Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 182300
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
700 PM EDT Thu Apr 18 2019

.SYNOPSIS...

Lifting warm front and showery conditions overnight makes way
for warm, humid conditions into Friday. A period of heavy rain-
fall is possible late Friday night into Saturday night as slow
moving low pressure approaches from the west. The low pressure
moves slowly across New England into early next week. Off and on
rain chances, perhaps a midweek dry period before additional
shower chances emerge late week into the following weekend.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...

7 pm update ...

Continued scattered shower activity as the warm front continues to
lift N. Low level isentropic ascent N brought per strengthening
S/SW flow ushering in a coincident blob of H7-9 higher theta-E.
Focused, lower cloud and visibility conditions sweeping N/E this
evening with scattered showers before drier air advects in across
the region just above the surface.

Increasing S winds, non-diurnal temperature trend overnight, rising
surface dewpoints. Expect improvement across the interior, however
along the coast adjacent to cooler ocean waters, expect lower dew-
point depressions, the persistence of patchy dense fog. Some
drizzle possible as well. Winds will be brisk, hence a bit less
confidence on any dense fog headlines. Thinking visibility will
be around 1 to 2 miles especially for SE-coastal New England
that includes the Cape and Islands.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...

Trough digging over the Eastern USA becomes a closed low Friday.
This further turns the upper flow out of the south, drawing
unseasonably humid air north. Precip Water values of 1.5 to 1.75
inches are forecast. This would be ho-hum for July, but not so
for mid-April. Average values for mid-April are 0.6 inches, and
high-end observed over the past 70 years have been around 1.4
inches.

Model time cross-sections concur, showing deep moisture
returning Friday afternoon and Friday night. Dynamics are less
helpful. Zone of favorable upper jet dynamics remains over NY/PA
during the period while low level jet and transport of moisture
doesn`t ramp up until Friday night. With this in mind, we will
maintain low-end chance pops much of the day, ramping up during
mid to late afternoon. Likely-to-categorical pops for Friday
evening and night, with highest values over Western MA and
Hartford County CT.

The low level jet, as mentioned, maintains a broad 50-knot zone
just offshore during the day. A 60-knot maxima then moves up the
coast late afternoon and evening with axis aimed mainly toward
our western areas. This supports the idea of higher pops in
Western sections during Friday night, as well as the start of
heavier rain especially in Western MA/Northern CT.

Mixing to 950 mb would support max sfc temps Friday in the mid
60s. Airmass over the southeast USA shows observed dew points in
the 50s and lower 60s. We will expect this for our area
Friday/Friday night in the warm sector south flow. So Friday
night min temps would be upper 50s and low 60s.

Gusty southwest winds continue due to the low level jet.
Mixing is limited, but even from 950 mb we would see 25-30
knots.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...

*/ Highlights ...

 - Saturday into Sunday, river flood concerns along with potential
   urban / poor drainage flooding ... also potential coastal
   flood issues along S-coastal New England

 - Monday into Tuesday, scattered shower activity along with
   cooler conditions prevails beneath the slow moving area of
   low pressure

 - Wednesday onward, cooler conditions with shower chance could
   possibly prevail

*/ Overview ...

Soaking rains. Echoes of a S Greenland block (+NAO) buckling the up-
stream pattern as progressive N Pacific flow maintains a continuous
feed of energy across the CONUS. Amplified H5 troughs, especially E
of the Rockies, dipping S, tapping the sub-tropics, pulling poleward
anomalous precipitable waters and much warmer air, slowed over the E
CONUS. H5 trough-ridge pattern, N Atlantic energy downshears by the
weekend into early next week yielding a rex-block that continues a
slowed upstream pattern. Depending on the aforementioned N Pacific
stream, could see a setup of on-again, off-again, decent rains that
are potentially soaking through the end of April. Sweeping frontal
boundaries, likely to see some temperature swings dictated by the
direction of the wind. Spring continues to prevail. Details as to
significant forecast concerns / threats and impacts below.

*/ Details ...

Saturday into Sunday ...

1.) Interior flooding:

High confidence mainstem CT per headwater snowmelt and forecast rain-
fall. MMEFS concerns also signaling some smaller streams throughout
S New England potentially seeing actionable to minor flood issues.

But not highly confident on significant interior flooding elsewhere.
Likely ponding on roadways and possible urban / poor-drainage flood
issues, but widespread impact is seemingly less with the latest 12z
suite of forecast guidance. Low undergoing full maturation towards
occlusion, dry air wrapping in within the mid-levels, losing deep
column moisture while it appears deep layer ascent is seemingly weak
to diffuse.

Widespread rain with embedded heavier showers and potential isolated
thunderstorms does appear likely and it looks like generally around
1-2 inches rainfall with locally higher amounts somewhere in S New
England. But not absolutely sure exactly where. Hopefully we`ll gain
greater confidence as convective allowing models become available as
to where and when heaviest rainfall will occur.

Focus Saturday, possibly lingering into Sunday, a slow moving system
up against a developing rex-block S of Greenland. Precipitable water
surge up around 1.5 inches beneath mid-level ascent and broader H3-5
diffluence aloft, but again, mid-level dry air could put the squash
on heavier rain outcomes. Lift/omega values up as high as around 20
micro-bars per second.

A feeling of a prolonged rain gradually adding up but not falling at
once, not exceeding flash flood guidance. FLOOD WATCH issued for the
W-half of S New England in collaboration with WPC and other WFOs. A
focus on Saturday to start coincident with EXCESSIVE RAINFALL out-
looks per WPC.

2.) Coastal flooding:

High astronomical tides plus longevity of S fetch, S-coast under the
greatest threat of inundation issues. MMEFS signaling tidal issues
at Westerly, RI. Particular focus within inner bays and sounds with
an eye on Narragansett and Buzzards Bay. Question whether if we push
over a foot of surge, then there`s concern. Continual monitoring,
but right now it appears most spots will fall short.

3.) Winds:

H925 S winds broadly speaking 50-60 mph over all S New England with
possibly 70 mph along the immediate S coast. As to how effectively
these winds mix-down to the surface will need to be watched closely,
as to whether precipitation drag or isolated thunderstorms will
bring momentum downward and pose wind-related hazards to the region.

Much stronger inversion along the coast with cooler waters beneath
much warmer air aloft, however the threat exists more so across the
interior, especially the high terrain. Nevertheless, persistence of
15 to 25 mph S winds since Friday will ultimately aid in S surge of
water along the S-coast and the threat of coastal flooding noted
above.

Monday into Tuesday ...

Beneath the low, showers swirl. Winds subside thus the coastal flood
threat subsides. Additional water into the system but should remain
fairly light. Mainstem rivers remain in flood, perhaps some smaller
streams as well. Lean cooler conditions as the rex-block evolves to
the immediate E blocking the escape of the low E.

Wednesday onward ...

Perhaps drying out, warming up ... or additional upstream Pacific
energy continues the threat of scattered showers and cooler weather.
But then again 18.12z indications that the eroding rex-block keeps a
persistent H5 trof lingering over SE Canada, additional energy and
shower chances dropping S along with cooler conditions. Forecast
guidance has been adjusting, no true confidence.

&&

.AVIATION /00Z FRIDAY THROUGH TUESDAY/...

Forecaster Confidence Levels...

Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Short Term /through Friday Night/...

Tonight...Moderate confidence.
Evening MVFR-IFR CIGs and VSBYs sweeping NE along with SCT SHRA
activity. Winds turning S, becoming brisk. An overnight interior
improvement, but likely IFR-LIFR settling in along the SE-coastal
terminals. Expect lower conditions fill back N across the interior,
becoming more widespread into Friday. LLWS threats as S/SW flow
2 kft agl will be increasing in excess of 40 kts, up to 50 kts
towards morning, especially over SE-coastal terminals.

Friday...Moderate confidence.
Widespread MVFR/IFR with patchy -RA/-DZ. SW gusts to 30 kt, more
so for interior, high terrain terminals close to the coastline
(i.e., ORH). Continued LLWS especially for the S-coast, Cape and
island terminals with S/SW winds 50-55 kt at 2 kft agl.

Friday night...Moderate confidence.
Widespread MVFR/IFR. Increased likelihood of -RA/RA, especially
from Worcester westward. Gusty winds with gusts to 30 kt. LLWS
continues.

KBOS terminal...
IFR CIG / MVFR-IFR VSBY issues through roughly 6z, thereafter an
improvement to IFR however CIGs remaining below 2 kft agl mostly,
continuing through Friday. LLWS threats through early Friday morning.

KBDL terminal...High
MVFR-IFR may be quick to clear out during the evening hours per
latest upstream trends and near-term forecast guidance. A brief
period of VFR, towards morning could see widespread MVFR-IFR
fill back in from the S with increasing S winds potentially
gusting as high as 30 kts.

Outlook /Saturday through Tuesday/...

Saturday: MVFR/IFR conditions possible. Windy with gusts up to
30 kt. RA, patchy BR.

Saturday Night: MVFR/IFR conditions possible. Windy with local
gusts up to 30 kt. RA likely, patchy FG.

Sunday: Mainly VFR, with local MVFR possible. Chance RA, chance
SHRA.

Sunday Night: VFR. Chance SHRA.

Monday: Mainly VFR, with local MVFR possible. Chance SHRA.

Monday Night through Tuesday: VFR. Chance SHRA.

&&

.MARINE...

Forecaster Confidence Levels...

Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.

Short Term /through Friday Night/...High confidence.

A low level jet about 2000 feet over the waters will persist
through Friday night. South-southeast winds this evening turn
from the southwest overnight. Sprinkles/drizzle/fog tonight and
Friday. Showers become more likely Friday night with reduced
visibility in showers and fog. Rough seas develop tonight and
linger through Friday night.

Outlook /Saturday through Tuesday/...

Saturday: Moderate risk for Small Craft Advisory winds with
gusts up to 30 kt. Rough seas up to 13 ft. Rain likely, patchy
fog. Local visibility 1 to 3 nm.

Saturday Night: Low risk for Small Craft Advisory winds with
local gusts up to 30 kt. Rough seas up to 13 ft. Rain likely,
patchy fog, slight chance of thunderstorms. Visibility 1 nm or
less.

Sunday: Winds less than 25 kt. Rough seas up to 12 ft. Chance
of rain, chance of rain showers, patchy fog. Local visibility
1 to 3 nm.

Sunday Night through Monday: Winds less than 25 kt. Areas of
rough seas. Chance of rain showers.

Monday Night: Winds less than 25 kt. Local rough seas. Chance
of rain showers.

Tuesday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Chance of rain showers.

&&

.TIDES/COASTAL FLOODING...

With the strong southerly winds aligned with astronomical high
tides due to the full moon late this week, there is a small chance
for that a few locations along the south coast may see minor
coastal flooding around high tide tonight. Most information
suggests some splashover but short of actual flooding. Will
monitor for potential coastal flood headlines.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...

CT...Flood Watch from late Friday night through Saturday evening
     for CTZ002>004.
MA...Flood Watch from late Friday night through Saturday evening
     for MAZ002>004-008>012.
RI...None.
MARINE...Small Craft Advisory until 8 AM EDT Saturday for ANZ231>237-
     250-251-254>256.
     Small Craft Advisory until 8 PM EDT this evening for ANZ230.

&&

$$

SYNOPSIS...WTB/Sipprell
NEAR TERM...WTB/Sipprell
SHORT TERM...WTB
LONG TERM...Sipprell
AVIATION...WTB/Sipprell
MARINE...WTB/Sipprell
TIDES/COASTAL FLOODING...WTB/Sipprell


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