BOX issues Area Forecast Discussion (AFD) at Aug 27, 8:04 PM EDT
030
FXUS61 KBOX 280004
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
804 PM EDT Tue Aug 27 2024
.SYNOPSIS...
An approaching cold front may bring a few showers or even a
thunderstorm later tonight into Wednesday. Cooler and drier air
will follow Thursday and Friday as high pressure builds into New
England. More unsettled by the weekend with a couple of cold
frontal passages bringing a risk for showers and thunderstorms but
no washouts are expected. Drying out early next week, with
indications toward cooler than normal temperatures.
&&
.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
800 PM Update:
Ridging aloft with surface high pressure over the southeast
waters is allowing for tranquil weather conditions across
Southern New England, but it is a little more humid tonight
with dewpoints in the lower to mid 60s. Looking at continued
quiet weather in many areas for most of the overnight hrs.
However a couple things we're monitoring for the overnight into
the early Wed AM hrs.
First is the risk for south-coastal RI/MA fog and stratus. The
continued lighter SW flow and high pressure overhead of these
locations would seem to favor fog development, though only the
HRRR, HREF and the 18z NAM as well as the LAMP MOS guidance.
Carried mention of patchy fog for the immediate South Coast,
Cape and Islands as well as the adjacent waters; it is likely
not to make it much further north.
Second is the potential for late-overnight to early-morning hit
or miss showers or garden-variety t-storms. Leading edge of a
pretty well defined warm front is evident from about Ithaca NY
northward into the Ottawa ON vicinity. This feature is on the
leading edge of steeper lapse rates in the 700-500 mb layer,
along with dewpoints in the upper 60s to low 70s. The resulting
increase in elevated instability has generated a line of
scattered showers and embedded thunderstorms in the Finger Lakes
region in NY. These are moving toward the SSE and will peel off
to our SW later tonight and not pose any threat to us. Toward
the pre- dawn hrs however, noted a rise in most-unstable CAPEs
into the 500-750 J/kg range around 09-12z Wed into western and
central sections of Southern New England. Thus far, most of the
higher-res guidance show little to no activity. However did
note the 21z HRRR was showing a modeled cluster of echoes coming
off Lake Ontario into the Albany NY area around 09-11z Wed; it
is possible that incoming model guidance may be playing catch-up
and will need to be monitored overnight for increasing trends
in development and coverage (if any). PoPs were more or less
left unchanged - we were already carrying 20% PoP for isolated
showers - but I opted to include a mention of thunder given the
rise in most- unstable CAPEs/elevated instability. If storms
develop they wouldn't become severe, but if they materialize, a
couple areas in central and western MA and into northern CT
could wake up to early-morning thunder.
Previous discussion:
Pesky fog bank that moved onshore in Boston Harbor was eroding
quickly and should be gone by late afternoon. Meanwhile, another fog
bank was affecting Nantucket. This area should slowly expand as we
approach sunset and should bring return of fog/low clouds to Cape
Cod and Islands tonight due to prevailing moist SW flow. Otherwise,
diurnal clouds that have formed inland will dissipate around sunset.
Later tonight, approaching short wave and cold front from Great
Lakes will bring increasing cloudiness to SNE. This higher cloud
cover should also cause fog/low clouds near Cape Cod and Islands to
erode somewhat. High-res models and ensembles are in good agreement
that any convection should pass far to our north, closer to stronger
dynamics and surface low. Nonetheless, we could see some scattered
showers and possibly an embedded thunderstorm as this weakening
activity arrives toward sunrise Wednesday.
&&
.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Things have sped up a bit with latest guidance which means a
much lower threat of thunderstorms. In fact, high-res models and
ensembles, including ML guidance, point to severe threat staying
outside of SNE (more like mid Atlantic region).
Cold front will cross SNE during day, with notable wind shift by
afternoon which will bring drier air into region. During morning
however, we can't rule out at least some scattered showers as
there is sufficient moisture still in place, but at that time
any instability is limited to locations near South Coast (where
we could see an embedded thunderstorm). However, in post-frontal
airmass Wed afternoon there is significant drying aloft and an
overall lack of forcing (subsidence behind departing short wave)
such that many areas should remain dry. In fact, some of the
high-res models show a complete shutout Wed when it comes to
rainfall.
High pressure then builds into New England Wed night with cooler
and drier airmass in place.
&&
.LONG TERM /THURSDAY THROUGH TUESDAY/...
Highlights:
* Dry weather with onshore breezes late week. Temps generally
seasonable, but cooler highs near the coast.
* Turns a little more humid this weekend, wit a couple frontal
passages Sat and later on Sun or early Mon bringing opportunities
for showers and t-storms. No washouts are expected.
* Cooldown toward below normal temps early next week?
Details:
Thursday and Friday:
500 mb heights build aloft late in the workweek, as a sprawling
1024+ mb high pressure area ridges southeast from central Quebec
Thurs into the Gulf of Maine Thurs aftn and remains in place for
Fri. This will supply dry weather both days, while also bringing an
enhanced onshore flow, allowing for coastal areas to be a little
cooler during the daytime hrs than well inland. Tried to better show
this vs the diurnal trend in NBM. Good radiational cooling expected
Thurs and Fri nights and adjusted temps towards MOS based
approaches. Could have some fog too, but the coverage of this is
uncertain and left out of the forecast for now. Highs mostly in the
70s, although upper 60s near the coast.
The Weekend into Monday:
Pattern change occurs aloft in the northern CONUS toward rather
pronounced mean troughing. Looking at a couple of cold frontal
passages this weekend, which will bring a return to unsettled
weather, with an eventual shot of cooler than normal temperatures as
we move into early next week.
First cold frontal passage occurs Sat into Sat night. While most
global model output show decent modeled QPF associated with this
initial frontal passage, after a couple days of easterly onshore
flow, I've got some reservations on the quality of moisture. That
said, PoPs in the Chance range remain advertised with the risk for
scattered showers and t-storms. Don't see too much necessarily
impactful with this activity at this point. Secondary front then
arrives later Sunday or early into Monday, though the timing still
needs to be pinned down. While unsettled, at least at the moment no
day(s) this weekend into Monday are necessarily washouts, but there
should be some showers and t-storms around to dodge. However several
hrs of dry weather still are anticipated too.
Saturday and Sunday look mild but also rather humid too with highs
in the upper 70s to mid 80s, then begin a steady cooldown for Monday
in the 70s with dewpoints dropping into the 50s.
&&
.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
00z TAF Update: High confidence.
Tonight: High confidence.
VFR, with an increasing coverage of cloudiness for the 2nd half
of the overnight. IFR/LIFR fog to develop over the waters and
spread into the Cape and Islands later tonight into early Wed
AM. It is currently viewed as low-probability, but will need to
monitor potential for ISO/widely SCT SHRA/TS early Wed AM,
perhaps as soon as 09z in western New England. Not set in stone
that any develop, but if they do, they would linger into the
morning hrs. S/SW winds around 10kt.
Wednesday and Wednesday Night: High confidence.
VFR Wed with SW winds 10-15kt shifting to NW by afternoon and
increasing to 15-20kt with gusts to 25kt, then diminishing later
in day. May see some scattered showers or even embedded TS in
the morning before drying out Wed afternoon. VFR Wed night with
N/NE winds.
KBOS TAF...High confidence in TAF. VFR with SE winds initially
becoming SW and increasing to around 10 kt tonight. Outside
chance at SHRA/TS 14-17z with frontal passage, which will bring
a windshift in the afternoon to W/NW around 10-12 kt.
KBDL TAF...High confidence in TAF. VFR. Low prob of SHRA/TS
09-15z Wed, then a cold frontal passage bringing a windshift to
NW/N around 10 kt.
Outlook /Thursday through Sunday/...
Wednesday Night through Thursday: VFR. Breezy.
Thursday Night through Friday: VFR.
Friday Night: Mainly VFR, with local IFR possible.
Saturday and Sunday: Mainly VFR, with local IFR possible.
Chance SHRA.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Overall high confidence through Wed night.
Cold front approaches later tonight and crosses waters Wed.
Tightening gradient will bring period of strong S/SW winds Wed and
we have posted Small Craft Advisories for 25kt gusts, mainly focused
around Cape Cod and Islands. Also expecting widely scattered
showers, especially in morning. Behind front, winds shift to N/NW
and diminish Wed night as high pressure builds over region.
Outlook /Thursday through Sunday/...
Thursday: Winds less than 25 kt. Seas up to 5 ft.
Thursday Night through Friday Night: Winds less than 25 kt.
Seas locally approaching 5 ft.
Saturday and Sunday: Winds less than 25 kt. Slight chance of rain
showers.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory from 8 AM to 2 PM EDT Wednesday for
ANZ230-250-251.
Small Craft Advisory from 6 AM to 5 PM EDT Wednesday for
ANZ231>234.
Small Craft Advisory from 6 AM to 2 PM EDT Wednesday for
ANZ235-255-256.
&&
$$
SYNOPSIS...Loconto/JWD
NEAR TERM...Loconto/JWD
SHORT TERM...JWD
LONG TERM...Loconto
AVIATION...Loconto/JWD
MARINE...Loconto/JWD
Source: BOX issues Area Forecast Discussion (AFD) at Aug 27, 8:04 PM EDT (https://mesonet.agron.iastate.edu/p.php?pid=202408280004-KBOX-FXUS61-AFDBOX)
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