Area Forecast Discussion
Issued by NWS Burlington, VT

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872 FXUS61 KBTV 211359 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 959 AM EDT Thu Jul 21 2016 .SYNOPSIS... Ridge of surface high pressure will remain over the North Country today, providing the region with warm, sunny, and dry weather. Periods of showers and thunderstorms are expected after midnight tonight through Saturday as a frontal system moves slowly through the region. Some of the storms will have the potential to become strong or severe. Another round of thunderstorms is possible on Monday. && .NEAR TERM /UNTIL 11 PM THIS EVENING/... As of 958 AM EDT Thursday...Any fog has burned off and early morning cool temperatures are warming quickly. Lake winds are a bit higher than earlier fcst and updating...otherwise all looks good. Previous discussion...Any early morning fog will burn off in the next hour, then it will be a very nice, warm, and sunny late July day with high temps 85 to 90. Large ridge of surface high pressure will remain over the region right through Thursday with dry weather. Some high level cirrus clouds are possible from time to time with NW flow aloft from thunderstorms up near James Bay early this morning. Highs today will be warmer than normal with ample sunshine and warming aloft as 850 temps rise to 16 to 18C. Expecting highs 85 to 90. Winds will become south to southwest and some gusts to 20 knots in the St. Lawrence Valley. Late Thursday night into early Friday morning A surface front in Ontario/Northern Great Lakes will be slowly approaching with increasing chance of showers and thunderstorms, mainly after midnight. Deep low-level layer moisture with PWATS >1.5 inches advecting ahead of front. There will be elevated instability with CAPE over 1000 J/kg in the St. Lawrence valley by 09-12Z Friday. The boundary layer should be stable with instability resulting from lifting parcels from the 900 mb level. There is also the remnants of an elevated mixed layer with 700-500mb lapse rates around 7C/km. Lift to be provided by a shortwave trough and associated QG forcing and divergence ahead of exit region of 300mb jet. They may be a mesoscale convective complex in northern Great Lakes/Ontario moving into the area during late Thursday night/early Friday morning as a result. Not really looking for much of a severe threat tonight but can`t rule out the possibility of a few stronger thunderstorm cells toward Friday morning. South to southwest winds will continue all night tonight and as a result low temps will only be in the mid 60s to lower 70s. && .SHORT TERM /11 PM THIS EVENING THROUGH SATURDAY NIGHT/... As of 421 AM EDT Thursday...As typical with a "Ring of Fire" situation (areas of convection developing along the fringe of a large upper level "heat" ridge, models struggle with the subtle but important details. Case in point, even the 3 to 6 hour forecast output from the 00z GFS and NAM do not depict the large clusters of convection located from Minnesota across into northwest Quebec. Thus any reliance on the exact details from models, especially the ones that explicitly forecast convection, will likely lead you astray. It also makes the forecast more difficult, and so this is one with low confidence, even though we are talking 24 to 36 hour forecast. Best I can do is work with the clues that the models give and merge that in with what we can see via satellite and radar and make an educated guess. So with that, here is what I envision. Friday: still see lots of ingredients that point toward the potential for significant convection. Source airmass at low levels by 18z was what was out across Illinois on Wednesday afternoon (temperatures well into the 80s, with dewpoints nearing 70). Airmass at 10,000ft or so will be what was in place across North Dakota into southern Manitoba (where we saw a cluster of strong/severe t-storms form last evening). This is shown in the models by a well defined EML (elevated mixed layer), high levels of surface instability (GFS has CAPE around 2000 J/kg, while the NAM has some seemingly a bit too high 3000 J/kg CAPE), robust shear suggesting storm organization. Still evidence that after an early morning round of showers/t-storms, mid levels will dry out considerably. Discussion with my mid-shift cohort leads us to still believe that we`ll still see some strong/severe t-storms develop, however they will be scattered in nature. Away from these storms, it will be a sunny/hot/humid day. The EML and instability maintains itself well into Friday evening. So it`s not out of the question that we`ll see repeated rounds of scattered strong thunderstorms develop anywhere and continue into the late evening. Primary threats still appear to be strong winds and hail. SPC continues to paint the area from Vermont and points east in a Slight Risk, with a marginal risk across northern NY. The differences between these levels of risk is pretty small. Bottom line message is that we will have a threat of isolated damaging thunderstorms. For those that want to geek out a little bit more, check out the CIPS Analogs webpage. Based on the top 15 analogs, about half indicate some sort of long lived convective cluster had developed, producing a long swath of damage. Most of those were across the Ohio Valley (with nothing up here -- which is a scenario that could happen!), however a couple of past similar events showed damage in New England. What past history indicates is that no scenario is off the table. Temperatures Friday are a bit tricky. If it`s mostly sunny with just a few storms, we could make a run at 90F. If we have a good deal of clouds and more widespread showers/storms around, it won`t be as warm. I leaned on the warm side, but lower than the warmest of the guidance. For PoPs, I went with a broadbrush approach, even though much of the time it likely won`t be raining in any one location. So 60-70% chance does not equate to how much of the day, but rather maximum coverage of the t-storms throughout the day. Saturday: We are not out of the woods with regard to t-storms. Guidance is fairly consistent that we`ll still have some instability about with another day of warm temperatures and high dewpoints. At upper levels, we`ll have a shortwave trough drop down from the northwest, probably during the first half of the day. Looks like the best chances for t-storms will be across Vermont, with chances lowering by later in the afternoon. SPC has the area in a "Marginal Risk" for severe thunderstorms. Agree with that assessment as the various ingredients are just not as well developed as they will be for Friday, but just enough that we may see one or two strong storms develop. && .LONG TERM /SUNDAY THROUGH WEDNESDAY/... As of 421 AM EDT Thursday...Given the focus on the Friday potential for severe weather, I stuck with a straight model blend for the remaining days. The only day that could be problematic still appears to be Monday. Both the GFS and ECMWF continue to show a potent upper trough and associated surface cold front zipping through the region at some point during the day. The 00z run is a little slower (especially the GFS) and has the system passing through more Monday night. In any event, some indication that we could see another round of strong (to perhaps isolated severe) thunderstorms develop. Still perhaps some residual showers around on Tuesday as some instability will be in place with troughiness aloft. Wednesday and Thursday look to be very nice days. Stayed with the blend for the temperatures, so have a forecast of more-or-less near normal conditions. && .AVIATION /14Z Thursday THROUGH Monday/... Through 12z Friday...VFR with mainly SKC or SCT250 skies. FG at MPV should be gone around 12z but could linger til 13Z. Clouds will gradually thicken tonight with cirrus streaming in from the NW and may see some mid clouds arriving at KMSS before 06Z. Have showers in the vicinity at that time and there is also a low chance of thunderstorm moving SE from Canada. Cigs/Vsby lower toward MVFR in showers and possible thunderstorms by 12Z Friday especially BTV west to SLK/MSS. Have left out TS for now as coverage and location not yet known. Light/calm winds will become light south to southwest today. KMSS could see some SW gusts to 20 knots during afternoon in tighter pressure gradient and channeling up the St. Lawrence Valley. Expecting winds to continue tonight as gradient strengthens everywhere. Have also included gusts over 20 kts at BTV for morning push and gradient increases and wind channeling starts in the Champlain valley. Outlook 12z Friday through Monday... Fri-Sat...VFR, with scattered brief local MVFR/IFR in showers and thunderstorms. Some thunderstorms may contain strong winds and hail. Sun...Mainly VFR under weak high pressure. Mon...Scattered MVFR local IFR in showers and thunderstorms. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Sisson NEAR TERM...Sisson/SLW SHORT TERM...Nash LONG TERM...Nash AVIATION...Sisson

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