
the afternoon as some clearing is expected after 12z. Models
suggest cape values between 600-1000 j/kg and 0-6km shear values
around 35kt. This marginal instability/shear, coupled with a
30kt 850mb jet and 85kt 250mb jet, may lead to some isolated
thunderstorm development so cant rule out chance of thunder
Thursday afternoon, however the best dynamics and moisture will
be to southeast of County Warning Area during this period. Temperatures on
Thursday will be in upper 70s with lows around 60. As the main
front moves east, drier air is expected overnight Thursday into
Friday with pop chances diminishing as cold air advection presides over area.
Widespread fog is not expected Thursday night/Friday morning as
north/northwest winds around 5-10mph should keep the surface well enough
mixed, however isolated fog may be possible in some sheltered
valley locations. Friday remains dry as surface ridge builds in.
Temperatures should be just below normal with highs in the mid
70s. Latest 12z trends have been to keep closed 7h/5h
circulation and associated surface low pres to our
south...resulting in dry conditions for Friday into Saturday.
Still some uncertainty on exact track of closed system and if
some showers reach our southern/central cwa late Friday.
Long term /Friday night through Tuesday/...
as of 324 PM EDT Tuesday...as the upper-level ridge continues to
build over area, dry conditions should persist through the
weekend. Temperatures should be near normal with highs in the
upper 70s to around 80 through Sunday. High pressure will move
east Monday, the chance for showers increases as conditions
become more unsettled by Tuesday. Westerly flow aloft combined
with southerly winds in the llvls will push warmer and increase
moisture into our fa. Have mention chc pops Tuesday with
isolated afternoon thunderstorms possible...as GFS indicates sfc
based cape values up to 1000 j/kg over the slv. Still plenty of
uncertainty on timing of boundary and magnitude of instability
out 7 days.
Aviation /02z Wednesday through Sunday/...
through 00z Thursday...clearing skies, building high pressure
and calm winds will make for good radiational cooling night
allowing widespread fog to develop. VFR conditions this evening
are expected to deteriorate to MVFR at 03z/kslk...07z/kbtv, kmpv
and kmss...possibly 08z/kpbg. Drainage flow expected to develop
at krut hindering widespread fog, but cannot rule out a brief
period of lighter winds during the early morning hours and MVFR
High confidence that kmpv and kslk will further deteriorate to
IFR-vlifr around 06z through 13z. Kmss may also see IFR vsbys as
this terminal is already clear and will have more time to
Fog that forms over the Winooski river could drift into btv
airfield. Expect a very light east wind at btv...advecting the
fog over the Airport mainly after 07z.
With continued high pressure, expect fog to erode between
1130-1330z. VFR conditions will prevail throughout the day
Wednesday night: VFR. Chance rain showers.
Thursday: VFR/MVFR. Likely rain showers...slight chance thunderstorms and rain.
Thursday night: VFR. Chance rain showers.
Friday: VFR. Slight chance rain showers.
Friday night: VFR. No sig weather.
Saturday: VFR. No sig weather.
Saturday night: VFR. No sig weather.
Sunday: VFR. No sig weather.