Area Forecast Discussion
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FXUS61 KBTV 211154
Area Forecast Discussion
National Weather Service Burlington VT
754 AM EDT Thu Jul 21 2016
Ridge of surface high pressure will remain over the North Country
today, providing the region with warm, sunny, and dry weather.
Periods of showers and thunderstorms are expected after midnight
tonight through Saturday as a frontal system moves slowly through
the region. Some of the storms will have the potential to become
strong or severe. Another round of thunderstorms is possible on
.NEAR TERM /THROUGH TONIGHT/...
As of 754 AM EDT Thursday...Any early morning fog will burn off
in the next hour, then it will be a very nice, warm, and sunny
late July day with high temps 85 to 90.
Large ridge of surface high pressure will remain over the region
right through Thursday with dry weather. Some high level cirrus
clouds are possible from time to time with NW flow aloft from
thunderstorms up near James Bay early this morning.
Highs today will be warmer than normal with ample sunshine and
warming aloft as 850 temps rise to 16 to 18C. Expecting highs 85
to 90. Winds will become south to southwest and some gusts to 20
knots in the St. Lawrence Valley.
Late Thursday night into early Friday morning A surface front in
Ontario/Northern Great Lakes will be slowly approaching with
increasing chance of showers and thunderstorms, mainly after
Deep low-level layer moisture with PWATS >1.5 inches advecting
ahead of front. There will be elevated instability with CAPE over
1000 J/kg in the St. Lawrence valley by 09-12Z Friday. The boundary
layer should be stable with instability resulting from lifting
parcels from the 900 mb level. There is also the remnants of an
elevated mixed layer with 700-500mb lapse rates around 7C/km. Lift
to be provided by a shortwave trough and associated QG forcing and
divergence ahead of exit region of 300mb jet. They may be a
mesoscale convective complex in northern Great Lakes/Ontario
moving into the area during late Thursday night/early Friday
morning as a result. Not really looking for much of a severe
threat tonight but can`t rule out the possibility of a few
stronger thunderstorm cells toward Friday morning.
South to southwest winds will continue all night tonight and as a
result low temps will only be in the mid 60s to lower 70s.
.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
As of 421 AM EDT Thursday...As typical with a "Ring of Fire"
situation (areas of convection developing along the fringe of a
large upper level "heat" ridge, models struggle with the subtle
but important details. Case in point, even the 3 to 6 hour
forecast output from the 00z GFS and NAM do not depict the large
clusters of convection located from Minnesota across into
northwest Quebec. Thus any reliance on the exact details from
models, especially the ones that explicitly forecast convection,
will likely lead you astray. It also makes the forecast more
difficult, and so this is one with low confidence, even though we
are talking 24 to 36 hour forecast. Best I can do is work with the
clues that the models give and merge that in with what we can see
via satellite and radar and make an educated guess. So with that,
here is what I envision.
Friday: still see lots of ingredients that point toward the
potential for significant convection. Source airmass at low levels
by 18z was what was out across Illinois on Wednesday afternoon
(temperatures well into the 80s, with dewpoints nearing 70). Airmass
at 10,000ft or so will be what was in place across North Dakota into
southern Manitoba (where we saw a cluster of strong/severe t-storms
form last evening). This is shown in the models by a well defined
EML (elevated mixed layer), high levels of surface instability (GFS
has CAPE around 2000 J/kg, while the NAM has some seemingly a bit
too high 3000 J/kg CAPE), robust shear suggesting storm
organization. Still evidence that after an early morning round of
showers/t-storms, mid levels will dry out considerably. Discussion
with my mid-shift cohort leads us to still believe that we`ll still
see some strong/severe t-storms develop, however they will be
scattered in nature. Away from these storms, it will be a
sunny/hot/humid day. The EML and instability maintains itself well
into Friday evening. So it`s not out of the question that we`ll see
repeated rounds of scattered strong thunderstorms develop anywhere
and continue into the late evening.
Primary threats still appear to be strong winds and hail. SPC
continues to paint the area from Vermont and points east in a Slight
Risk, with a marginal risk across northern NY. The differences
between these levels of risk is pretty small. Bottom line message
is that we will have a threat of isolated damaging thunderstorms.
For those that want to geek out a little bit more, check out the
CIPS Analogs webpage. Based on the top 15 analogs, about half
indicate some sort of long lived convective cluster had developed,
producing a long swath of damage. Most of those were across the Ohio
Valley (with nothing up here -- which is a scenario that could
happen!), however a couple of past similar events showed damage in
New England. What past history indicates is that no scenario is off
Temperatures Friday are a bit tricky. If it`s mostly sunny with just
a few storms, we could make a run at 90F. If we have a good deal of
clouds and more widespread showers/storms around, it won`t be as
warm. I leaned on the warm side, but lower than the warmest of the
guidance. For PoPs, I went with a broadbrush approach, even though
much of the time it likely won`t be raining in any one location. So
60-70% chance does not equate to how much of the day, but rather
maximum coverage of the t-storms throughout the day.
Saturday: We are not out of the woods with regard to t-storms.
Guidance is fairly consistent that we`ll still have some instability
about with another day of warm temperatures and high dewpoints. At
upper levels, we`ll have a shortwave trough drop down from the
northwest, probably during the first half of the day. Looks like the
best chances for t-storms will be across Vermont, with chances
lowering by later in the afternoon. SPC has the area in a "Marginal
Risk" for severe thunderstorms. Agree with that assessment as the
various ingredients are just not as well developed as they will be
for Friday, but just enough that we may see one or two strong storms
.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
As of 421 AM EDT Thursday...Given the focus on the Friday
potential for severe weather, I stuck with a straight model blend
for the remaining days. The only day that could be problematic
still appears to be Monday. Both the GFS and ECMWF continue to
show a potent upper trough and associated surface cold front
zipping through the region at some point during the day. The 00z
run is a little slower (especially the GFS) and has the system
passing through more Monday night. In any event, some indication
that we could see another round of strong (to perhaps isolated
severe) thunderstorms develop. Still perhaps some residual showers
around on Tuesday as some instability will be in place with
troughiness aloft. Wednesday and Thursday look to be very nice
days. Stayed with the blend for the temperatures, so have a
forecast of more-or-less near normal conditions.
.AVIATION /12Z Thursday THROUGH Monday/...
Through 12z Friday...VFR with mainly SKC or SCT250 skies. FG at
MPV should be gone around 12z but could linger til 13Z.
Clouds will gradually thicken tonight with cirrus streaming in
from the NW and may see some mid clouds arriving at KMSS before
06Z. Have showers in the vicinity at that time and there is also
a low chance of thunderstorm moving SE from Canada. Cigs/Vsby lower
toward MVFR in showers and possible thunderstorms by 12Z Friday
especially BTV west to SLK/MSS. Have left out TS for now as
coverage and location not yet known.
Light/calm winds will become light south to southwest today. KMSS
could see some SW gusts to 20 knots during afternoon in tighter
pressure gradient and channeling up the St. Lawrence Valley.
Expecting winds to continue tonight as gradient strengthens
everywhere. Have also included gusts over 20 kts at BTV for
morning push and gradient increases and wind channeling starts in
the Champlain valley.
Outlook 12z Friday through Monday...
Fri-Sat...VFR, with scattered brief local MVFR/IFR in showers and
thunderstorms. Some thunderstorms may contain strong winds and
Sun...Mainly VFR under weak high pressure.
Mon...Scattered MVFR local IFR in showers and thunderstorms.