Area Forecast Discussion
Issued by NWS Burlington, VT

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200 FXUS61 KBTV 230549 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 149 AM EDT Tue May 23 2017 .SYNOPSIS... A weak cold front crossing New York and Vermont through this evening will bring isolated showers, along with lingering low clouds and areas of fog overnight. Will see clearing Tuesday with a weak area of high pressure in place across the northeastern U.S. and temperatures moderating back into the upper 60s to lower 70s. A few afternoon showers are possible Wednesday, but high temperatures will continue to moderate into the low to mid 70s. A large mid-level trough across the Tennessee and Ohio Valleys and associated developing surface low pressure will bring increasing chances for widespread rainfall across the North Country on Thursday and Friday. && .NEAR TERM /UNTIL 2 PM THIS AFTERNOON/... As of 702 PM EDT Monday...Minor adjustments to the previous forecast for this evenings update, mainly to reduce pops and tweak sky cover. Frontal showers have quickly exited the region to our east, and while a few stray showers may be around through the evening, in general the remainder of the night will be dry. Forecast challenge for sky cover remains though with subsidence building in aloft and an abundance of low level moisture around. Current thinking is that sky cover will be widely variable through the night with some low stratus hanging tough over higher elevations, while many valley locales may clear out to a scattered/overcast mid/high deck. Can`t rule out some areas of fog, especially in any clearing so have left that in, but signals are trending towards less of a chance of dense fog and more BR-ish with winds just off the deck hanging in the 10-15 knot range. Lows remain on track for mid/upper 40s to low 50s. Previous Discussion... Will see some gradual mid-level drying this evening (above 750mb) with WSWLY flow aloft, but pronounced low- level inversion should continue to hold low stratus in areawide with overcast skies and cool temperatures. Afternoon highs only expected to range from the lower 50s east of the Green Mtns, to near 60F at BTV, and upr 50s to lower 60s across nrn NY. The KCXX vad wind profile still indicates 25-30kt flow at 2-3kft AGL. Won`t fully see these winds at low elevations due to stability, but occasional gusts to 20-25 mph are possible thru this aftn. Expect winds remaining 20-25kts over the broad portion of Lake Champlain, and the Lake Wind Advisory continues there. Tonight through Tuesday a weak surface ridge builds into the region. Lowering inversion and light surface winds, along with moist ground from recent rain will promote fog formation in the usual areas. During this time flow aloft remains southwesterly, and low pressure over the Great Lakes will ride through the 500 mb ridge, suppressing the ridge & shifting it east. The low passes well enough north that the surface ridge will win out with no rain during this time, but will still be quite a few clouds around. Tuesday about 10 degree warmer than monday with highs in the 60s/near 70. && .SHORT TERM /2 PM THIS AFTERNOON THROUGH WEDNESDAY NIGHT/... As of 315 PM EDT Monday...12z guidance suite still indicating that Wednesday will be the warmest day of the entire weak. Weak ridging aloft will put the region into a col area (light flow regime). Hi-res models do hint at just enough low level moisture and surface instability will exist to pop a few showers, primarily across the higher terrain in the afternoon, though with an inversion around 15,000ft not expecting any t-storms to develop. Winds from 10,000ft down to the surface will be generally 10 knots or less, so look for onshore lake breezes to develop. 925mb temperatures will be running about 17C, which given a decent amount of sunshine will result in lower elevation temperatures within a couple of degrees of 80F. Wednesday night flow starts to turn south/southeast as low pressure moves toward the Great Lakes. Models all show an area of deeper moisture moving toward the region, so as we move toward daybreak Thursday it appears precipitation will be starting to overspread the region. Have depicted a ramping up of PoPs up well after midnight, though most of the night will be dry. && .LONG TERM /THURSDAY THROUGH MONDAY/... As of 315 PM EDT Monday...12z guidance came in fairly close to previous runs, thus confidence remains rather high for the overall scenario of unsettled weather through much of the period. Looks like the primary rounds of more significant weather will be Thursday and again Monday (Memorial Day), with "okay" weather squashed in between. Comments on each day follow... Thursday: Fairly sizable low pressure will be moving across the Ohio Valley with a deepening upper level trough also in place. This will result in a tightening pressure gradient with a low/mid level east-southeast jet developing during the day. The GFS indicates about 40-45kts of wind at 850mb, with the 12z euro having a few grid points of 50kts of wind. Fairly typical situation in which we`ll probably have some breezy conditions along the western slopes of the Green Mountains. At this point think we could max out with some gusts in the 30-40 mph range in the favored areas. Lower resolution global models don`t have a very good depiction of these localized effects, so did try to manually introduce some gustier winds into the gridded database. Just an aspect of the forecast we will have to monitor. Otherwise, system will have a pretty good moisture tap (precipitable water values about 1.3"), so expect fairly widespread rainfall to occur during the day with rainfall totals probably around 1/2". Shouldn`t lead to any hydro issues, but also another aspect to monitor. Kept with a model blend for temperatures, which has primarily 60s across the region (lower 60s across southeast VT to upper 60s around 70 for the northern Champlain Valley into the St Lawrence Valley), however it may end up cooler than that if we inject more Atlantic airmass into the region. Friday: The upper trough turns into a closed low and moves across the region during the day. Basically means a continuation of unsettled conditions with spotty showers at times. Lots of clouds will keep it on the cooler side once again, generally in the 60s. Saturday: Trough and surface low move off to our east, putting us into a northwest flow situation. 850mb temperatures start off around 5-7C and by afternoon rise to 9C. This should support highs in the upper 60s to lower 70s, though if we can see more sunshine, mid 70s are possible. Still can`t rule out an isolated shower, but should be pretty good for most outdoor activities. Sunday: GFS hints at a weak back-door cold front trying to come at us from the east as surface high pressure moves over Maine and sets up some low level easterly flow, with the chance for a few light showers. The 12z ECMWF has no such feature, and is putting the region into a developing southerly flow situation ahead of Monday`s weather maker. However, with that deeper southerly flow, it`s got some deeper moisture. The net result from either type of scenario is that showers can`t be ruled out for Sunday at this point. Not as much confidence in the temperature forecast however, and given the variation in scenarios we could be anywhere from the upper 60s to lower 80s for highs. Stuck with a blend for this go around. Monday: Alas, Memorial Day isn`t looking the best for those outdoor BBQs/games/parades etc if you want to avoid any rain. Another fairly robust trough will be swinging across the region, though the GFS and ECMWF differ on details relating to placement of surface low and timing. In either model`s scenario, we`ll have scattered showers around. GFS is more robust in pushing some more humid airmass into the region from the south, with enough instability to fire off a few non-severe t-storms during the afternoon. ECMWF has much less instability, but still a signal that a rumble of thunder can`t be ruled out. Have gone ahead and placed some slight chance probability of thunderstorms in the forecast. Stuck with the blended approach for high temperatures, with highs generally around normal (upper 60s to lower 70s). Not great confidence on that aspect at this point. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/...
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Through 00Z Wednesday...A challenging aviation forecast continues thru 12z this morning with fog product and obs showing multi cloud layers with categories ranging from ifr at rut/mpv to vfr elsewhere. Thinking ifr conditions become mvfr/vfr at rut/mpv shortly as winds help with mixing. The combination of yesterdays rainfall and lighter winds after 08z...and several locations going 3 to 6 degrees below cross over temps...thinking some fog/br will develop between 08z-11z. Have utilized tempo group at rut/mpv and slk to highlight fog/br potential early this morning. Overall confidence in IFR potential is on the low side...given 10 to 15 knots of winds around 975mb. After 12z...expecting cigs/vis to become VFR with winds shifting to the north at 4 to 8 knots...with some localized terrain driven gusts. Some high clouds possible today as coastal system passes to our east. Outlook... Wednesday: VFR. NO SIG WX. Wednesday Night: VFR. Chance SHRA. Thursday: VFR. Breezy. Likely RA. Thursday Night: MVFR with areas of IFR CIGS MPV. Breezy. Likely RA. Friday: MVFR with IFR MPV/SLK. Likely RA. Friday Night: VFR. Chance SHRA. Saturday: VFR. Slight Chance SHRA.
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&& .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...Banacos NEAR TERM...Banacos SHORT TERM...Nash LONG TERM...Nash AVIATION...Taber

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