.NEAR TERM /THROUGH TONIGHT/...
...Significant flash flood and severe weather event likely to
occur in much of the area today and tonight...
First concern to address is precipitation/convection this morning,
with long-lived storms continuing to affect northern Virginia, the
DC area, and adjacent portions of central Maryland. Although the
environment appears less favorable for our area, could see some
strong/severe slow-moving storms move into Delmarva and adjacent
portions of far southeast Pennsylvania and southern New Jersey this
morning. Will be monitoring this potential closely the next few
The remnants of Ida will be affecting the region today and tonight,
with significant impacts expected for the area. Model trends early
this morning continued a slight northward nudge of the highest QPF
axis, with the GFS/NAM/ECMWF in good agreement that 3+ inches of
rainfall will occur near/northwest of the Fall Line (with embedded
totals 6+ inches somewhere in the southern Poconos and/or Lehigh
Valley and vicinity). The CMC remains farther south (almost to
outlier status), with the main axis generally between I-76 and I-80.
The convection-allowing models (CAMs) are in between, and these are
generally in line with our current thinking this morning.
There are a few reasons for favoring a slightly farther south
solution to the main QPF axis. For one thing, models tend to shunt
warm sectors too far northward in advance of these systems,
particularly ones that are in the process of intensifying via large-
scale baroclinic processes. Additionally, models tend to bias
convective precipitation too far north, in association with the
synoptic forcing, rather than with the ambient/pooling instability
(which tends to win out with convectively-enhanced events like
these). Finally, suspect upslope contributions to precipitation are
being underrepresented farther southeast near the Fall Line to the I-
95 corridor. For these reasons, our latest QPF forecast did not
adjust totals as far north as the consensus of the coarser NWP
models. However, these discrepancies do lead to higher uncertainty
with forecast totals along the I-95 corridor. For example,
Philadelphia may end up anywhere between 1 and 5 inches of rain
based on the array of guidance available.
However, the risk of flash flooding is not only tied to total
rainfall but also duration, and the CAM solutions would suggest that
much of the rainfall south of the high-QPF axis will be occurring in
a relatively short period of time. Excessive rainfall rates (via PWs
well north of 2 inches) are likely with the main convective show
late this afternoon into the early overnight hours, and will likely
lead to several instances of flash flooding near the urban corridor.
Therefore, despite somewhat lowered QPF for the I-95 corridor
southeastward, the risk of flash flooding remains quite elevated
because of the rainfall rates. The HREF guidance provides some
insight here, with the 00z ensembles indicating probabilities of 1-h
rainfall accumulations exceeding flash flood guidance (FFG) above 50
percent in virtually all areas northwest of I-95 in our CWA during
the evening hours, despite a range of 3-5 inches in total QPF.
Notably, the probabilities of 6-h rainfall accumulations exceeding
FFG are above 90 percent in much of the same area, which conveys the
seriousness of the flooding threat for our area quite well.
Of course, that is not all. With the warm sector expected to shift
northward into at least the southern half of the CWA, the ambient
preconvective environment will be quite favorable for severe
weather. Model soundings indicate MLCAPE 1000-2000 J/kg (closer to
500-1000 J/kg near the warm front), 0-6 km bulk shear nearing 40 kt,
and SRH > 200 J/kg in the late afternoon and early evening hours.
This is an environment supportive of rotating storms, with CAM
simulations suggesting a mixed mode of short line segments and
cellular storms. Damaging wind gusts and tornadoes are the main
threats, and the SPC has nudged the enhanced risk slightly farther
north given model trends this morning. Given the coverage of
convection expected, the favorable environment is quite concerning,
with the severe potential continuing to look more and more
impressive. The main time window for severe weather is likely in the
20z to 06z time frame from west to east. Individual cells will
likely move quickly north-northeast, but the overall system will
move only slowly eastward, so training convection poses a flash
flood risk in the southern half of the CWA.
Models continue to trend faster with precipitation moving out of the
region, with most areas likely drying out by daybreak Thursday.