Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 230827
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
427 AM EDT Sun Jul 23 2017

.SYNOPSIS...
Through Monday...low pressure will slowly track east along a nearly
stationary frontal boundary draped across our region...and this will
set the stage for periodic showers and thunderstorms. Some of these
could produce locally heavy rainfall...with a few of the storms also
possibly becoming strong to severe across western New York this
afternoon and evening. Following the passage of this system...Canadian
high pressure will then build across our region Tuesday...ushering
in a return to mainly dry and cooler weather along with much more
comfortable humidity levels.

&&

.NEAR TERM /THROUGH TONIGHT/...
Through the course of today...a compact but also fairly potent mid-
level shortwave trough will steadily press southeastward across the
Great Lakes...with its attendant broad surface low over Lake Erie
slowly meandering its way into western New York. In the process...
this latter feature will help to push a quasi-stationary frontal
boundary a little further northward into our region...with the
front roughly straddling our southernmost two tiers of counties
this afternoon.

While we can expect some continued spotty light showers to persist
through this morning out in advance of the above features...things
will begin to change by midday/early this afternoon...as increasing
large-scale forcing and lift supplied by the front/low/mid level
trough spreads across our region...and interacts with our humid
airmass and building diurnally-driven instability. This will result
in scattered to numerous showers and thunderstorms likely initiating
across far western New York sometime in the 16Z-18Z time frame...
before growing more numerous/widespread and pushing northeastward
across the Finger Lakes and into Central New York through the rest
of the afternoon and early evening hours. In the forecast...this has
been covered with a mix of likely to lower-end categorical PoPs
across areas south of Lake Ontario this afternoon and early this
evening...while we have maintained chance-level probabilities for
much more scattered convection across the North Country.

As has been previously highlighted...there continues to be a risk
for both heavy rainfall and some strong to severe storms this afternoon
and early this evening. Precipitable water values will surge to between
1.5 and 1.75 inches...which will be supportive of localized heavy
rainfall and flood concerns if any areas of training cells manage to
materialize. In addition...plenty of shear will also be in place...with
0-6 km bulk shear values of 40-45 knots in play...and vertical wind
profiles suggesting that this will be comprised of both speed and
directional shear...with a low level southeasterly/southerly flow out
ahead of the surface low underlying an increasingly fast westerly flow
at the mid and upper levels. Such levels of shear are supportive of at
least a conditional risk for some stronger to severe storms...with the
caveat that this risk will be highly dependent on the amount of instability
that actually develops...which will be a function of both daytime
heating (cloud cover) and the position of the main surface front.

Compared to what was seen 24 hours ago...the 00z/23 guidance suite has
trended a bit stronger with instability levels south of Lake Ontario...
with the NAM/GFS/GEM all now suggesting that MUCAPE values will at least
reach the 1000-2000 J/kg range from about the western Finger Lakes
westward...and the former two models suggesting the potential for MUCAPE
levels in excess of 2000 J/kg across the western Southern Tier...which
makes sense given the expected position of the main surface front. Should
all this actually come to fruition...this would place the greatest risk
for strong to severe storms across the western Southern Tier this afternoon
and evening...with a gradually lowering risk then extending northeastward
across the remainder of the region south of Lake Ontario. This meshes
up very well with the latest SPC Day 1 Convective Outlook...which
still features a Slight Risk area across portions of far western New
York...and a surrounding marginal risk area that extends northeastward
to Oswego County.

Given the above parameters...at this point the primary severe weather
threats still appear to be damaging winds and to a lesser extent large
hail...with the above mentioned veering wind profiles and presence of the
main frontal zone also supportive of a MUCH smaller (but still non-zero)
risk for an isolated tornado. With this in mind...will continue to
highlight this potential within our Hazardous Weather Outlook...as well
as carry enhanced wording in our routine forecast products where
appropriate.

As we move into tonight...the axis of the mid-level shortwave trough
will settle across nearby Southern Ontario...while forcing the surface
low to slowly lift northeastward across far western New York. This
will result in the most widespread shower and thunderstorm activity
steadily shifting eastward into the Finger Lakes and the North Country
over time...with convection becoming more scattered across far western
New York. In general the greatest thunderstorm coverage and risk for
strong to severe storms will be found south of Lake Ontario during the
early evening hours...before diminishing instability leads to a rapid
reduction in both thunder coverage and severe storm potential for the
balance of the night.

The above stated...concerns for locally heavy rainfall will actually
INCREASE through the night across the North Country...where a deepening
southerly feed of moisture and increasing lift will set up out ahead of
the surface wave and mid level trough. This could potentially result in
a period of moderate to heavier rain setting up across this latter region
later tonight and then persisting into if not right through Monday...which
could in turn lead to basin average rainfall amounts of 1 to 1.5 inches
per the 00z/23 guidance suite...with some individual models (most notably
the NAM12 and regional GEM) suggesting the potential for as much as 2-3
inches of rainfall. Were the latter to be realized...this would obviously
lead to some flood concerns. With this in mind have toyed with the idea of
issuing a flood watch for our eastern Lake Ontario counties...though given
lingering model discrepancies and resultant forecast uncertainty have
elected to hold off on this for now...in favor of continued enhanced
wording in the forecast and mention in the HWO.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
Widespread showers and thunderstorms will be ongoing as we start the
day Monday as well defined mid level wave is forecasted to propagate
across the eastern great lakes sending its attendant surface low
into western New York monday morning. Potential exists for heavy
rainfall, especially across central New York as precipitable water
values approach 1.5 inches as low level deformation deformation
increase ahead of approaching surface low and associated low level
boundary. The most favorable surface based instability will be
Monday afternoon across central New York. SPC has included a
Marginal risk area on Monday. These potential hazards will continue
to be highlighted in the Hazardous Weather Outlook.

Convection will trend downward Monday night as the surface low moves
to a position off the New England coast with high pressure in the
process of building back across the Great Lakes.

Tuesday and Tuesday night will feature dry and quiet weather as high
pressure settles over western New York.

Temperatures will run cooler Monday and Tuesday topping out below
normal late July levels. Highs run generally reach into the lower to
mid 70s in the cool northerly flow on the backside of the passing
low Overnight lows will bottom out in the 50s Monday and Tuesday
nights with cool and dry Northerly flow.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
Upper level ridging will crest across the region on Wednesday,
providing for one more fair weather day for western and north-
central New York, with seasonable temperatures in the upper 70s to
lower 80s. After a couple of cooler nights earlier in the week,
mugginess will return Wednesday night as surface high pressure moves
off the New England coast, opening up the lower Great Lakes to
warm and moist southerly flow. This will be reflected in lows
that will run in the mid to upper 60s. Along with this increase
in humidity, we will see a chance of showers and thunderstorms
towards daybreak, as a prominent upper level trough approaches
the area.

Model consensus continues to indicate the passage of the
aforementioned upper level trough on Thursday. This should result in
widespread showers and thunderstorms Thursday into Thursday evening
as a cold front sags south across the area. A secondary reinforcing
shortwave may end up aiding in the development of diurnal showers
Friday afternoon, particularly across the southern half of the
forecast area, closer to the hung up frontal boundary. nominally
cooler air filtering in behind the front may yield a milder day
Friday, with highs in the mid to upper 70s.

&&

.AVIATION /09Z SUNDAY THROUGH THURSDAY/...
For the most part...the pre-dawn hours will continue to feature
VFR conditions under increasing amounts of mid and high cloud cover...
with just a few spotty light showers dampening otherwise mainly dry
conditions. The exception to this will be found across the western
Southern Tier (including KJHW) where low level moisture advecting in
from the Ohio Valley will lead to the development of LIFR/IFR stratus...
with this possibly lowering enough to lead to the development of fog/
IFR vsbys across the higher terrain.

As we move into the daylight hours...any LIFR/IFR in low stratus/
fog over the Southern Tier should improve back to MVFR/VFR during
the morning. Otherwise...approaching low pressure and increasing
diurnally-driven instability will lead to the development of scattered
to numerous showers and thunderstorms across far western New York by
early afternoon...with these then becoming more widespread and pushing
eastward across the Finger Lakes and into Central New York through the
rest of this afternoon and tonight. General MVFR to VFR conditions
out ahead of the convection will deteriorate to IFR/MVFR within the
showers and thunderstorms...with some of the storms capable of producing
locally heavy rainfall. In addition...a few stronger to severe storms
with strong wind gusts and hail will also be possible across western
New York this afternoon and evening.

As tonight progresses...expect flight conditions to lower to a mix of
MVFR across the lower elevations and IFR across the higher terrain as
low level moisture increases across our region...and leads to the
development of fairly widespread lower ceilings.

Outlook...
Monday and Monday night...Showers and scattered thunderstorms
should gradually diminish from west to east...though widespread
IFR/MVFR ceilings will likely persist in lingering lower stratus/
stratocumulus.
Tuesday...Improvement back to VFR with any leftover scattered
showers ending early.
Wednesday...VFR.
Wednesday night and Thursday...Mainly VFR with a chance of
showers and thunderstorms.

&&

.MARINE...
As low pressure approaches our region today and this evening...
easterly to east-northeasterly winds will increase to between 15
and 25 knots across the western two-thirds of Lake Ontario. While
the strongest overall winds will remain out over the open waters...
recent guidance trends toward slightly stronger winds and a subtle
backing of the flow for a time this evening both favor conditions
reaching advisory criteria from the Niagara River eastward to about
Irondequoit Bay. With this in mind...a Small Craft Advisory has
been issued as outlined below.

While gradient winds and waves will be lower across the rest of
the region today and tonight...scattered to numerous thunderstorms
developing this afternoon and evening could also produce locally
strong wind gusts and correspondingly higher waves.

&&

.TIDES/COASTAL FLOODING...
As low pressure approaches our region Sunday and Sunday evening...
a low-level easterly to east-northeasterly flow will briefly
strengthen across Lake Ontario with this potentially
strengthening enough to generate some lakeshore flooding
concerns along the south shore of the lake from roughly Niagara
county east to Monroe County. Latest guidance keeps the
strength of the winds just below a critical threshold to drive
waves high enough for the issuance of a Lakeshore Flood Watch at
this point but will continue to be closely monitored.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Beach Hazards Statement from 2 PM EDT this afternoon through
     this evening for NYZ001>003.
     Lakeshore Flood Watch from this afternoon through late tonight
     for NYZ001>003.
MARINE...Small Craft Advisory from 2 PM this afternoon to 8 AM EDT
         Monday for LOZ042-043.

&&

$$

SYNOPSIS...JJR
NEAR TERM...JJR
SHORT TERM...TMA
LONG TERM...WOOD
AVIATION...JJR
MARINE...JJR
TIDES/COASTAL FLOODING...JJR



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