Area Forecast Discussion
Issued by NWS Buffalo, NY

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000 FXUS61 KBUF 241517 AFDBUF Area Forecast Discussion National Weather Service Buffalo NY 1117 AM EDT Tue Apr 24 2018 .SYNOPSIS...
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Broad low pressure extending from the Ohio Valley into the Southeast states will strengthen while moving to the Mid Atlantic region tonight. This system will spread rain showers into western and north- central New York from southwest to northeast later this afternoon and evening. Periods of rain will then continue tonight through Wednesday night before ending from west to east on Thursday as the low moves into New England. High pressure will bring a return to drier weather later Thursday.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Low pressure is centered over the Savannah River Valley near midday with an inverted trough extending north into the Ohio Valley. Mid/high clouds continue to increase across western New York as moisture advects north ahead of this system. Skies are mainly sunny across north-central NY but cloud cover should increase through the afternoon. KBUF radar is largely clear of returns with the exception of some light virga shifting north across western NY. Dewpoint depressions of 20 deg or more are keeping any showers from reaching the surface. Later this afternoon and evening a strong area of DPVA, moisture, and forcing for ascent will move north ahead of the storm system. Upper level divergence will increase by this evening as the right entrance region of a 100+ knot upper level jet moves across the eastern Great Lakes. Expect showers to reach the western Southern Tier around mid afternoon, then spread into the rest of Western NY and the Genesee Valley during the late afternoon and early evening, before reaching the eastern Lake Ontario region by late evening. Periods of rain will then continue tonight as the Ohio Valley closed low begins to open up and interact with a northern stream trough digging into the central Great Lakes. This will maintain moisture transport and large scale ascent across the region. The low levels will begin to saturate overnight with the ongoing rain, allowing some patchy fog to develop. The fog will be most prevalent across the higher terrain where low stratus will intersect the hilltops, and possibly the south shore of Lake Ontario from Rochester westward with low level moisture condensing as it crosses the cold lake waters on developing northeast flow. It will be another warm day today prior to the arrival of the rain. Expect highs in the mid or even upper 60s from the Niagara Frontier eastward across the Genesee Valley to the eastern Lake Ontario region. The western Southern Tier will only be in the mid 50s with an earlier arrival of thicker cloud cover. Across Western NY, these highs will occur during the early afternoon before temperatures begin to fall during the mid to late afternoon. Lows tonight will pull back into the mid 40s in most locations.
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&& .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... A cutoff low will be over the Carolinas by Wednesday morning while a robust northern stream wave dives from the upper Great Lakes toward western NY. The weakening cutoff low will be absorbed by the northern stream wave as it moves up the east coast to New England by Thursday morning. This complex interaction will slow the progression of the northern stream wave, while strengthening it into a closed off low as it tracks across New York state Wednesday night. The most reasonable forecast solution remains the blend of the EC/GEM/NAM, which slow the progression of the wave as the merger occurs, while the operational GFS remains an outlier among the solutions with a very progressive and farther north solution. The operational GFS is even an outlier among its own ensemble members, which prefer a slower evolution. Thus the official forecast continues leverage this EC/GEM/NAM blend. The result of this complex interaction will be periods of fairly steady moderate rain through the day Wednesday as anomalous easterly flow ahead of the cutoff low moving up the coast directs deep Atlantic moisture back across WNY. This moisture will be fed around the cold conveyor belt of the strengthening surface low moving up the east coast, and into an area of enhanced forcing under diffluent flow aloft ahead of the closing upper-level low and the inverted surface trough. The greatest rainfall totals will depend on exactly where the deformation zone sets up, but this would like be somewhere between WNY and the Finger Lakes. Rain will weaken in intensity and become more showery into Wednesday night and Thursday as the upper-level low moves over and eventually east of the forecast area, shutting down the optimal synoptic dynamics. However, the strengthening northwesterly flow across the region as the surface low moves into New England, along with ample deep wrap around moisture, will support lingering rain showers. These showers will be most enhanced south and east of the Lake Ontario where the combination of low-level convergence off the lake and orographic enhancement over the higher terrain provide added lift. Following with the expected slower evolution of this system, this will mean rain showers tapering off from west to east late Wednesday night through the day Thursday as the surface low moves into the Canadian maritimes and a surface high builds in from the west. Any lingering showers should end by late Thursday night giving a dry start to Friday. Have also introduced a chance of snow to the forecast across the higher terrain of the western Southern Tier for Thursday morning. However, this would be a narrow window where, by the time the cold advection brings in enough cold air to allow some wet flakes to mix in, the precipitation will be shutting down, and thus any impact or accumulation would be essentially zero. Rainfall totals will likely be over 3/4" for most locations, with 1" to 1.5" quite possible for some, depending on where the deformation zone sets up, and when factoring in lingering upslope enhancement south and east of Lake Ontario. With the recent bout of drier weather across the region, expect the area creeks and rivers will be able to handle the rainfall without issue. Daily high temperatures will be held to the low to mid 50s with ample cloud cover and rain across the region. Nighttime lows will be in the 30s to low 40s. && .LONG TERM /FRIDAY THROUGH MONDAY/... A shortwave ridge will move across the region Friday behind the mid- week storm. This will bring a break of dry weather and mild temperatures (upper 50s to low 60s) through much of the day Friday. An upper trough dropping through the Great Lakes will bring a cold front through the region during the late Friday or early Saturday period, bringing another round of rain showers. This will leave a spell of cooler weather in its wake for the weekend, with daytime highs only in the low to mid 50s. Forecast models are coming into good agreement about a ridge building across the region to start next week. Exactly how fast this happens remains is in question, but we could be looking at mid to upper 60s by Monday and 70s by Tuesdays along with dry and mainly sunny weather. && .AVIATION /15Z TUESDAY THROUGH SATURDAY/...
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Mid and high level clouds continue to increase across western NY near midday ahead of low pressure broad low pressure extending from the Ohio Valley into the Southeast states. A widespread area of rain showers are expected to arrive in the western Southern Tier around mid afternoon, then spread northeast across the rest of western and north-central New York from late afternoon through the evening as the low pressure and associated forcing and deep moisture approach. CIGS/VSBY will remain VFR through much of this afternoon. CIGS/VSBY will then drop to MVFR this evening as the low levels begin to saturate, with IFR becoming more likely across the western Southern Tier by mid evening, and across the remainder of Western NY overnight. East of Lake Ontario, VFR will trend towards MVFR later tonight, with IFR developing late tonight across higher terrain. Outlook... Wednesday...MVFR/IFR with occasional rain. Thursday...MVFR/IFR improving to VFR with showers ending. Friday...VFR. Saturday...VFR to MVFR in scattered showers.
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&& .MARINE...
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Broad low pressure extending from the Ohio Valley into the Southeast states this afternoon will move to the Mid Atlantic region by Wednesday morning before exiting across the Gulf of Maine on Thursday. South winds this afternoon will become easterly by tonight, and northeast on Wednesday as the surface low passes by to our south and east. Wind speeds will generally remain under 15 knots through this period. A tighter pressure gradient will develop Wednesday night and Thursday in the wake of this system, with northwest winds becoming west. This may bring a period of Small Craft Advisory conditions to the eastern Great Lakes.
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&& .FIRE WEATHER...
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High pressure will drift east off the New England coast this afternoon while a trough of low pressure moves slowly through the Ohio Valley. Dry conditions will continue east and southeast of Lake Ontario with afternoon minimum relative humidity dropping to around 25 percent with some wind gusts occasionally approaching 20-25 mph or near Red Flag. Farther west, minimum relative humidity will briefly drop to around 35 percent across the Niagara Frontier, Genesee Valley, and western Finger Lakes before increasing again late today as showers arrive. Periods of rain will then bring widespread wetting rain tonight through Wednesday.
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&& .BUF WATCHES/WARNINGS/ADVISORIES... NY...None. MARINE...None. && $$ SYNOPSIS...Hitchcock/Smith NEAR TERM...Hitchcock/Smith SHORT TERM...Church LONG TERM...Church AVIATION...Hitchcock/Smith MARINE...Hitchcock/Smith FIRE WEATHER...Hitchcock/Smith

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