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Area forecast discussion
National Weather Service Buffalo New York
629 am EDT Tue Aug 20 2019

Synopsis...
behind a weak cool front, a dry day is expected across our region
with increasing levels of humidity this afternoon. A few showers or
thunderstorms will form near the New York and Pennsylvania state
line this afternoon, with scattered convection expanding across the
southern tier this evening, and eventually across most of the region
by Wednesday morning. A cold front will cross the region tomorrow,
with additional showers and heavy thunderstorms before much drier
and cooler weather expands across our region for Thursday.

&&

Near term /through tonight/...
GOES satellite imagery displays the region clear this morning, with
just a few passing cirrus clouds. The river valleys of the southern
tier are filling with fog, as seen from both GOES satellite imagery
and supported by surface observations.

High pressure at the surface is over our region this morning with
light surface winds. Sunny skies and daytime mixing of 850 hpa
temperatures in the mid-teens celsius will bring afternoon
temperatures into the low to mid 80s.

As this surface high slides eastward a southerly flow will begin to
draw deeper moisture from the south northward. A few showers or
thunderstorms near the New York/PA state line this late afternoon will
push deeper into the southern tier this evening as
moisture/instability increases from the south. A convective
shortwave that is sparking T-storms over Iowa this morning will rotate
close to our region late tonight...with models tracking this feature
a tad farther to the south.

This shortwave should again Blossom showers and a few thunderstorms
late tonight and towards dawn Wednesday morning. Highest chances
will be towards the southern tier which will lie closer to the
shortwave and deeper lift ahead of the feature. There will be plenty
of instability to end the night, with MUCAPE values of 1000 to 1500
j/kg across wny by the end of the night. Also with the warm cloud
layer increasing, a few of these thunderstorms could produce
localized heavier rain.

&&

Short term /Wednesday through Friday night/...
a significant pattern change will bring a much cooler, fall-like
airmass across the lower Great Lakes late this week. Zonal upper
level flow in place to start this week will be replaced by strong
troughing across much of the Great Lakes Thursday into Friday. The
change will be accompanied by strong to severe thunderstorms on
Wednesday out ahead of primary cold front that swings through
Wednesday evening. At the least, should see scattered storms on
Wednesday with gusty winds and heavy downpours as a more humid
airmass streams in ahead of the cold front.

Starting the day on Wednesday, a leading shortwave, likely enhanced
by upstream convection just starting to develop from the Central
Plains to the middle Mississippi River valley, will be tracking into
western Pennsylvania and maybe as far north as southern tier during
the morning. If anything, recent model trends show this wave staying
mainly to our south, so will keep that going theme in our forecast
with only some small pops across the southern tier early in the day.
Since the wave will only graze our area, or so it appears at this
time, subsidence behind the wave and/or lingering cloud cover
probably will not be significant enough to wipe out convection
chances late morning Onward as daytime heating boosts MLCAPES to
1500-2500j/kg.

Though effective shear of 20-25 kts initially is on the weaker side
for organized convection, arrival of very strong upper jet over 110
kts should ramp up effective shear toward 40 kts by mid to late
aftn. There is also hints of additional convective shortwaves or an
mesoscale convective vortex along with a pre frontal sfc trough moving into western New York late
in the day just ahead of main cold front. All in all, overall theme
of forecast remains the same, but is now supplemented by a marginal
to slight risk of severe storms per latest Storm Prediction Center day 2 outlook. The
Bump up in risk from previous day3 (general thunder) to new day 2
looks very reasonable. Have kept mention of heavy rain in grids
(with precipitable waters rising toward 1.5 inches) and added gusty winds as well.
Strongest storms could also produce hail, though wbzero heights over
11kft in warm, moist airmass should limit that potential.

Slightly slower cold frontal passage compared to previous model
runs, so kept mention of some thunder in for Wednesday evening. By
late evening and overnight, threat of thunder should end and will
just be left with diminishing shower chances. Main chances for
showers late Wednesday night into Thursday will occur to east of
Lake Ontario closer to edge of broad upper level troughing and where
could be some embedded shortwaves working through and where WNW
winds provide additional upslope lifting. There is even a small
chance of seeing some lingering showers toward western NY, mainly in
the morning. Chances too low attm to put some pops in the forecast.
If some showers do occur, would be mainly in the morning. Otherwise,
strong low-level inversion around 5kft along with daytime heating of
the cooler airmass aloft and moisture below the inversion should
result in plenty of stratocu clouds and a partly to mostly cloudy
sky cover late morning into the afternoon. After a couple days with
high temps reaching the low to mid 80s through Wednesday along with
humid conditions, high temps on Thursday will only reach the low to
mid 70s. Should be pretty quick transition to less humid conditions
on Thursday with dewpoints dropping into the lower 50s by Thursday
afternoon. Will be breezy on Thursday especially closer to the Lake
Ontario shore.

Real taste of fall occurs Thursday night through Friday night as
overnight mins will range from the mid 50s near the lakes to the 40s
in the cooler southern tier valleys and the typical cooler spots to
east of Lake Ontario in Jefferson and Lewis counties.

&&

Long term /Saturday through Monday/...
good model consensus that general ridging will be found over the
region through this period. The resulting fair dry weather will be
accompanied by subtle day to day warming, so that near to below
normal temperatures at the start of the period will climb above
normal by the start of the new work week.

&&

Aviation /12z Tuesday through Saturday/...
for the 12z tafs VFR flight conditions are found, with the
exception of River Valley fog forming in the southern tier. Here
IFR flight conditions are likely through an hour or two past
sunrise. The region will remain dry with light winds as a
surface high passes over the region. This feature will exit to
the east later today, allowing for an isolated shower or
thunderstorm to creep north of the state line and into the
southern tier. As moisture builds and instability increases
tonight showers will become more in numbers, with scattered
activiy likely across wny by the end of tonight. Flight
conditions will likely persist at VFR levels...though some MVFR
flight conditions in ceiling heights are possible across the
southern tier with the deepening low level moisture.

Outlook...

Wednesday...VFR with areas of MVFR/IFR in thunderstorms. Some storms
may be strong with gusty winds and hail.
Thursday through Saturday...mainly VFR.

&&

Marine...
high pressure will continue light winds and waves on the lakes today
and tonight.

A southerly flow will increase Wednesday ahead of a cold front with
waves building to 2-3 feet on Lake Erie. A cold front will swing
across the lower lakes Wednesday evening and early overnight...with
waves and winds increasing within the cold air advection
environment.

West-northwest winds over the shorter fetch of Lake Erie will build waves 2-3
feet Wednesday night and Thursday, while a longer fetch and slightly
stronger wind flow over Lake Ontario could bring waves 5 feet or
greater on the Southern Lake Ontario shoreline. Small craft
advisories may be needed later this week for these higher winds and
waves on Lake Ontario.

&&

Tides/coastal flooding...
a strong cold front will cross the lower Great Lakes Wednesday
evening. Strengthening northwesterlies in the wake of the front will
increase wave action and combine with already High Lake levels to
produce the potential for significant shoreline erosion and flooding
Wednesday night into Thursday evening. A Lakeshore Flood Watch
continues from Niagara County to Oswego County.

&&

Buf watches/warnings/advisories...
New York...Lakeshore Flood Watch from Wednesday evening through Thursday
evening for nyz004>006.
Lakeshore Flood Watch from Wednesday evening through Thursday
afternoon for nyz001>003.
Marine...none.

&&

$$

Synopsis...Thomas
near term...Thomas
short term...jla

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