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Area forecast discussion
National Weather Service Buffalo New York
228 PM EDT sun Jul 22 2018

Synopsis...
low pressure will exit into southern Ontario through this
evening with mainly dry weather outside of a few scattered
showers and thunderstorms. A weak surface trough will pivot
northwestward across our region tonight, with humid and rather
unsettled conditions to follow for much of the upcoming week.

&&

Near term /through tonight/...
at 2 p.M. An area of low pressure was centered near Hamilton
Ontario with this bringing a generally southerly flow across
our region. This has resulting in a partial clearing with radar
showing mainly dry conditions across the area. This will build
instability some, but will be offset with the departure of a
shortwave and with the downslope flow. It looks like any showers
or thunderstorms will hold off until late afternoon or early
evening. The warmest temperatures this afternoon will be east of
Rochester with temperatures in the mid 80s with mid to upper
70s elsewhere.

Tonight the inverted trough will pivot northwestward and across
the remainder of the region. Model consensus brings convection
developing across eastern Pennsylvania into central New York
this evening before weakening late tonight. The wave driving
this is subtle, but with the convection already there model
guidance should do well advecting this feature. It's difficult
to rule out a shower at any location as this moves through this
evening into the early morning hours, with slightly lesser
chances late tonight behind the wave. Even some embedded
thunderstorms are possible with forecast soundings maintaining
marginal instability throughout the night.

Otherwise, temperatures will not drop off much overnight with
a fair amount of cloud and winds lasting all night. Expect lows
in the mid to upper 60s inland and around 70 across the lake
plains.

&&

Short term /Monday through Wednesday night/...
..muggy with frequent showers through the period...

An unusual mid Summer blocking pattern should prove very beneficial
to our region during this period. An anomalously deep trough locked
in place over the eastern half of the country will provide US with
frequent showers and possibly some thunderstorms. The axis of the
stationary trough will extend from Lake Huron and lower Michigan
southwards across The Heart of the Ohio Valley to the Panhandle of
Florida. It will not be until late Wednesday and Wednesday night
that a powerful shortwave plowing across the Canadian prairies will
finally dislodge The Block over the eastern half of the continent.

The true block within this pattern though will be found downstream
where a strong ridge will be anchored in place over the Canadian
Maritimes. This is the feature that will prevent the aforementioned
trough from being naturally progressive (unlike a closed low) and
will be much more atypical in its mid Summer strength and position.
The deep southerly flow generated between this robust ridge and its
counterpart trough will fuel the unsettled weather...as a steady
stream of moisture laden air will make its way northward across the
lower Great Lakes. Precipitable water values that will start off around 1.5" on
Monday will climb to 2" as a ribbon of true tropical moisture will
back across Pennsylvania and eastern New York to our forecast area
by Monday night. This will not only set the stage for some heavier
showers...but it will make this entire forecast period uncomfortably
humid. Now for some day to day details.

As we open this period on Monday...a deep southeasterly flow will
advect increasing amounts of sub tropical moisture across western
and north central New York. While difficult to time jetlets moving
through a stationary axis of strong h25 winds could help to support
some passing showers during the morning and midday...the bulk of our
shower and thunderstorm activity will be diurnally based...that is
to say driven by thermodynamics. Given the wealth of cloud cover...
it will be tricky to gauge exactly how much instability will be
available for any true convection. That being said...there should be
a minimum of 500-1000 j/kg of cape simply based on the mid levels
alone. Any significant break in the cloud coverage would allow for
upwards of 1500 j/kg of SBCAPE. As was so astutely pointed out in
previous discussions though...this airmass will only offer tall
skinny cape profiles. Since the freezing level will be in the vcnty
of 15kft and there will be a lack of any true focusing boundaries in
the region...the tall skinny cape profiles will favor some 'heavy
rainers' and not much more. While the majority of the day will be
rain free...will maintain the trend of higher pops during the
afternoon with likely pops being used for all but the iag Frontier.
Otherwise...it will be warm and quite humid. 850 mb temps in the vcnty
of 17-18c will support afternoon temperatures of at least the mid
80s across the lake plains and in the valleys. Dew points in the
lower 70s will make it feel some 5 degrees warmer.

The mainly diurnally driven convection will die down during the
course of Monday evening...although some showers will still be
possible overnight. This will mainly be the case from The Finger
lakes to the Eastern Lake Ontario region under the plume of richest
tropical moisture. It will certainly be a night for air conditioners
as the Mercury will only drop into the lower 70s across the lake
plains while readings will bottom out in the mid to upper 60s across
most of the srn tier and parts of the north country.

On Tuesday...there appears to be very little difference in the
forcing and overall thermal profiles across the region. The only
subtle change is that the tropical moisture should be a little
deeper and there could be the presence of a passing shortwave within
a more southerly flow. Again...the bulk of the shower and
thunderstorm activity will be diurnally driven with thin cape
profiles and negligible 20kt bulk shear. While this will support
likely pops for the afternoon...the majority of the day should be
rain free with plenty of clouds and uncomfortably high humidity.

As a strong shortwave crosses the Canadian prairies Tuesday night...
mid level energy should start to eject out of the persistent trough.
These impulses will push across our region during the overnight and
will likely support more widespread showers and thunderstorms. Will
maintain the likely pops from continuity while forecasting basin
average quantitative precipitation forecast of a quarter to a half inch.

Wednesday should be the most unsettled day of this 'short term'
period. The once stationary trough over the Ohio Valley will finally
start to weaken and lift out in response to the strong shortwave
that will be approaching the upper Mississippi Valley. While the
various guidance packages have different solutions as to how fast
this takes place...they are all in general agreement that the
overall synoptic forcing will noticeably increase. The largest two
lifting mechanisms will be a 100kt h25 jet over Quebec (positioning
US under the rr jet entrance region) and a streak of channelled
vorticity established within the axis of the transitioning mid level
trough. This deep lift will interact with the much discussed sub
tropical airmass so that increasingly widespread showers and
thunderstorms will have the potential to produce significant
rainfall. Basin averages should exceed a half inch on Wednesday...
especially for areas east of the Genesee Valley.

The mid level trough will continue to advance across our region
Wednesday night...while the deep moisture field will be shoved to
the east across New England and extreme eastern New York. This will
encourage the widespread showers and thunderstorms from the evening
hours to gradually taper off to just scattered showers during the
wee hours of Thursday morning...especially over the far western
counties. Have slowed this process by some 3-6 hours from
continuity.

&&

Long term /Thursday through Saturday/...
the remnants of the mid level trough that was partly responsible for
the plentiful shower activity leading into this period will complete
its journey across our region on Thursday. The associated shower
activity will largely be confined to The Finger lakes and Eastern
Lake Ontario regions...as much drier air in the mid levels will
overspread the far western counties.

A strong shortwave crossing the upper Great Lakes Thursday night and
Friday will then drive at least one cold frontal boundary across our
forecast area. While this could touch off a few showers...relatively
dry air in the mid levels should enable the bulk of this twenty four
hour period to be rainfree. Will thus shave back a bit on the
pops by going with low chc Thursday night and slgt chc Friday.

The synoptic pattern will have some fall like qualities to it Friday
night and Saturday. While a cyclonic flow from h925 through the
boundary layer will be found across the lower Great Lakes...surface
based ridging and a swath of relatively dry air will build across
the area. This will promote dry weather to end the forecast
period...although had to use slgt chc pops on Saturday due to some
differences in the medium range ensembles in regards to the
amplitude (depth) of the overlying mid level trough.

As far as temperatures are concerned...this period will start off
quite warm with 850 mb temps in the mid teens supporting Thursday
temperatures in the mid 80s f. In the wake of at least two cold
frontal passages though...850 mb temps should drop to nearly 10c. This
will lead to a day to day lowering of temperatures so that we may
actually be a couple degrees below normal for Saturday.

&&

Aviation /18z Sunday through Friday/...
low pressure will track into far southern Ontario with a
southerly flow across the region this afternoon. This flow tends
to downslope and help dry the boundary layer with mainly VFR
conditions expected outside of patchy areas of MVFR or lower
conditions.

A wave of convection will move across central New York this
evening, with some hit or miss showers and storms as it moves
through. Expect mainly VFR conditions outside of localized
storms. The exception is the southern tier and jhw where low
moisture may result in a IFR/MVFR cloud deck late tonight into
Monday morning.

There will continue to be a risk of a few showers or
thunderstorms Monday morning, but these will be fairly scattered
in nature with otherwise VFR conditions.

Outlook...
Monday afternoon through Tuesday...scattered to occasionally
more numerous showers and thunderstorms with attendant
MVFR...otherwise VFR.
Wednesday...VFR/MVFR in showers and thunderstorms.
Thursday...VFR/MVFR with scattered showers and thunderstorms.
Friday...VFR.

&&

Marine...
winds have shifted to the south as low pressure tracks to our
west and into southern Ontario province. It still will take a
few hours for waves to diminish across Western Lake Ontario
where small craft headlines remain in place.

Looking further ahead...a prolonged period of unsettled weather
is expected for most of next week...as a warm and humid airmass
will support frequent opportunities for scattered to occasionally
more numerous showers and thunderstorms...along with attendant
locally higher winds and waves. Outside of these...winds and waves
will generally remain below Small Craft Advisory levels.

&&

Buf watches/warnings/advisories...
New York...beach hazards statement until 5 PM EDT this afternoon for
nyz001-002.
Marine...Small Craft Advisory until 5 PM EDT this afternoon for loz042-
043.

&&

$$

Synopsis...apffel/jjr

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