An unseasonably mild air mass was in place over much of New
York and New England for
December. Temperatures were running a good 15-20 degrees above normal prior to
the severe weather event. A backdoor cold front moved southward across the
forecast area Friday morning between 05Z-12Z FRI. The shallow cold air made it
far enough southward to roughly Hudson, NY.
Temperatures on the north side of the boundary fell into the mid 30s to lower
40s...except over the Berkshires, where temps stayed in the 60s the entire time
(Fig. 1). An 18Z KALY sounding was done (Fig. 2). It should be noted the depth
of the cold/cooler air was less than 1500 ft deep. This boundary would lift north
of ALY towards 23Z (Fig. 3), as an impressive line of convection was forecast
to develop ahead of the strong cold front approaching from the west. The WRF
model did the best job forecasting the backdoor surface cold frontal passage
through ALY and hanging it up north of KPOU. The cold air had the most
difficult time scouring out of the valleys prior to the severe convection.
Figure 1. MSLP and station plot.Note the extreme temperature and dew point
gradient through PA, NY and New
England associated with
Figure 2.ALB sounding at 1800 UTC 1 December.Note the extreme low level inversion and
conditional instability above 925 Mb and winds of 40-70 Kt between 850 Mb and
Figure 3.MSLP and station plots at 2100 UTC 1
December.Note the continued extreme
temperature and dew point gradient along the front through eastern NY and New
England, and the low pressure centered near SYR.
An exceptionally strong mid level trough quickly moved N/NE from the TN and OhioValley during the late morning. The
500 mb jet streak was around an impressive 120 kts. Portions of the ALY
forecast area would get into the left front quadrant of the jet streak...as the
upper level trough and its incipient sfc low pressure system moved across
western and northern NY in the late afternoon and early evening. The upper
level trough strengthened and turned negative pushing the main cold front
through the forecast area with multiple lines of severe convection ahead of the
front (Fig. 4).
Figure 4.Infrared satellite picture and lightning
overlay, 2200 UTC 1 December.Note the
frequent lightning associated with the thunderstorms well north into the cold
air, and the enhancement of the clouds, indicating deep convection.
At 1300 UTC..SPC upgraded the entire forecast area to a moderate risk (Fig. 5).
It actually was the first one for the ALY forecast area, since the significant
squall line of 6 June 2005. The last one before that event was 21-22 JUL 2003,
which was ALYs last big tornadic event.
The mesoscale environment was very volatile. It was a classic high shear-low
CAPE environment. 0-1 km bulk shear values of 50 kts
were forecasted. The deep layer shear (0-6 km) was progged to be in excess of
75 knots. 0-3 km helicity values off model hodographs produced values well in
excess of 500 m^2/s^2. A strong southerly 850 hPa low-level jet in excess of 60
kts advected in low-level theta-e rich air (Fig. 6). Surface dewpoints were in
excess of 15C in the southern portion of the forecast area. DCAPE values in
excess of 1000 J/kg were forecaseted by the GFS and WRF model runs at 12Z and
18Z. LCLs were low under 3kft. All signals of a possible widespread wind
damage/isolated tornado threat.
Figure 5.StormPredictionCenter Severe Weather Outlook for day one showing
a moderate risk for severe weather over much of the northeastern U.S., extremely rare, especially for December.
Figure 6.KENX Velocity Azimuth Display wind profile at
2200 UTC 1 December.Note the
directional and speed shear present in the lowest few thousand feet and winds
of 50-80 Kt between 4000 feet and 10000 feet.
A severe thunderstorm watch box was put up to the west of ALY forecast area
around noon to 1 pm. Jeff Evans, SPC lead forecaster called ALY, PHL and OKX to
coordinate a new severe thunderstorm watch box between 3-330 pm. Everyone
agreed a watch was necessary and the possibility of isolated tornadoes was
mentioned due to the dangerous low-level shear and 0-3 km hodographs. Jeff
initially had Hamilton and Warren counties out of the WCL proposed. He listened
to our suggestions to put them in. Adding these counties worked out well...as
multiple Quasi Linear Convective Systems (QLCSs) approached eastern NY. The
QLCSs coalesced into two main lines between 5-730 pm as they pushed through the
forecast area (Figs. 7-11).
(Click on the next 3 images for
Figure 7.Regional radar mosaic of base reflectivity at
the 0.5° elevation angle with lightning overlay at 2100 UTC 1 December.Click on figure for movie showing blinking
Figure 8.KTYX base reflectivity at the 0.5° elevation
angle with lightning overlay at 2130 UTC 1 December.Click on figure for movie showing alternate
reflectivity and base velocity.Note the
extreme winds indicated in the base velocity displays associated with the
A highly concentrated tornado watch box was proposed by Rich Thompson, SPC lead
forecaster, at approximately .
The tornado threat was increasing based on tornado warnings issued and damage
coming in possibly related to them in Luzerne and Pike counties. Ulster,
Dutchess and Litchfield counties were put in the Tornado Watch box (a watch
that would last less than 4 hours).
Figure 9.KBGM base reflectivity at the 0.5° elevation
angle with lightning overlay at 2145 UTC 1 December.Click on figure for movie of alternating
reflectivity and base velocity. Note the extreme winds indicated in the base
velocity displays associated with the bowing segments.
Figure 10.KENX base reflectivity at the 0.5° elevation
angle at 2327 UTC 1 December.Note the
lines of thunderstorms with bowing segments over the region.
Figure 11.KENX echo tops with lightning overlay at 2200
UTC 1 December.Note the frequent
lightning throughout the area of thunderstorms, and the echo tops between 30000
feet and 40000 feet, which is very deep convection for early December.
Many more SVRs were issued with lots of wind damage from bowing segments in the
strongly forced convective line(s). The initial line weakened a bit over the
eastern Capital District possibly due to the slightly cooler/more stable air in
the boundary layer. However...the line refired heading into western New England
and the mid Hudson Valley. Three Tors were issued. One was in the mountainous
terrain of Ulster County...where beam blockage occurs. Modest rotation existed
with a weak mesocyclone detected. The circulation strengthened passing into
western Dutchess county with several scans of a meso detected with a TVS (Fig.
12). At one point the cell exhibited 29 kts of rotation gate to gate with a
VR-shear value of 0.033 s-1 on the 0.5 elevation angle of VCP-12. From past
research this was on the low or marginal end for a tornado warning and very
justifiable. A tornado warning was issued based on this. The last tornado
warning was issued in LitchfieldCounty
after an SVR was issued. OKX and ENX radars showed modest mid-level rotation
with the radar beams up at 5-6 kft. It was a tough call on this one.
Figure 12.KENX Storm Relative Velocity (SRM) at the 0.5°
elevation angle with TVS and legacy mesocyclone overlay at a) 2323 UTC 1
December and b) 2327 UTC 1 December.Note the TVS and mesocyclone indicated in addition to the rotation in
the SRM display.