Area Forecast Discussion
Issued by NWS Burlington, VT

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000 FXUS61 KBTV 260203 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 1003 PM EDT Tue Jul 25 2017 .SYNOPSIS... Upper level low pressure will slowly pull east of the area this afternoon, allowing skies to clear. After high pressure brings a short window of fair and dry weather on Wednesday, shower and thunder chances return by Thursday into Thursday evening as a weak front crosses the area. Dry conditions then return by next weekend as surface high pressure builds across the northeast. && .NEAR TERM /THROUGH WEDNESDAY NIGHT/... As of 1001 PM EDT Tuesday...Just adding to below...made a small tweak to KSLK for 41 vs. 39 for a low with low clouds precariously close to them and expected fog development. Hrly obs match with this change I made 3 hours ago, thus no changes to current fcst. Trends are occurring as expected, gradual drying and diurnal clouds dissipating. Right now...just a wait/see on if these current/expected trends continue which we believe will and then its waiting for fog to develop overnight. Afternoon discussion...The sunshine is coming, I promise. The thick deck of strato- cumulus clouds are slowly pushing southeast as well as eroding due to drier air from the northwest and subsidence associated with a building ridge. Latest GOES-16 satellite visible imagery shows a lot of breaks in the clouds across northern sections of VT and NY. These trends will continue such that the clearing skies move from north to south. Most of the region should be clear before sunset, the exception perhaps in south-central Vermont where some clouds will linger latest. As the sun comes out, temperatures will pop up a few degrees rather quickly, thanks to strong summer sunshine. Radar not showing any precipitation of consequence, so have taken out any mention of showers for the remainder of the day. Still looking at a high pressure, light wind and mostly clear skies type of night. Given recent rains and wet soil, that strongly suggests the development of locally dense fog in normally fog prone valleys, and even a light fog in many other areas. Even the hi-res models, such as the 3km NAM, seems to have quite a realistic projection of the fog areas in it`s visibility fields. With a radiational cooling night, temperatures will vary quite a bit from location to location, with 40s in the cooler locations to 50s in the lower elevations. Still an argument going on between the NAM MET MOS guidance and GFS MAV MOS guidance with respect to how cold it will get in Saranac Lake. NAM says 37. GFS says 47. I`m confident the final result will be in between those two values, and think perhaps a little closer to the NAM than GFS. It will ultimately depend on when the fog forms there. The earlier the fog, the slower the temperature drop will be. Wednesday still looking to be a great outdoor activity type of day. Plenty of sunshine, though the next weather maker will be starting to spread some clouds in late in the day across northern NY. With 850mb temperatures about +12c, that should support high temperatures topping out around +27c (about 80F) in the lower elevations. Wednesday night will start dry, but as trough aloft approaches the region, we`ll see showers develop from west to east -- especially across the northern half of the region. Not very strong dynamics, nor any instability to speak of, so at this point it looks like run of the mill showers. Rainfall totals should be mostly less than 1/4" prior to daybreak Thursday. Have PoPs ramp up into the 55-65% chance range after midnight across the north, and 35-50% for the southern half of the area. The clouds and precipitation will keep temperatures much more mild, ranging from the upper 50s to lower 60s. && .SHORT TERM /THURSDAY THROUGH FRIDAY/... As of 324 PM EDT Tuesday...A digging mid/upper-level trough with associated pre-frontal energy will enter the St. Lawrence valley early Thursday morning. This leading energy will move west to east across the CWA Thursday morning helping to initiate some showers across northern NY and northern VT. Instability chances will be low in the morning increasing slightly during the afternoon as some clearing is expected after 12z. Models suggest CAPE values between 600-1000 J/kg and 0-6km shear values around 35kt. This marginal instability/shear, coupled with a 30kt 850mb jet and 85kt 250mb jet, may lead to some isolated thunderstorm development so cant rule out chance of thunder Thursday afternoon, however the best dynamics and moisture will be to southeast of CWA during this period. Temperatures on Thursday will be in upper 70s with lows around 60. As the main front moves east, drier air is expected overnight Thursday into Friday with PoP chances diminishing as CAA presides over area. Widespread fog is not expected Thursday night/Friday morning as N/NW winds around 5-10mph should keep the surface well enough mixed, however isolated fog may be possible in some sheltered valley locations. Friday remains dry as surface ridge builds in. Temperatures should be just below normal with highs in the mid 70s. Latest 12z trends have been to keep closed 7h/5h circulation and associated surface low pres to our south...resulting in dry conditions for Friday into Saturday. Still some uncertainty on exact track of closed system and if some showers reach our southern/central cwa late Friday. && .LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/... As of 324 PM EDT Tuesday...As the upper-level ridge continues to build over area, dry conditions should persist through the weekend. Temperatures should be near normal with highs in the upper 70s to around 80 through Sunday. High pressure will move east Monday, the chance for showers increases as conditions become more unsettled by Tuesday. Westerly flow aloft combined with southerly winds in the llvls will push warmer and increase moisture into our fa. Have mention chc pops Tuesday with isolated afternoon thunderstorms possible...as GFS indicates sfc based cape values up to 1000 j/kg over the slv. Still plenty of uncertainty on timing of boundary and magnitude of instability out 7 days. && .AVIATION /02Z WEDNESDAY THROUGH SUNDAY/... Through 00Z Thursday...Clearing skies, building high pressure and calm winds will make for good radiational cooling night allowing widespread fog to develop. VFR conditions this evening are expected to deteriorate to MVFR at 03Z/KSLK...07Z/KBTV, KMPV and KMSS...possibly 08Z/KPBG. Drainage flow expected to develop at KRUT hindering widespread fog, but cannot rule out a brief period of lighter winds during the early morning hours and MVFR conditions developing. High confidence that KMPV and KSLK will further deteriorate to IFR-VLIFR around 06Z through 13Z. KMSS may also see IFR vsbys as this terminal is already clear and will have more time to radiate. Fog that forms over the Winooski River could drift into BTV airfield. Expect a very light east wind at BTV...advecting the fog over the airport mainly after 07Z. With continued high pressure, expect fog to erode between 1130-1330Z. VFR conditions will prevail throughout the day Wednesday. Outlook... Wednesday Night: VFR. Chance SHRA. Thursday: VFR/MVFR. Likely SHRA...Slight Chance TSRA. Thursday Night: VFR. Chance SHRA. Friday: VFR. Slight Chance SHRA. Friday Night: VFR. NO SIG WX. Saturday: VFR. NO SIG WX. Saturday Night: VFR. NO SIG WX. Sunday: VFR. NO SIG WX. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...JMG NEAR TERM...Nash/SLW SHORT TERM...LaRocca LONG TERM...LaRocca AVIATION...KGM

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