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Area forecast discussion 
National Weather Service Burlington Vermont 
958 am EDT Friday Jul 18 2008 


Synopsis... 
the potential for strong to severe thunderstorms will exist 
across the north country...especially areas near the Canadian 
border later this afternoon and evening. These thunderstorms will 
be capable of producing frequent lightning...damaging 
winds...hail...and localized very heavy rainfall this afternoon. 
Meanwhile...weak high pressure will build into the region later 
tonight and this will bring a relatively dry day on Saturday and 
into the first part of Sunday. However...an upper level trough of 
low pressure will begin to develop over New England Sunday 
afternoon and will be slow to move east. Thus expect an unsettled 
period Sunday afternoon through early next week. Temperatures will 
be at or slightly above normal today through Sunday. 


&& 


Near term /through tonight/... 
as of 1000 am EDT Friday...forecast challenge today will be areal 
coverage/intensity of storms...followed by amount of clring and 
impacts on temperatures/instability parameters this afternoon. Radar shows first 
line of storms associated with middle/upper level vorticity moving across 
stationary boundary over nny into central/northern Vermont at this time. This 
boundary...along with several outflow boundaries from this 
mornings convection and terrain...will serve as a low level focus for more 
storms this afternoon. In addition...strong surface heating differential 
boundary will develop across central/southern Vermont this afternoon...helping 
to enhance more lift. Given...plenty of boundaries across our 
County Warning Area...next question will be amount of surface heating and development of 
surface based instability parameters. Visible satellite pic shows clring 
developing across slv and western dacks at this time...and laps data shows 
surface based cape values already climbing above 1000 j/kg. Feel this 
clring will expand east...based on weak ribbon of subsidence/dry air 
aloft behind first short wave energy. This clring will allow temperatures to 
quickly warm into the 80s across central/southern Vermont and most of 
nny...creating surface based cape values between 1000-1500 j/kg. In 
addition to surface instability the low to middle level wind fields are 
very impressive across our central/northern County Warning Area this afternoon. Soundings 
show 700mb winds near 50 knts...with 800mb winds around 40 to 45 
knts. These strong wind profiles combined with middle level drying and 
steep surface to 800mb lapse rates support the potential for strong 
and damaging thunderstorm wind gusts this afternoon. Furthermore...low level 
boundaries will enhance the 0-1km shear parameters and support some 
organized cells...with potential for large hail...given wet bulb 
zero hghts around 10kft. Will mention large hail/damaging winds in 
zones...along with heavy rain. Final adjustment to zones will be to 
increase clouds across our central/northern County Warning Area...and decrease temperatures 
by several degrees based on expected cloud cover. 


Water vapor imagery shows shortwave trough moving out of southeast 
Ontario province and into southwest Quebec early this morning. 
This is enhancing convection across southeast Ontario and with 
westerly flow...expect this activity to move across northern New 
York and northern Vermont through the morning hours. Once this 
moves out of the area...there should be enough time for the air 
mass to recover. Expect temperatures in the 80s along with low to 
middle 60 dew points to create sufficient instability to generate 
some thunderstorms. Not much in the way of steep lapse rates in 
the middle levels...thus looking at maybe 500-1000 j/kg. Dew points 
in forecast soundings are way too high...lower 70s...and thus feel 
that type of instability will never really materialize. A moderate 
to strong westerly flow aloft will exist across the area creating 
favorable deep layer shear across the north country. This combined 
with sufficient instability will lead to the development of 
organized storms later this afternoon and evening. Unidirectional 
shear suggests strong gusty winds from linear convection will be 
the main threat. In addition...higher precipitable water air will 
lead to heavy downpours as well. Will use enhanced wording in the 
zones to highlight potential for strong to severe storms. Threat 
of convection should end around midnight. 


&& 


Short term /Saturday through Sunday/... 
as of 438 am EDT Friday...northwest flow aloft develops on 
Saturday which will help dry the area out and limit potential for 
convection. 850 mb temperatures will not drop off too much...thus 
still looking at high temperatures generally in the 80s. The much 
anticipated development of an upper level trough over the 
northeast United States looks like it will start Sunday 
afternoon. As a result...an increased threat for showers and 
thunderstorms will exist as deeper moisture moves into the region 
and dynamic support increases. Will mention a chance of showers 
and storms for Sunday with temperatures only a couple of degrees 
cooler on Sunday versus Saturday. 


&& 


Long term /Sunday night through Thursday/... 
as of 438 am EDT Friday...continue to expect unsettled weather 
for the extended portion of the forecast. Slow moving low 
pressure system which affected the area Sunday will stick around 
through Monday. Upper level flow becomes more progressive on 
Sunday night and trough takes shape over the Great Lakes on Monday. 
On Monday evening surface low moves off the coast of New England 
and upper level trough begins to push east. Some residual moisture 
remains across the area on Tuesday and upper level low and 
associated trough become more organized over the Great Lakes at 
this time. Low becomes very vertically stacked and gains strength 
on Tuesday night and approaches the area on Wednesday...with lots 
of moisture streams into the area. The surface low will be 
centered over our County Warning Area on Wednesday night and be very slow to move 
away from the area. Low begins to depart New England on Thursday 
and wrap around moisture will affect the area...upper level low 
moves away from the area as well. As low moves east chance for 
showers will continue across the area...high pressure will build 
into the area on Friday. Through the extended there will 
generally be a chance for showers during the day...showers and 
thunderstorms in the afternoon and evening. 


&& 


Aviation /14z Friday through Tuesday/... 
through 12z Saturday...rain showers and thunderstorms and rain moving across the area this 
morning will continue to move through the area...then there will 
be a break in precipitation before another round of rain showers and thunderstorms and rain this 
afternoon. Gusty winds are possible in and around these 
storms...along with lowered ceilings and visibilities. Chances 
for rain showers/thunderstorms and rain increase during the day today...with best chances 
being between 14z-20z as shortwave moves across area...especially in our 
northern zones. Possible MVFR conditions in any shra/tsra. Light 
winds tonight will become S-SW around 10 kts Friday. Have also 
included haze for humid conditions across the area. Have included 
br for mpv...mss and slk...our typically fog prone areas. Any 
terminal that gets a soaking today will be more likely to produce 
br and fog overnight. 


12z Saturday through Tuesday...unsettled weather will continue 
through the period. Impulses will ride along the westerly flow 
aloft through Sunday...bringing scattered showers/thunderstorms to the 
region...especially to northern sections due proximity with 
lingering boundary near the international border. Threat of more 
showers/thunderstorms continues Monday into Tuesday as deep upper level 
trough develops over grtlks/northeast. Prevailing conditions for 
much of the period will be VFR/MVFR...but IFR is possible in any 
precipitation. 


&& 


Equipment... 
the Massena ASOS will be out of service until further notice. 


&& 


Btv watches/warnings/advisories... 
Vermont...none. 
New York...none. 


&& 


$$ 
Synopsis...evenson 
near term...Taber 
short term...evenson 
long term...Jan 
aviation...Jan 
equipment... 

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