Scientific Forecaster Discussion
fxus61 kbtv 222336
Area forecast discussion
National Weather Service Burlington Vermont
736 PM EDT Mon may 22 2017
a weak cold front crossing New York and Vermont through this evening
will bring isolated showers, along with lingering low clouds and
areas of fog overnight. Will see clearing Tuesday with a weak
area of high pressure in place across the northeastern U.S. And
temperatures moderating back into the upper 60s to lower 70s. A
few afternoon showers are possible Wednesday, but high
temperatures will continue to moderate into the low to mid 70s.
A large mid-level trough across the Tennessee and Ohio valleys
and associated developing surface low pressure will bring
increasing chances for widespread rainfall across the north
country on Thursday and Friday.
Near term /through Tuesday night/...
as of 702 PM EDT Monday...minor adjustments to the previous
forecast for this evenings update, mainly to reduce pops and
tweak sky cover. Frontal showers have quickly exited the region
to our east, and while a few stray showers may be around through
the evening, in general the remainder of the night will be dry.
Forecast challenge for sky cover remains though with subsidence
building in aloft and an abundance of low level moisture
around. Current thinking is that sky cover will be widely
variable through the night with some low stratus hanging tough
over higher elevations, while many valley locales may clear out
to a scattered/overcast mid/High Deck. Can't rule out some areas
of fog, especially in any clearing so have left that in, but
signals are trending towards less of a chance of dense fog and
more br-ish with winds just off the deck hanging in the 10-15
knot range. Lows remain on track for mid/upper 40s to low 50s.
will see some gradual mid-level drying this evening (above
750mb) with west-southwesterly flow aloft, but pronounced low- level
inversion should continue to hold low stratus in areawide with
overcast skies and cool temperatures. Afternoon highs only
expected to range from the lower 50s east of the green mtns, to
near 60f at btv, and upr 50s to lower 60s across nrn New York. The
kcxx VAD wind profile still indicates 25-30kt flow at 2-3kft
above ground level. Won't fully see these winds at low elevations due to
stability, but occasional gusts to 20-25 mph are possible thru
this aftn. Expect winds remaining 20-25kts over the broad
portion of Lake Champlain, and the lake Wind Advisory continues
Tonight through Tuesday a weak surface ridge builds into the
region. Lowering inversion and light surface winds, along with
moist ground from recent rain will promote fog formation in the
usual areas. During this time flow aloft remains southwesterly,
and low pressure over the Great Lakes will ride through the 500
mb ridge, suppressing the ridge & shifting it east. The low
passes well enough north that the surface ridge will win out
with no rain during this time, but will still be quite a few
clouds around. Tuesday about 10 degree warmer than Monday with
highs in the 60s/near 70.
Short term /Wednesday through Wednesday night/...
as of 315 PM EDT Monday...12z guidance suite still indicating
that Wednesday will be the warmest day of the entire weak. Weak
ridging aloft will put the region into a col area (light flow
regime). Hi-res models do hint at just enough low level moisture
and surface instability will exist to pop a few showers,
primarily across the higher terrain in the afternoon, though
with an inversion around 15,000ft not expecting any T-storms to
develop. Winds from 10,000ft down to the surface will be
generally 10 knots or less, so look for onshore lake breezes to
develop. 925mb temperatures will be running about 17c, which
given a decent amount of sunshine will result in lower elevation
temperatures within a couple of degrees of 80f.
Wednesday night flow starts to turn south/southeast as low
pressure moves toward the Great Lakes. Models all show an area
of deeper moisture moving toward the region, so as we move
toward daybreak Thursday it appears precipitation will be
starting to overspread the region. Have depicted a ramping up of
pops up well after midnight, though most of the night will be
Long term /Thursday through Monday/...
as of 315 PM EDT Monday...12z guidance came in fairly close to
previous runs, thus confidence remains rather high for the
overall scenario of unsettled weather through much of the
period. Looks like the primary rounds of more significant
weather will be Thursday and again Monday (memorial day), with
"okay" weather squashed in between. Comments on each day
Thursday: fairly sizable low pressure will be moving across the
Ohio Valley with a deepening upper level trough also in place.
This will result in a tightening pressure gradient with a
low/mid level east-southeast jet developing during the day. The
GFS indicates about 40-45kts of wind at 850mb, with the 12z Euro
having a few grid points of 50kts of wind. Fairly typical
situation in which we'll probably have some breezy conditions
along the western slopes of the Green Mountains. At this point
think we could Max out with some gusts in the 30-40 mph range in
the favored areas. Lower resolution global models don't have a
very good depiction of these localized effects, so did try to
manually introduce some gustier winds into the gridded database.
Just an aspect of the forecast we will have to monitor.
Otherwise, system will have a pretty good moisture tap
(precipitable water values about 1.3"), so expect fairly
widespread rainfall to occur during the day with rainfall totals
probably around 1/2". Shouldn't lead to any Hydro issues, but
also another aspect to monitor. Kept with a model blend for
temperatures, which has primarily 60s across the region (lower
60s across southeast Vermont to upper 60s around 70 for the northern
Champlain Valley into the St Lawrence valley), however it may
end up cooler than that if we inject more Atlantic airmass into
Friday: the upper trough turns into a closed low and moves
across the region during the day. Basically means a continuation
of unsettled conditions with spotty showers at times. Lots of
clouds will keep it on the cooler side once again, generally in
Saturday: trough and surface low move off to our east, putting
US into a northwest flow situation. 850mb temperatures start off
around 5-7c and by afternoon rise to 9c. This should support
highs in the upper 60s to lower 70s, though if we can see more
sunshine, mid 70s are possible. Still can't rule out an isolated
shower, but should be pretty good for most outdoor activities.
Sunday: GFS hints at a weak back-door cold front trying to come
at US from the east as surface high pressure moves over Maine
and sets up some low level easterly flow, with the chance for a
few light showers. The 12z European model (ecmwf) has no such feature, and is
putting the region into a developing southerly flow situation
ahead of monday's weather maker. However, with that deeper
southerly flow, it's got some deeper moisture.
The net result from either type of scenario is that showers
can't be ruled out for Sunday at this point. Not as much
confidence in the temperature forecast however, and given the
variation in scenarios we could be anywhere from the upper 60s
to lower 80s for highs. Stuck with a blend for this go around.
Monday: alas, Memorial Day isn't looking the best for those
outdoor bbqs/games/parades etc if you want to avoid any rain.
Another fairly robust trough will be swinging across the region,
though the GFS and European model (ecmwf) differ on details relating to placement
of surface low and timing. In either model's scenario, we'll
have scattered showers around. GFS is more robust in pushing
some more humid airmass into the region from the south, with
enough instability to fire off a few non-severe T-storms during
the afternoon. European model (ecmwf) has much less instability, but still a
signal that a rumble of thunder can't be ruled out. Have gone
ahead and placed some slight chance probability of thunderstorms
in the forecast. Stuck with the blended approach for high
temperatures, with highs generally around normal (upper 60s to
lower 70s). Not great confidence on that aspect at this point.
Aviation /00z Tuesday through Saturday/...
through 00z Wednesday...challenging forecast in the 00z-12z time
frame with a mix of VFR/MVFR/IFR dependent on terminal and
varying time frames. Any lingering light shower activity to end
at eastern terminals by 02z with overall cig/vsby forecast
remaining low to moderate. Main idea is for valley locales of
kbtv/kpbg/kmss to generally trends toward VFR after some brief
MVFR early. For krut/kmpv/kslk trapped lower moisture will
likely keep areas of MVFR/IFR around a good portion of the night
before a trend toward VFR after 12z Wed. Some patchy MVFR/IFR
visibilities in the 1-2sm range also possible at these terminals
in the 08-12z time frame, with highest confidence at kmpv. Krut
cigs may lift earlier than current forecast. Winds generally
light south to southwesterly through the period, though
southeasterly gap flow to 10 kts at krut expected overnight.
Tuesday: VFR. Patchy morning fog.
Tuesday night: VFR. No sig weather.
Wednesday: VFR. Slight chance rain showers.
Wednesday night: VFR. Chance rain showers.
Thursday: VFR. Breezy. Likely rain showers.
Thursday night: VFR/MVFR. Likely rain.
Friday: MVFR. Likely rain.
as of 145pm Monday...southerly winds 15-25kts across Lake Champlain
will diminish and shift into the southwest early this evening.
The lake Wind Advisory will likely be cancelled late this
afternoon or early this evening.