Discussions from Mt. Holly and Upton.
.NEAR TERM /THROUGH TUESDAY/...
This remains a low confidence but potentially high impact forecast.
The main features to watch over the next 24 to 36 hours are the
closed low currently over the mid Mississippi Valley, the warm front
currently bisecting our region, and a mid level short wave trough
which was over AL/GA as of the 00Z soundings. The interaction of
these features is what is expected to result in cyclogenesis later
today off the GA/SC coast which will become the coastal storm to
impact our region tonight continuing through Tuesday.
As mentioned above, this is a low confidence forecast. The warm
front is much slower than any of the models had depicted even with
the 00Z runs. I am also concerned that at least some models appear
to have initialized with the center of the mid level low too far
south and east compared to what 00Z soundings and satellite trends
would suggest. If there is in fact more separation (distance wise)
between the mid level short wave trough and the closed low, then it
is possible the coastal low will develop further off shore.
Cyclogenesis occuring entirely off the coast is already a
notoriously high uncertainty pattern, and the previously mentioned
trends just add to my uncertainty with this forecast.
That being said, this has the potential to be high impact for our
region, so here is a summary of the potential hazards through
Heavy rain: Heavy rain at this point is my biggest concern, and the
area of concern hasn`t changed much, generally favoring coastal NJ.
There is potential that the axis of heaviest rain could be slightly
further south along the southern NJ shore, so Cape May and Atlantic
counties were added in to the watch. Additionally, based on guidance
trends, increased the QPF slightly.
Severe Thunderstorms: Limited instability, but significant shear
(both speed and directional shear) means that there is potential if
any thunderstorms (or even just showers with a stronger updraft)
develop, that stronger winds could mix down to the surface.
As far as timing, expect most of today to be dry, but some initial
showers could move in or develop (especially if the warm front is
still in the area) by late in the afternoon. The bulk of the rain
should be from late this evening through much of the day on
.SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
Major storm just off the coast will be in the process of pivoting
eastward Tuesday night and Wednesday as a digging vort max reaches
the Mid-Atlantic. As it reaches the base of the longer-wave trough
around 12z Wednesday, the orientation of the vorticity maximum will
allow for the increase in eastward momentum of the main upper low by
Wednesday afternoon. Of course, a number of questions remain about
this process, since the interactions among the various perturbations
and the predecessor impacts of widespread precipitation/convection
in tandem with the rapidly deepening low are generally low-
predictability phenomena. Nevertheless, with the notable model
trends on keeping the low closer to the coast and for a little while
longer, suspect stronger winds will linger near the coast for a time
Tuesday night and Wednesday morning, with gusts exceeding 35 to 40
mph probable along the immediate coast. Not out of the question a
wind advisory will be needed for these areas. Additionally, showers
will likely rotate south-southeastward on the west side of the low
in much of the area Tuesday evening, slowly shifting eastward as the
low begins its inevitable acceleration offshore. By Wednesday, most
if not all of the precipitation should be done for the region, with
breezy north winds also gradually declining during the day. Highs on
Wednesday should range from the mid 50s in the Poconos to the mid
60s in Delmarva.
Sky cover may diminish Wednesday night as north to northeast winds
slowly decrease, which may be favorable for some stronger cooling
than on the previous night. Still, think winds will not completely
decouple, so forecast lows are mainly in the mid 40s to around
.NEAR TERM /THROUGH TONIGHT/...
With a warm front north of the area for today, some mid level
ridging will be allowed to momentarily take place. A deep layered
southwesterly flow regime sets up for today allowing temperatures to
warm. As a result dew points readings will get into the lower and
even a few middle 60s, which is rather humid for late October. There
will be some instability into the afternoon and early evening,
however due to an overall lack of forcing not expecting really
anything impactful in terms of sensible weather through the day.
By this evening the jet axis begins to round the base of the upper
level trough off to our west as it swings east. As this occurs the
stronger mid and upper level forcing will begin to get underway.
This should result in moderate to heavy rain to develop towards and
after midnight across the area. Much of the guidance has a potent
mid level shortwave pivoting through during the the overnight
period, followed quickly by strong PVA at 500 mb into Tuesday
morning. The winds at 850 mb also begin to back from the south to
more out of the east. A strong LLJ gets into the region into the day
on Tuesday with a double barrel low type structure. The exact
placement and interactions of the low(s) will determine the
intensity of the LLJ, thus determining where the axis of heavier
rainfall will take place. Also there is the potential for strong
winds. With the low levels becoming more stabilized it will be
difficult to get the stronger winds down to the surface, at least
initially. With some elevated instability and a strong LLJ
developing into Tuesday morning expect more organized heavy rainfall
along with embedded convective elements, a few rumbles of thunder
are possible, if not likely. Over an inch of rain widespread across
the area is looking increasingly likely from midnight to the Tuesday
.SHORT TERM /TUESDAY THROUGH WEDNESDAY/...
Strong forcing due to a strong WAA pattern and the potential for 50
kt or greater easterly winds at 850 mb, along with strong PVA as the
upper level trough axis and 500 mb low gets just south of the area.
This should keep the best lift over the area with periods of heavy
rain throughout much of the day on Tuesday. Some guidance does have
some drying occurring in the mid levels late Tuesday into Tuesday
evening. This will depend on the exact placement of the area of low
pressure. If the low gets further north then some dry slotting may
occur which could bring some of the heavier rain to an end maybe a
few hours earlier than expected. In any event, a significant
rainfall event looks like a proverbial lock due to the deep and
strong WAA / Carlson conveyer belt set up. A strong LLJ gets going
during the day on Tuesday, with the model guidance of the LLJ
placement and intensity varying. Have issued a Wind Advisory for far
eastern sections of CT and LI for late Tuesday and Tuesday night.
There is the potential as the low deepens further and one low begins
to take over that the winds could peak at 45 to 50 mph across far
eastern sections with the winds more out of the northeast and north.
The mostly likely time for this would be during Tuesday evening and
into the first half of the overnight as the low may get slightly
further to the east. The heavier rain would then end from west to
east based on the latest timing. With the center of circulation
potentially getting further east as the low gets more vertically
stacked the storm should get to its peak in terms of intensity and
lower surface pressure readings. Rainfall totals as this time look
to be on the order of 2 to 4 inches. HREF 24 hr. PMM QPF has
localized amounts of 5 to 7 inches which is tough to come by.
Hopefully this scenario does not come to fruition as this would lead
to widespread flooding issues. Thus continuing with the Flood Watch
across the entire area for tonight into the day on Tuesday.
On Wednesday the low will gradually get further offshore, especially
by the afternoon. The current thinking has chance POPs giving way to
slight chance POPs from west to east through the day as the back
edge of any rainfall begins to pivot off to the east. With
completely dry conditions returning by the late day and evening with
mostly cloudy skies giving way to some clearing during this time
frame. A gusty northerly flow will have temperatures running near
normal with mainly lower 60s for afternoon highs.