Area forecast discussion
National Weather Service Burlington Vermont
1042 am EDT Friday may 24 2013
Synopsis...
a developing coastal system will bring additional rainfall and much
cooler temperatures to the north country this weekend. The additional
rainfall will cause significant rises on local waterways...with some
river flooding possible. Temperatures will be 15 to 20 degrees below
normal with some high elevation accumulating snowfall possible on
Saturday into Sunday. A warming and drying trend will occur by early
next week.
&&
Near term /until 6 PM this evening/...
as of 1042 am EDT Friday...minor updates to T/TD/pops as we head
into the late morning and afternoon hours. Temperatures generally
steady from here on out...generally holding in the 40s. Did opt to
raise probability of precipitation to nearly categorical in all areas (i.E. > 80%) per
latest radar coverage and hi-res model 0-12 hour output. Rain will
certainly not be as excessive as we saw yesterday in
spots...nonetheless it will be a raw...breezy and wet day for most
locales with an additional 0.10 to 0.50 inches of quantitative precipitation forecast possible by
day's end. Prior discussion follows...
Updated forecast to drop temperatures by 3 to 5 degrees across the slv and 1 to
3 degrees over the cpv. Also...made several tweaks to probability of precipitation based
on current radar data. The combination of low level cold air advection and upslope follow will
produce occasional rain across most of the forecast area today. Highest
probability of precipitation/quantitative precipitation forecast will be along the western slopes and northern dacks.
Additional rainfall amounts today will be between 0.10 and 0.40". Temperatures
will remain near steady in the l/M 40s mountains to near 50 slv/cpv.
Also...gusty northwest winds of 10 to 20 miles per hour can be
anticipated...adding an extra chill to the air.
A long duration rain with some accumulating mountain
snow event expected today through Sunday. Additional rainfall
amounts of 1 to 2 inches with localized 3 inches can be expected
by Sunday...with sharp rises expected on area waterways.
Water vapor shows a complex digging middle/upper level trough across the eastern
Continental U.S. This morning...as both northern/southern stream energy are
phasing. Noticing large plume of sub-tropical moisture advecting into
the middle Atlantic/southern New England as trough becms negatively tilted. Also...developing
middle/upper level dry slot is rotating across the southeast Continental U.S.. these features
combined with a dual 25h jet structure...similar to a winter-time
synoptic system will produce a complex weather scenario across our forecast area this weekend.
Meanwhile...at the surface a sharp cold front is moving across our
County Warning Area...helping to enhance the low level thermal gradient. This will eventually
push toward the coast...with coastal low pressure moving along the boundary.
Forecast challenge today will be areal coverage/intensity of
rainfall...associated with tight thermal gradient and Post-frontal
moisture. Expect occasional widespread light rain today...as forecast area is between
systems. GFS/NAM show best 7h fgen forcing and associated q-vector
convergence developing across our County Warning Area after 18z. In addition...warm moist
conveyor belt should be east of our County Warning Area...along with heaviest quantitative precipitation forecast. Will
still mention Cat probability of precipitation...but quantitative precipitation forecast amounts should range between 0.10 and
0.40" today. Temperatures today with breezy north winds will struggle in the
u30s to m40s mountains to l50s cpv/slv to near 60f at vsf.
&&
Short term /6 PM this evening through Sunday/...
as of 430 am EDT Friday...potent northern stream short wave energy will round middle/upper
level trough base and develop surface low pressure near Cape Cod by 00z this
evening. This system will quickly becm vertically and negatively
stacked as 7h/500 mb circulations deepen and become closed. The
cyclonic circulation will advect plenty of deep Atlantic moisture
back into our forecast area between 850 and 700mb...while a very cold air mass is
pulled into the system from central Canada. This very tight
thermal gradient...combined with favorable rrq of 25h jet will
produce a widespread moderate to heavy precipitation event across most of
our County Warning Area...especially eastern dacks into Vermont. This synoptic scale
coastal system will have a very winter-time thermal/jet structure.
Both GFS/European model (ecmwf) in good agreement with placement of backside
deformation band of enhanced 850 to 500mb relative humidity and associated deep
layer Omega from 18z today through 00z Sunday. This band of favorable
lift/moisture moves very little over the next 12 to 24 hours...which
will produce a long duration precipitation event. The fgen forcing and
deformation zones northwest of the closed system is very similar to what
you see in a winter time coastal...along with favorable backside
upslope parameters.
Rainfall...expect storm total quantitative precipitation forecast amounts to range from 0.25 to 0.75"
for the slv/western dacks to 1.0 to 2.0" cpv/eastern dacks to downslope
regions of the nek to 1.50 to 2... localized higher amounts mountains
of north/central Vermont by 00z Monday. These anticipated rainfall amounts
over a 24 to 48 period will cause additional rises on local
waterways...with some additional river and low lying flooding possible.
Note...this will be a long duration event with steady/moderate rainfall
rates...which is different from Thursday event. Still some uncertainty
with regards to timing and placement of heaviest quantitative precipitation forecast...therefore no
Flood Watch will be issued at this time.
Snowfall...still having a hard time believing accumulating
snowfall is possible across the mountains above 2500 feet on Sat into
Sunday. GFS/NAM BUFKIT soundings and European model (ecmwf) profiles have indicated
this potential for the past couple of days...as colder air is
wrapped into the system. Prognosticated 850 mb temperatures drop blw 0c by
tonight...with 1000 to 500mb thickness values blw 540dam. This
combined with some cooling from strong dynamics will help to produce
some accumulating snowfall across the dacks and Green Mountains above
2500ft. Will try to show in grids...but impacts will be minimal given
the high snow levels. Thinking a dusting to several inches of
heavy wet snow will be possible...with 6 inches or so for the
summits from Montana Mansfield to Jay Peak and across parts of the
eastern dacks by Sunday. Best chance for accumulating snowfall will be
Saturday night/Sunday morning.
Temps/winds...prognosticated 850 mb temperatures near 0c...combined with plenty of
clouds/precipitation and a north wind will result in highs only in the M/u 30s
mountains to M/u 40s slv/cpv on Sat/Sunday. The trends have been cooler and
cooler in the guidance. Expect little change in temperatures on Sat
night...with precip/clouds...thinking l/M 30s mountains to near 40f cpv/slv.
Expect a cold/blustery and wet day on Sunday for Marathon
activities in Burlington.
&&
Long term /Sunday night through Thursday/...
as of 430 am EDT Friday...upper low finally starts to lift northeast out
of the area Sunday night and Monday...thus bringing any
precipitation quickly to an end. Surface high pressure builds in
for a return to dry weather. Northwest flow aloft will exist early
next week for a gradual warming trend...but upper ridging builds
in by midweek and this will allow for a more noticeable warming
trend with above normal temperatures expected. Frontal boundary
lifts northeast into the area by midweek and combined with
increasing instability from the warmer and more moist air
mass...potential for some showers and thunderstorms will exist
Wednesday into Thursday.
&&
Aviation /15z Friday through Tuesday/...
through 12z Saturday...expect an extended period of MVFR and IFR
conditions across northern New York and Vermont due to low
ceilings and visibilities as well as light rain. There will also
be brief periods of LIFR ceilings as abundant low level moisture
will exist over the area. The wind will become northwest all areas
at speeds of 10 knots or less through much of the period.
Outlook 12z Saturday through Tuesday...the combination of a surface
boundary and associated low pressure will keep showers in the taf
sites through the weekend. These showers will produce periods of MVFR
ceilings/visibilities with some IFR conditions possible...especially at
night. In addition...breezy northwest winds will develop as surface
low pressure tracks into the Gulf of Maine on Sunday. A slow clearing
trend is anticipated on Monday...W/ VFR conditions Tuesday.
&&
Hydrology...
as of 430 am Friday...a long duration moderate to heavy rainfall event
is anticipated from today through Sunday...with significant rises
on large Stem rivers. Expect storm total quantitative precipitation forecast amounts to range from
0.25 to 0.75" for the slv/western dacks to 1.0 to 2.0" cpv/eastern
dacks to downslope regions of the nek to 1.50 to 2...
localized higher amounts mountains of north/central Vermont by 00z Monday.
This rainfall combined with recent rain from convection will produce
additional minor low lying flooding...with the potential for
several rivers to approach flood stage by Sunday. No Flood Watch
will be issued with this package...but if current trends continue
another Flood Watch maybe needed for the weekend.
&&
Btv watches/warnings/advisories...
Vermont...none.
New York...none.
&&
$$
Synopsis...Taber
near term...jmg/Taber
short term...Taber
long term...evenson
aviation...evenson/rjs
hydrology...btv
