Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Burlington Vermont
259 PM EDT Friday Aug 29 2014
surface high pressure translating eastward across New York and New
England will bring pleasant and dry weather conditions to the north
country today and tonight. Warmer and more humid weather will return
for the weekend as high pressure shifts east of the region and south
to southwest flow develops. Saturday should be precipitation free...
but showers and thunderstorms are likely Sunday afternoon as an
upper level disturbance moves into the region from the Great Lakes.
Some thunderstorms on Sunday are expected to produce brief heavy
downpours. Above normal temperatures...with valley highs
generally in the 80s...will continue through most of next week.
Near term /until 11 PM this evening/...
as of 1240 PM EDT Friday...the early overcast stratus that was
once trapped in the Champlain Valley this morning has since
evolved into fair weather cumulus. Expecting partly to mostly
sunny skies for the remainder of the day under these cumulus
clouds. No change to high temperatures of 70-75f.
Previous discussion from 954 am Friday...
high pressure will continue to crest over the region today
bringing a mostly sunny day to much of the area. Still have a low
stratus deck trapped under the subsidence inversion and northerly
flow in the Champlain Valley and along the western slopes of the
Green Mountains. This has been beginning to shrink over the past
1-2 hours. Also starting to see other areas mix out as indicated
by daytime cumulus forming in parts of eastern Vermont. Thus
expecting the Champlain Valley stratus to clear out around 16z
(noon). Latest rap/hrrr/4km NAM support this timing as well.
Temperatures warming up nicely in areas that are seeing sun...so
highs from 70 to 75 still look good.
Short term /11 PM this evening through Sunday/...
as of 329 am EDT Friday...surface high pressure remains in control
tonight will drifting eastward toward the Gulf of Maine after midnight.
As a result...should see some southerly return flow develop
late...especially in the Champlain Valley with channeling effects.
Winds generally around 10 miles per hour in the Champlain Valley...but with
steep lapse rates over Lake Champlain due to Warm Lake
waters...will see the development of 15-25 knots south winds by 09z
over the open waters of Lake Champlain. Models suggest increasing
relative humidity in 300-500mb layer...so will likely see an increase in cirrus
from west to east overnight. No precipitation is expected. Fog
will be most prevalent across the valleys of central and eastern Vermont
where winds just above the planetary boundary layer will be lightest and cirrus clouds
will arrive a bit later. Overnight lows will generally be in the
low-middle 50s...with a few upper 40s in the northern Adirondacks and far
South to southwesterly low-level flow will
be in place across the north country on Saturday...between surface
high pressure departing to the east and a weak surface low and middle-
level trough across the Great Lakes. Moderate low-to-mid level warm air advection
is prognosticated by 00z NAM/GFS...with 850mb thermal ridge of +15 to
+16c building into the region by 00z Sunday. Issue on temperature
will be clouds...as high 500-300mb relative humidity suggests filtered sunshine
through cirrus/altostratus layer. Adjusted a bit above MOS
consensus...with highs in the 80-83f for most valley
sections...but not quite as warm as it could be under full sun. S-SW
winds generally 10-15 miles per hour. With middle-level ridging still in place
and not much evidence for low- level convergence...it appears
Saturday will remain dry areawide...and will keep probability of precipitation below 15
percent. Dewpoints will slowly climb...likely reaching the upper
Weak middle-level height falls commence Saturday night as middle-level
shortwave trough progresses eastward across the Great Lakes. Should see
mostly cloudy skies...with a chance for a few showers developing
across northern New York...and perhaps far northern Vermont toward daybreak Sunday.
Clouds and continued light south winds Sat night...combined with
warmer air mass will result in very mild overnight lows...ranging
from the upper 50s across the CT River Valley...to 65-68f in the
Champlain and St. Lawrence valleys.
Sunday looks like the best chance for precipitation over the next
several days. Shortwave trough provides middle-level upward vertical velocity while weak
surface-based instability develops with daytime heating (sbcape
values of 500-1000 j/kg). Deep layer shear is not particularly
impressive...with surface-6km bulk shear of only 20-25 kts. Combined
with only weak instability with abundant cloud cover in place...not
anticipating any severe weather activity. That said...shower and
thunderstorm activity will gradually become more widespread toward
afternoon with peak heating. While not a "washout"...these showers
could be locally heavy...as precipitable water values approach 2.00" in both the NAM
and GFS. Have included heavy downpour mention in the hazardous
weather outlook and in the forecast grids. Definitely would be
good to have an indoor backup plan for any planned outings on
Sunday giving potential for brief heavy rainfall. High temperatures
will range from the upper 70s to lower 80s.
Long term /Sunday night through Thursday/...
as of 329 am EDT Friday...fairly active weather expected for the first
part of the extended forecast period...Sunday night through
Tuesday night...with drying taking place Wednesday and Thursday.
Cold front moves across the area Sunday night and thus going
forecast of likely showers and thunderstorms looks real good. Not
much air mass change behind the front on Monday and thus
sufficient instability should develop Monday afternoon for some
showers and storms to develop. A brief break in the convection
takes place later Monday night and Tuesday morning before another
cold front moves into the region Tuesday afternoon. Thus looking
at another chance of showers and thunderstorms through Tuesday
evening before drier westerly flow aloft develops over the area
Wednesday and Thursday. Thermal profile throughout the extended
suggests daytime highs will be at or a few degrees above seasonal
Aviation /19z Friday through Wednesday/...
through 18z Saturday...the earlier MVFR ceilings primarily in the
Champlain Valley have lifted. Have scattered-broken ceiling made up of
shallow daytime cumulus, but VFR with cloud bases generally
3500-4000ft. Anticipate VFR for the rest of the day with
light/variable surface winds. Should be VFR overnight as well.
Guidance indicates slk and mpv will both fog in with LIFR
conditions. Think the combination of relatively dry air near the
surface, increasing middle-level clouds (especially for northern ny)
and strengthening winds a few hundred feet above the surface will
preclude much in the way of fog. Can't totally rule it out in the
valleys of eastern Vermont, so that means mpv could be impacted.
At this point think it's just an MVFR situation from roughly
06-12z. On Saturday, VFR area wide with increasingly gusty
south/southwest winds, especially in the Champlain Valley. Could
see gusts over 20kt there, less elsewhere.
Outlook 12z Saturday through Wednesday...
18z Sat Onward...mainly VFR, though MVFR/brief IFR in scattered
showers and thunderstorms that will be most numerous Sunday and
perhaps again late Tuesday.
south winds will increase sharply on Lake Champlain toward
daybreak Saturday as the pressure gradient increases between high
pressure departing east of New England...and a developing low
pressure system across the central Great Lakes.
On Saturday south winds will increase from 10-20 knots in the morning to
15-25 knots during the afternoon. Waves over the broad portion of
Lake Champlain will build to 2 to 3 feet.
Saturday night winds will increase slightly higher as Champlain
Valley funnelling and nocturnal low level jet set up so winds may
nudge a bit higher 20 to just shy of 30 knots overnight. A few
waves over 4 feet may occur on the north half of the Open Lake.
There will be an increasing chance of showers after midnight.
Sunday south winds will taper off to 10 to 20 knots and waves will
subside to 1 to 3 feet. Looking for widespread showers and a chance