Scientific Forecaster Discussion
fxus61 kphi 220823
Area forecast discussion
National Weather Service Mount Holly New Jersey
423 am EDT Sat Jul 22 2017
a very warm and humid air mass will remain over the area through
this weekend and possibly into Monday. Meanwhile, low pressure
along a roughly west to east frontal boundary will result in
periods of showers and thunderstorms. By Tuesday, high pressure
will build over the area bringing somewhat milder and drier
conditions through through the middle of next week. A seasonally
strong cold front may cross the area later in the week.
Near term /until 6 PM this evening/...
concerns for today include temperatures/heat and then potential
for severe weather.
First, storms continue to develop/progress through
central/southern Delaware, Delaware Bay, and far southeast New
Jersey early this morning in an environment characterized by
MUCAPES > 2000 j/kg, pws near or above 2 inches, and substantial
isentropic lift in the low levels downstream of a weak
perturbation in the southern mid-Atlantic. Rain rates have been
impressive with the strongest storms (1-3 inches per hour), with
totals exceeding 0.5-1.0 inch(es) in portions of central
Delaware. Storms are not moving very quickly, so accumulations
greater than an inch are likely in southeastern Cumberland and
Cape May counties during the next hour or so. Storms should move
offshore after 6 am or so. Wind gusts, so far, have reached
30-35 mph with the strongest cores, and this should likely be
the case for the rest of the night.
Meanwhile, the forecast today has been updated to reflect latest
thinking on timing of convection moving through during the late
afternoon. Hi-res guidance is beginning to converge on the most
favorable timing generally after 3 PM from west to east across
the area. The 00z NAM nest is hinting at the potential for
multiple rounds of convection, with perhaps another convective
complex this evening. This increases Hydro concerns,
particularly given the troubling high quantitative precipitation forecast output from the NAM
nest (and nam), rap, UKMET, and CMC. Don't think this is Flood
Watch material (yet), but with more convection expected later
this weekend ... think the concerns will increase with time.
Would not be surprised to see a few flooding issues this
afternoon/tonight as well, though this may be more of the
urban/poor-drainage variety, for the most part.
Chance of severe storms today is certainly present. Forecast
soundings show 2000+ MLCAPE and deep-layer shear of 30+ kts this
afternoon, with effective shear approaching/exceeding 40 kts by
early evening. With moist midlevels and a well-mixed boundary
layer this afternoon, wet microburst potential is present, with
cold-pool organization likely given degree of instability/shear.
Shear looks to increase this evening, so the potential will be
present for embedded mesovortices within any convective
lines/clusters that develop. Think the risk of severe is high
enough for inclusion in the grids at this point.
Regarding temperatures, statistical guidance is generally in the
90-94 range for kphl today, with dew points around 70 during Max
heating. With uncertainty regarding maximum heating from
approaching cloud cover, there is simply not enough confidence
to justify issuing another heat advisory or excessive heat
warning. The best potential appears to be for the Delmarva
Peninsula, but even here, forecast heat indices look to be
around or just below yesterday's values...which were shy of
advisory criteria. Nevertheless, it will be uncomfortable again
outside, and heat stress from prolonged outdoor exposure or
strenuous activity is possible.
Short term /6 PM this evening through 6 am Sunday/...
low-confidence forecast overnight with potential for a round or
two of storms in a moderately unstable environment. With
adequate shear for severe storms, mention of this potential
remains through the early morning hours. However, with the first
round of convection likely to move through during the early
evening, there is some question as to what occurs thereafter. If
another convective complex enters the region, threat of heavy
rainfall (with some suggestion of this in most model output
currently) will be elevated, especially north of the Mason-Dixon
line (as a weak surface boundary slowly lifts northward).
However, there is a decent chance there will be too much
stabilization with the first round of storms to support
additional convection. Decreased pops after midnight but did not
eliminate them anywhere given the uncertainty.
With a slow-moving boundary, midlevel flow oriented parallel to
it, very high pws (2+ inches, generally), and potential for
training/upwind propagation effects, locally excessive rainfall
is possible. Too much uncertainty exists for a Flood Watch at
this time, especially given the relatively dry antecedent
conditions. Nevertheless, some local instances of poor-
drainage/urban flooding are probable and isolated flash flooding
Temperature forecast very uncertain given likely impacts from
precipitation. Large errors in the hourly forecast should be
anticipated if/when convection passes through, and low
temperatures may be augmented by rain-cooled air. Currently have
lows just below values observed this morning.
Long term /Sunday through Friday/...
this long term section has not had the opportunity to develop
confidence on the forecast scenario. Was working the thunderstorms
the that produced at least 1.5" of rain in parts of Cape May
County and reports of gusts near 30 kt early this morning.
500mb: a -1sd short wave in the Great Lakes region Sunday morning
crosses New York state and New England Monday with a short wave ridge to follow
Tuesday-Wednesday before short waves carve out a -1sd trough over
Quebec and New England next Thursday - Friday.
Temperatures: the first 21 days of the month were averaging about a
degree above normal near Mount Pocono to about 2.5 degrees above
normal elsewhere except almost 5 degrees above normal vicinity
Georgetown Delaware. Overall, expect an above normal month of July
temperature wise, despite a cool down to near normal next week.
Calendar day averages: should be 3 to 7 degrees above normal
Sunday and Monday then near normal Tuesday and Wednesday,
several degrees above normal next Thursday and possibly Friday.
Forecast basis: a 50 50 blend of the 00z/21 GFS/NAM MOS was applied
Sunday-Monday, then 00z/21 GFS mex MOS Monday night and Tuesday
before relying heavily on the 05z/21 wpc d4-8 elements for
Tuesday night- Friday. Exception: probability of precipitation were basically continuity
from yesterdays 330pm Mount Holly forecast through day 5, then
wpc probability of precipitation days 6-7.
Sunday...considerable cloudiness. A quasi-stationary east-west
frontal boundary is expected across our area. Low pressure along the
front will favor low-level convergence and areas of showers and
thunderstorms. Precipitable water values near 1.75 inches north and over 2 inches
Delmarva with 2400j mlc just to our west will support some heavy
downpours and possible flooding. Also Storm Prediction Center has a slight risk of
severe for Sunday. Timing is tricky.
Monday...partly sunny, very warm and still a bit humid (dewpoints
upper 60s to mid 70s) with 1100j of mlc. Severe potential. A cfp
should generate scattered afternoon and evening thunderstorms?
Tuesday and Wednesday...should be relatively pleasant with ridging
aloft and surface high pressure moving across the area. The high is
associated with a somewhat cooler and drier air mass. Partly to
mostly sunny Tuesday with a light northeast wind and partly to
mostly sunny Wednesday with a southeast wind.
Wednesday night...looks like some low-level warm advection develops
and the forecast carries a chance of showers at that time. South
Thursday...partly sunny. Chance of rain increases on Thursday as
another shortwave and associated cold front approach from the
northwest. Mlc increases to 2300j vicinity i95 and dewpoints are
near 70. Small chc of svr...timing dependent. Southwest wind shifts
to northwest at night.
Friday...uncertainty and followed wpc.
Aviation /08z Saturday through Wednesday/...
for the 06z tafs...VFR through the morning, except for
potential/temporary visibility restrictions at kmiv/kacy
through 12z. Clouds increase through the day, but cigs should
remain VFR outside convection. Storms expected to move east
through the area late this afternoon into early evening, with
timing generally between 21z and 03z. More storms are possible
thereafter, but confidence is low. Additionally, sub-VFR
cigs/vsbys may develop after storms move through, though
confidence is too low at this point for taf mention. Expect
frequent updates to the taf today as details become clearer.
There is potential for strong to severe storms today with gusty
to damaging downburst winds and torrential rainfall.
Sunday through Monday...generally VFR conditions expected, but
temporarily lower cigs/vsbys in scattered showers and thunderstorms
are expected at times.
Tuesday and Wednesday...VFR conditions with a light northeast
wind Tuesday becoming and then southeast Wednesday.
storms will impact the southern New Jersey and Delaware coastal
waters, as well as Delaware Bay, early this morning. Wind gusts
around 30 kts or so can be expected with the strongest storms,
as well as frequent lightning and torrential downpours. More
storms are expected this afternoon and tonight, with a higher
chance of severe weather. Strong winds are likely with the
strongest storms. Keep a close eye on the weather today if
venturing to sea.
Outside of convection, winds should generally be under 15 kts
but may be quite variable. By this afternoon, some favoring of a
south direction is likely. After storms pass tonight, some
preference for a west or northwest wind may be observed. Seas
should be 2-4 feet through the period, with locally higher
winds/seas near any thunderstorm.
Generally sub-Small Craft Advisory conditions are expected Sunday through Wednesday.
However winds and seas may be locally higher in scattered thunderstorms
over the waters from Sunday through Monday. Small chance of 5 foot
hazardous seas advisory Sunday evening.
The 10-12 second dominant period has returned early this
morning, and there are indications that this will continue
through the day. As such, the rip current risk is expected to be
in the moderate category today, especially for the New Jersey
coastal waters. The risk remains lower for the Delaware coastal
dew point readings at kdov continue to measure too high
compared to surrounding locations and should be treated as
unrepresentative of the area.