Tropical Storm Warning 1003, NWS New Orleans, LA

Area Forecast Discussion
National Weather Service New Orleans LA
423 AM CDT Thu Jun 17 2021

.SYNOPSIS...

Surface boundary separating dew points in the 60s from much more
humid air is currently over our coastal waters and is reasonably
depicted on radar by a narrow band of showers and a few
thunderstorms. Upper ridging still over the southern Rockies, with
an upper trough over New England. Skies mostly clear across the
area, with comfortable temperatures in the mid and upper 60s
across most of the northern half of the area, and in the 70s for
most of the southern areas. New Orleans metro remains a little
warmer, as northeast winds are coming off Lake Pontchartrain water
temperatures that are in the upper 80s.

&&

.SHORT TERM (through Saturday night)...

Eyes will be on the Bay of Campeche today, monitoring the area of
disturbed weather in that area. Should start to see some northward
movement by late in the day, and if something is going to
organize, that should start happening overnight. Today will be
rather quiet regarding precipitation, with any showers and storms
remaining offshore. Dew points will remain in the 60s in most
areas, making conditions fairly comfortable, considering it's
mid-June. While NBM high temperature values today are a couple of
degrees above operational guidance, somewhat concerned that those
values may not even be warm enough, considering most of our CWA
was in the mid 90s yesterday. We're in line with our neighbors
right now, so will brief the day shift to keep an eye on it.

As the Bay of Campeche disturbance begins moving northward
tonight, so will the surface boundary, and moisture will rush back
to the north. BUFR soundings for New Orleans show precipitable
water values less than an inch this afternoon already back to 1.6
by Friday morning, and over 2 inches by midday Friday.
Precipitation chances will increase during the day on Friday from
south to north with likely to categorical PoPs by Friday evening.
Main threat of heavy rain will occur overnight Friday night
through Saturday. The area of most significant threat for heavy
rain and severe weather will be displaced to the right of any
circulation center. Still quite a bit of uncertainty on a track,
but there are enough indications that some portions of our area
will see very heavy rainfall over about a 36 hour period,
potentially 5-10 inches with locally higher amounts. Precipitable
water values expected to end up somewhere around 2.5 inches, which
is at the upper end of climatology for late June, so those amounts
are certainly achievable, even with a system like this. Right
now, best estimates would be from about Interstate 55 eastward,
but that definitely could shift one way or the other. As a track
becomes better defined, Flash Flood Watches will likely be issued
later today for portions of the area late Friday into Saturday.

Isolated tornadoes could definitely be an issue with rainbands to
the east of the low. NAM BUFR soundings showing 0-3KM helicities
well over 400 east of the track.

Onshore winds will push tide levels up as well. Fortunately, we're
near neap tide ranges. Current indications are surge levels should
be in the 1-2 foot range, but again, that'll be dependent on
strength and track of the system.

System should track northeast away from the area Saturday night
with the heavy rain threat diminishing.

Temperature guidance for Friday/Saturday looks reasonable
considering precipitation trends. 35

&&

.LONG TERM (Sunday and beyond)...

The primary concern for the first half of next week will be the
passage of a northern stream longwave trough and associated
frontal boundary next Tuesday into Wednesday. In advance of this
feature, a largely zonal flow regime in the upper levels will
combine with weak low to mid-level ridging to produce warmer and
more diurnally induced convection for Sunday and Monday.
Additionally, a lingering theta e axis and region of higher
precipitable water values could keep rain chances a bit more
elevated than expected for the overlying upper level pattern on
Sunday.

Starting with Sunday, the low pressure system impacting the area
late this week will be quickly departing to the east and northeast
with the primary forcing and rain impacts advecting into the Mid-
Atlantic states.  However, an axis of higher precipitable water
values is expected to trail behind this low pressure system and
stretch in a southwest oriented fashion back into the Gulf of Mexico
on Sunday.  Precipitable water values will be lower than observed on
Saturday, but should still remain fairly elevated in the 1.75 to 2
inch range through Sunday evening.  Although upper level support
will be trending downward, there should be enough residual omega in
place to support scattered to numerous showers and thunderstorms
through the day, and the threat of some locally heavy downpours and
street flooding issues will persist.  Temperatures will be near to
slightly below normal given the expected amount of shower and
thunderstorm in place through the day.  By Sunday night, conditions
will begin to improve as instability decreases and some weak
negative vorticity advection develops aloft.

By Monday, a more typical Summer pattern of diurnally induced
convection is expected to take hold.  The flow pattern aloft will be
zonal in nature, and a deep layer southwest flow pattern will be in
place in the low to mid-levels.  The one caveat is that precipitable
water values will remain elevated as an axis of deeper moisture
remains over the region with readings of around 2 inches expected.
This moisture will tap into a fairly unstable airmass to produce
scattered showers and thunderstorms throughout much of the day, and
once again some locally heavy downpours and isolated street flooding
events will be the primary concern.

The threat of stronger storms will increase on Tuesday and Wednesday
as a northern stream trough axis and an associated surface front
push through the area.  A band of convection could accompany the
front on Tuesday, and have higher POP in the likely range of around
60 percent to reflect this risk.  Temperatures are also forecast to
be a few degrees cooler than average due to the expected increase in
cloud cover and thunderstorm activity.  After reviewing model
soundings, the profile looks favorable for a few wind events to
potentially occur, and this is supported by DCAPE values in excess
of 1000 J/KG both Tuesday and Wednesday afternoons.  This pattern
and the potential for a few strong to possibly severe thunderstorms
will need to monitored over the weekend.  PG

&&

.AVIATION...

VFR conditions will persist at all of the terminals this morning.
Winds will generally be from the east and northeast through the
morning hours, and a shift to the southeast is expected during the
afternoon hours. Overall, no significant impacts to aviation are
expected through 12z Friday, but will expect deteriorating
conditions 12z-18z Friday for at least the southern terminals.

&&

.MARINE...

Seeing borderline Small Craft Exercise Caution conditions early
this morning in some of the waters, but those winds are on a
downward trend and won't carry the headlines. Conditions are
expected to deteriorate late tonight, with Small Craft Advisories
likely to be necessary Friday through Sunday if we don't end up
with tropical headlines.

&&

.PRELIMINARY POINT TEMPS/POPS...
MCB  91  68  87  72 /   0   0  50  50
BTR  91  71  87  74 /   0   0  70  60
ASD  92  73  86  75 /   0  10  80  70
MSY  93  78  87  77 /   0  20  80  80
GPT  91  75  85  76 /   0  20  80  70
PQL  91  73  85  74 /   0  20  70  70

&&

.LIX WATCHES/WARNINGS/ADVISORIES...
LA...None.
GM...None.
MS...None.
GM...None.
&&