Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 241517
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
1117 AM EDT Tue Apr 24 2018

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH TONIGHT/...
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.

&&

.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/...
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.

&&

.MARINE...
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.

&&

.FIRE WEATHER...
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.

&&

.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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