Area Forecast Discussion
Issued by NWS Burlington, VT

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861 FXUS61 KBTV 200825 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 425 AM EDT Wed Jul 20 2016 .SYNOPSIS... Ridge of surface high pressure will remain over the North Country through Thursday, providing the region with sunny and dry weather. Temperatures will warm above seasonal normals again towards the end of the week. Afternoon showers and thunderstorms are expected on Friday. && .NEAR TERM /THROUGH TONIGHT/...
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As of 425 AM EDT Wednesday...little significant change with this update. Already down near 40 in parts of the NEK and Adirondacks. Dew points falling with with the temps and there may be some patchy fog around but doesn`t look too widespread. Any fog that forms will lift following sunrise, and another sunny and dry day is expected today. There may be just some scattered fair weather cumulus clouds around today, especially around the mountains and NEK but no precipitation is expected. Prevailing flow is northwest aloft with surface high pressure off to our south and west today. 850 temps rise to to 10-13 deg C by 21z so expect surface temperatures will rise quickly and reach upper 70s to lower 80s which is just about normal for late July. Another mainly clear night is expected tonight as flow aloft becomes more westerly and surface high pressure drifts to the mid-Atlantic region. Some warm air advection starts aloft so some high clouds may show up after midnight. Some patchy fog is possible but depends on the clouds so left it out for now. Low temps not as cool as this morning mainly in the 50s with perhaps a few cold spots in the upper 40s such as SLK.
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&& .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
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As of 425 AM EDT Wednesday...0z guidance suite did not offer too much in the way of changes, though there are differences between models on some of the specific details. for thursday, NAM was more aggressive with afternoon convection across northern NY and have discounted that aspect. for Friday, I stuck with an overall blend of models. So details are still on the fuzzy side given the averaging method of these differences. Some of the forecast thoughts are as follows: Thursday: surface high moves off to the east, so a low level south/southwest flow develops. mid level flow is still from the west/northwest, however some of the fringes of the intense heat found across the Central US will be leaking into the region on that westerly flow. 925mb temperatures will rise to around 22C & 850mb temperatures to about 16C, and given a good deal of sun, we`ll easily be able to develop a deep mixed layer. Rough rule of thumb in these situations is to add 10C to the 925mb temperature and 15C to the 850mb temperature to estimate the high temperature in the valleys. Thus the math gives us 31-32C (or 88-90F), which is more for the Champlain & St Lawrence Valleys. Slightly cooler elsewhere due to elevation. Despite the warmth, we should see mainly sunny skies thanks to the warm layer aloft/inversion. However by late in the day, guidance indicates an elevated mixed layer (EML) advecting in from the west. Could mean an isolated shower/storm across northern NY, though chances are generally less than 20%. The NAM is more aggressive with this feature and thus develops more scattered late afternoon t-storms across the western half of the forecast area. As mentioned above, have discounted this. Thursday Night: EML and associated instability aloft moves across the area. We should see scattered showers and isolated thunderstorms develop because of this. It will be turning more humid, and with all of the clouds, lows will not drop much -- mid/upper 60s. Friday: Interesting day. Still looking like we could see some strong or isolated(?) severe t-storms. However, the various ingredients don`t quite come together all at the same time. Models also have timing differences between them which makes the confidence a little lower. Best guess at this point is we`ll have surface instability (temperatures well into the 80s, perhaps 90F again)resulting in CAPE values pushing 2000 J/kg. We`ll also have increasing shear aloft of 35-45kts which is good for storm organization potential. We`ll have still a little bit of an EML around. We`ll also have a good amount of dry air aloft, which will both act to reduce the overall coverage of convection but to enhance the downburst potential. We`ll also be having a shortwave skip through at some point during the day. Model soundings also indicate high equilibrium levels and semi-low wet bulb zero (10-11k ft) levels which suggest a hail threat. What I`m thinking right now is that we`ll see scattered thunderstorms develop by late morning or early afternoon. Although there won`t be widespread coverage, those that do form could be strong. NAM indicates scattered convection will continue well into the evening as well. At this point kept PoPs in the 55-65% range. Did not include any enhanced characteristics to the weather at this point. However, Friday is a "Stay Tuned" kind of weather day, especially if you are planning to do outdoor things.
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&& .LONG TERM /SATURDAY THROUGH TUESDAY/...
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As of 425 AM EDT Wednesday...Stuck with a blend of model guidance as there is general agreement. The weekend will continue to be warmer than normal. 850mb temperatures Saturday will be in the 15-17C range, and perhaps a degree warmer on Sunday. Went a couple of degrees warmer than the blend, as I like the values shown by the 00z GFS MEX guidance as we should be solidly in the 80s, perhaps touching 90F here and there. A shot of a thundershower across Vermont Saturday and Northern NY on Sunday. Monday could be another active thunderstorm day. Fairly robust shortwave embedded in the westerly flow aloft should be coming through during the time of peak heating. At the surface, low pressure will pass by well to our north, but a cold front will sweep through the region probably late in the day. Both the 00Z GFS and ECMWF indicate decent amounts of instability (CAPE of 1000-1500 J/kg), and increasing 0-6km shear later in the day. This far out, subtle changes in timing could mean big differences in what we actually see. Thus will not go crazy with things, and just paint in 30-45% chance of thunderstorms. Depending on how much sun we get, it could be another day where we make a run for 90F. Tuesday - looks dry and cooler, with temperatures back to near normal.
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&& .AVIATION /08Z Wednesday THROUGH Sunday/... Through 06z Thursday...Mainly VFR except MVFR/IFR possible KSLK 09-12Z in fog. Left fog out of KMPV as temp/dewpoint spread not conducive for fog but confidence is marginal with several more hours of cooling and small bodies of water steaming away with air temps in the 40s to lower 50s. Most areas calm or light/variable overnight, however KRUT has its usual southeast drainage wind around 5 kts. During the day expect some lake breeze interaction around Lake Champlain, with KPBG turning easterly and westerly component at KBTV. Scattered cumulus at 5-6K ft expected to develop over the mountains and NE VT. Should be mainly clear and calm tonight through 06Z. Outlook 06z Thursday through Sunday... Thu...VFR under high pressure. Overnight IFR fog possible, especially at KSLK and KMPV. Thu night-Sat...VFR, with scattered MVFR in showers and thunderstorms. Sun...Mainly VFR under weak high pressure. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Sisson NEAR TERM...Sisson SHORT TERM...Nash LONG TERM...Nash AVIATION...Sisson

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