BOX issues Area Forecast Discussion (AFD) at Aug 8, 8:25 PM EDT626
FXUS61 KBOX 090025
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
825 PM EDT Thu Aug 8 2024
.SYNOPSIS...
A few showers possible into tonight, with increased chances for rain
Friday. Debby makes its closest approach to our area Friday night
into Saturday. Debby's passage will bring showers with pockets of
heavier rains and a low risk for severe weather across interior
southern New England. Other than a period of light to moderate rains
Friday night into Saturday morning, little to no impacts are
expected for eastern MA and RI. Rapid improvement Saturday
afternoon. Increasing west breezes will herald an extended stretch
of pleasant and dry weather Sunday into midweek.
&&
.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
Made some tweaks to the forecast this evening based on regional
radar trends and the last few runs of the HRRR and RAP, which
seemed to have the best handle on the current conditions.
Temperatures were a little lower than forecast, so adjusted
those as well over the next few hours. Dew points were generally
in the lower 60s, so that should be the lower limit for low
temperatures tonight. Expecting rounds of showers, especially
across the western half of southern New England overnight.
Previous Discussion...
After being generally dry north/wet south we're seeing the high
pressure slowly begin to retreat as the remnants of Debby push
further north toward the mid Atlantic. As this happens more moisture
is being advected north in the form of more widespread cloudcover
and scattered showers. This is the result of a weak mid level warm
front that is lifting north and will bring isolated to scattered
showers the rest of the evening. Overall the lower part of the
atmosphere will be quite saturated with low clouds and fog expected
to expand overnight.
&&
.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
The surface warm front then approaches from the south on Friday,
crossing SNE and pumping up the humidity (dewpoints will be back
into the tropical-feel category, in the low to mid 70s). This comes
as the remnants of Debby begin to pass to our west. This environment
will be somewhat conducive to isolated/scattered showers and
thunderstorms though not seeing a lot of breaks in the clouds during
the day; not expecting coverage to be very widespread until the
better forcing arrives from the west late in the day. More on the
threats from Debby's remnants below:
Heavy rainfall and localized flood potential:
Given the track of the surface low well west of southern New England
the heavy rain and flood threat is greatly diminished. However,
given that we expect up to a few inches of rain to fall over far
western MA/CT, locations that end up beneath prolonged downpours
could see some localized street flooding. Western SNE continues to
be in a Slight Risk (level 2 of 4) of excessive rainfall, so
something to keep an eye on. Rain comes to an end for western zones
by 8AM, moving off of the eastern MA coast through the afternoon.
Severe Weather:
Environment becomes favorable for short-lived tornadoes as remnants
of Debby track to the west of New England later Friday afternoon and
evening. These situations often feature very high shear in lowest
levels and low LCLs but in order to support tornado formation we
need instability in lowest levels (0-3km CAPE) and either surface
boundary or mesolow to focus low level spin.
In this case, we're seeing several favorable signals in model
guidance, especially for western thirds of MA/CT. First, there is
sufficient 0-3km CAPE (over 200 J/kg) as warm front lifts through
during late afternoon and early evening. Local CSTAR research has
shown that we only need to see values of 25-75 J/kg in tropical
remnant environments so that should easily be attainable. Second,
environment itself favors high amounts of shear in lowest levels and
in this case we have over 200 m2s2 of 0-1km shear, much of which is
focused in 0-500m layer. Research has also shown the 0-500m layer
can be a good discriminator for tornadoes vs non tornado
environments (values of 95 or higher). Lastly, LCL heights are down
to 500m which is below the typical height of about 700m.
While these factors line up very well and have support from SREF
Tornado Ingredients, it's not always a lock in these situations.
Certainly we'll be on the eastern edge of more favorable area
(region closer to the actual remnant low itself). But we have to
watch its warm front extending eastward as well as possibility of
mesolow formation along boundary to help enhance spinup. It's also
possible to have all of this occur without any lightning.
It's important to stress that these types of circulations, while
potentially supportive of tornadoes, tend to be very shallow in
nature and do not last very long. Radar detection can be difficult
especially at farther distances from the radar. These situations are
usually handled by issuing Severe Thunderstorm Warnings with the
"Tornado Possible" tag since it is difficult to track these
transient couplets. However, any longer-lived couplets that do
manage to form can be handled with Tornado Warnings.
Marine/Rip Currents:
Southerly winds will be on the increase Friday night and
Saturday morning leading to some gusty winds, mainly over the
waters. A Gale Watch is in effect for the southern waters with
Small Craft Advisories elsewhere. Building wave heights and the
southerly flow will lead to a risk for increased surf and rip
currents for south- facing beaches Friday night into Saturday.
&&
.LONG TERM /SATURDAY THROUGH THURSDAY/...
Highlights:
* Rains ends for all areas by Sat morning, with rapid improvement in
conditions and increasing west breezes Sat afternoon.
* Extended stretch of pleasant weather for Sat night through
midweek, with slightly below normal temperatures, dry weather and
low humidity.
Details:
Quite the pattern shift expected for this portion of the forecast.
Expecting a broad mid level trough to linger in the vicinity of the
Great Lakes this weekend, before slowly moving off to the NE and
reaching the Maritimes late Tuesday. At the surface, the remnants of
Debby should move into the Saint Lawrence Valley by Saturday
morning, with a cold front sweeping across our region in the
afternoon and overnight. Some question as to how fast this front
will move, but expecting this front to be well offshore by Sunday
morning. After then, a large high pressure over central North
America will slowly build in from the west.
Lingering showers Saturday morning, especially across RI and the
eastern half of MA. These showers end from west to east during the
day, with all areas dry by Saturday evening. This rain-free weather
then expected to continue through at least Tuesday. Some hints for
some diurnal showers Wednesday and Thursday. However, those details
will need time to come into better focus.
Plenty of sunshine should lead to near to slightly below normal
temperatures. Pressure gradient also looks to be weak enough, where
seabreezes are possible each day from Tuesday to Thursday.
&&
.AVIATION /00Z FRIDAY THROUGH TUESDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Tonight...Moderate Confidence.
MVFR to IFR from south to north, with a few showers spreading
north across the region. Light east to southeast winds.
Friday...Moderate Confidence.
MVFR/IFR to start, improving trend to VFR/MVFR from south to
north with showers and thunderstorms moving in during the
afternoon from west to east. Increasing southerly winds 10 to 15
knots with gusts up to 25 knots along the south coast, Cape, and
Islands.
Friday night...Moderate Confidence.
IFR in SHRA/+SHRA overnight. S winds 10-15 kts
gusting 20-30 kts.
KBOS TAF... High confidence in TAF trends. Moderate confidence
in timing.
MVFR dropping to IFR 04-06z with periods of showers. Continued risk
for scattered showers Friday.
KBDL TAF... High confidence in TAF.
MVFR lowering to IFR with periods of showers overnight.
Outlook /Saturday through Tuesday/...
Saturday: Mainly VFR, with local MVFR possible. Windy with
local gusts up to 30 kt. Chance SHRA.
Saturday Night: VFR.
Sunday: VFR. Breezy.
Sunday Night through Tuesday: VFR.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Through Friday night...High Confidence.
Winds become SE tonight with seas 2-4 feet. Friday into Friday
night seas will build, especially over the southern waters
reaching 8-9 feet by Friday night.
Outlook /Saturday through Tuesday/...
Saturday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Rough seas up to 10 ft. Rain showers likely.
Saturday Night: Winds less than 25 kt. Local rough seas.
Sunday: Winds less than 25 kt. Areas of seas approaching 5 ft.
Sunday Night: Winds less than 25 kt. Seas locally approaching
5 ft.
Monday through Tuesday: Winds less than 25 kt.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 8 PM Friday to 10 AM EDT Saturday
for ANZ230-236.
Small Craft Advisory from 8 PM Friday to 2 PM EDT Saturday for
ANZ231-250-251.
Gale Watch from Friday afternoon through Saturday afternoon
for ANZ232-254-255.
Gale Watch from Friday afternoon through Saturday morning for
ANZ233>235-237-256.
&&
$$
SYNOPSIS...Belk/BW
NEAR TERM...BW
SHORT TERM...BW
LONG TERM...Belk
AVIATION...Belk/BW
MARINE...Belk/BW
Source:
BOX issues Area Forecast Discussion (AFD) at Aug 8, 8:25 PM EDT----------------
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