Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Burlington Vermont
739 PM EDT Thursday Aug 21 2014
a low pressure area over the Great Lakes will bring showers and a chance
of thunderstorms over northern New York tonight. There will be a
slight chance for showers on Friday. A ridge of high pressure will
build south from Canada Friday night and will remain over the
north country into early next week...with fair and dry weather
expected this weekend into early next week.
Near term /until 7 am Friday morning/...
as of 728 PM EDT Thursday...modest update as of early
evening...primarily to raise probability of precipitation across the South/West half of
the forecast area overnight. Latest hi-res/cam models show showery
activity continuing and/or blossoming across Essex County New York into
central/southern Vermont overnight as middle level shear zone and
diffluence slowly pivot across our area. As alluded to
earlier...lower probability of precipitation further north and east across NC/NE Vermont but
still some scattered activity here as well (case in pt showers across
Caledonia city currently). Rainfall rates in general coming down
and while some areas will still see a good soaker this
evening/overnight...with loss or lack of instability feel any
threat of flash flooding will be minimal. Temperatures remain on
track...generally bottoming out near seasonal late Summer norms in
the upper 50s to lower 60s. Prior discussion follows. Have a great
afternoon mesoanalysis depicts a surface low over the northern shore of
Lake Ontario...associated with a deamplifying upper low over the
northern Great Lakes. Both of these features will shift slowly
eastward...passing over the north country overnight tonight.
Regional radar this afternoon depicts scattered to numerous showers and
thunderstorms across St. Lawrence County...with additional
thunderstorms in central New York. Unstable air mass with laps convective available potential energy
around 2000 j/kg. Expect these to generally pivot northeastward
through early evening with a continuing threat of locally heavy
rainfall from St. Lawrence Valley and Adirondacks into southern Vermont.
I've continued enhanced wording for this general area through early
evening. Forecast quantitative precipitation forecast generally around an inch tapering sharply with
Later tonight into the overnight...rain coverage then becomes more
nebulous/spotty. Decreasing trend to probability of precipitation overnight to chance
across southern Vermont and slight chance elsewhere. Overcast
tonight with patchy areas of mist outside of any showers. Felt both
12z mav and met MOS were both too cold with lows given cloud
cover...opting to keep temperatures in the upper 50s to Lower/Middle 60s.
Short term /7 am Friday morning through Saturday night/...
as of 335 PM EDT Thursday...upper low then continues to shift
eastward into the coastal waters of New England Friday. NAM and
GFS depict a weak shortwave trough rotating around and passing
through the north country during peak heating. Therefore...expect
an increase in diurnal showers Friday with chance probability of precipitation forecast
during the afternoon hours...trending drier by evening. Mostly
cloudy/overcast trends to partly/mostly cloudy by afternoon from south
to north with more significant clearing expected after midnight.
Highs Friday are coolest in southern Vermont and Essex County New York and
warmest near the Canadian border...upper 60s to middle 70s with lows
in the upper 40s to upper 50s.
High pressure continues to build in from the north with dry weather
for Saturday. While clearing skies and warmer temperatures are expected
for most of the north country...easterly trajectories and saturated
NAM/btv-4 low-level relative humidity forecasts suggest potential for marine
layer/stratus in eastern Vermont. With 850 mb temperatures from +9c in eastern
Vermont to around +11c elsewhere...looking at highs only around 70 in
eastern Vermont to middle/upper 70s for the rest of the north country. Good
radiational cooling Saturday night allows lows to range from upper
40s to upper 50s.
Long term /Sunday through Thursday/...
as of 335 PM EDT Thursday...no big changes to previous forecasts
for the time. Much of the period will be dominated by strong
ridging aloft and at the surface (aka: ridgosaurous). 12z suite of
model guidance remains fairly similar to each other and prior
runs. Noted that the 12z European model (ecmwf) trended toward the GFS for the
scenario on Wednesday and is now showing a stronger ridge into middle
week. Models diverge for Thursday, with Euro showing a faster/more
amplified trough breaking down the ridge. The differences continue
for beyond the official period i'm forecasting for, but an early
look at guidance for Labor Day weekend seems to indicate it will
feature seasonable temperatures and mostly dry conditions. We'll
see. As for the daily specifics, just read on:
Sunday...surface high just a little north of here so continued
light easterly flow. 850 miles per hour temperatures rise to 11-13c (warmest
in western half), which will support highs in the middle 70s east to
low 80s west. Strong subsidence will keep any clouds to a minimum.
At night, clear and calm so look for patchy valley fog.
Monday...surface high overhead, so hardly a breeze. 850mb
temperatures up another degree to about 12-14c, so add about 1-3f
for the high temperature from Sunday -- lower 80s pretty much area wide.
Monday night, more clear/calm/patchy fog.
Tuesday...high south of here, so a bit of a west/southwest flow
gets established. 850mb temperatures up another degree. With west
flow, this should also help with some downslope into the Champlain
Valley so i'd expect middle 80s to be a solid bet in the valleys. The
Euro runs several degrees cooler, so have leaned more toward the
warmer GFS temperatures. Overnight will feature clear/calm/patchy fog.
Wednesday...high south of here, so the area is solidly in
west/southwest flow. 850mb temperatures pushing 15-16c which
support highs in the middle to upper 80s in spots. Add in some
downsloping off the Adirondacks, and I think parts of the
Champlain Valley could make a run at 90f. I did increase
temperatures several degrees over the blend of guidance which was
biased a bit cool due to the Euro and climatology in the MOS. That blend
gave ME 81f for btv. Mex guidance was 83f. I went 85f. We'll see
how future guidance GOES and if the trend to be warmer is needed.
Wednesday night again clear/calm/patchy fog.
Thursday...this is when it looks like the pattern will begin to
break down. As mentioned earlier, the Euro is more aggressive with
a shortwave. At this point, best course is to take a blend. Have
painted in chance level of probability of precipitation for scattered thunderstorms.
Temperatures may again be very warm if the GFS is right. Or cooler
if the Euro is right. Stuck with the blend. Which still features
lots of lower 80s.
Aviation /00z Friday through Tuesday/...
through 00z Saturday...essentially looking at VFR and MVFR
conditions through about 12z with plenty of clouds...showers...and
fog lingering over the area tonight through the early morning
hours. After 12z any precipitation comes to an end and
visibilities improve with broken ceilings above 3000 feet. Winds
will have an easterly component to them tonight...but at speeds
generally under 10 knots. The exception to this will be at krut
where southeast winds will gust to 20 knots through about 06z.
Outlook 00z Saturday through Tuesday...VFR with the following
exceptions -- brief MVFR in any showers through 06z Saturday.
Localized IFR in late night fog (mainly an issue for mpv and slk).