BOX issues Area Forecast Discussion (AFD) at Sep 4, 1:25 PM EDT328
FXUS61 KBOX 041725
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
125 PM EDT Wed Sep 4 2024
.SYNOPSIS...
Dry weather prevails through Thursday accompanied by seasonably
warm days and cool nights. Cooler than seasonable temperatures
on Friday with prolonged onshore flow and stratus. Shower
chances return later on Saturday into Sunday with seasonable
temperatures. Turning drier early next week.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
115 PM Update...
Forecast remains on track. No changes in latest update.
Previous discussion...
High pres remains in control today as it shifts east of New Eng
this afternoon. Despite a weak shortwave approaching from the
west, the column is dry and will support abundant sunshine again
today. Low level temps are a few degrees warmer than yesterday
so highs should reach the upper 70s with a few locations near
80, but cooler along the immediate coast where sea-breezes
develop. A spectacular late summer day with seasonably warm
temps, light winds and low humidity.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/...
Tonight...
The center of the high pres shifts to the east, but surface
ridging will persist across New Eng with clear skies and light
winds. Another cool night with good radiational cooling and
used colder MOS guidance to derive the low temps, which range
from upper 40s to mid 50s. Patchy late night fog may develop in
the CT valley.
Thursday...
Not much change as surface ridging and subsidence brings
sunshine and seasonably warm temps. Lower clouds will be
increasing south of New Eng as easterly flow becomes
established with increasing low level moisture, but these
clouds will hold off until Thu night. Highs will reach upper
70s again, but cooler along the immediate coast as E-SE flow
develops. Comfortable humidity levels continue with dewpoints
in the low- mid 50s.
&&
.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
Highlights
* Cool on Fri with stratus and prolonged onshore flow. Could see
some spotty drizzle, but overall anticipating a dry forecast.
* Next shot for more widespread showers comes later on Sat into Sun.
Dry weather returns early next week.
* Higher surf Fri into Sat and perhaps lingering into Sun could
result in moderate to high rip currents across eastern facing
beaches.
Thursday Night through Friday...
A shortwave ridge will initially be over the eastern Great Lakes
with a meandering shortwave over New England Thu evening. The
shortwave lifts toward northern New England/Nova Scotia by late Fri,
while the shortwave ridge builds into New England. During this
timeframe a coastal storm develops well offshore to our southeast,
and lifts east of SNE by late Fri. High pressure nudges in from Nova
Scotia and northern New England through this period.
Generally expecting dry and quiet weather to dominate through this
timeframe. Though given we will have prolonged onshore flow am
anticipating stratus to advect in starting Thu Night and continuing
through much of Friday, especially across eastern areas per lowest
1000-850 hPa RH values. The latest GFS keeps us dry whereas the
ECMWF/GEM indicating some light shower activity. Ensembles also
leaning toward some light QPF per moderate to high (30-70+ percent)
probs of 24 hr precip AOA 0.01 inches. Given the weak forcing and
PWATs between 0.75 to 1 inch am leaning toward spotty drizzle being
possible. Still sticking with a dry forecast at this point. Should
note that the NAM guidance appears to be an outlier, so have thrown
out, but this brings the coastal storm into our area by late Fri.
This would bring stronger winds and better chances for rain.
Main change to the forecast for this timeframe was to significantly
increase sky cover and lower temperatures as NBM was much to
low/warm given this setup. For now stuck with the 25th percentile
for high temps on Fri and generally blended up sky cover based on
the consensus of low level RH values. The result are highs in the
70s. Though suspect these temps may need to be lowered further in
future updates, especially if more cloudiness is anticipated.
Saturday through Sunday...
A trough/cutoff will initially be situated over the central Great
Lakes/Ontario late on Fri. The trough rotates southeastward into the
central/eastern Great Lakes by late Sat, which swings a shortwave
into southern New England on Sat. The cutoff/trough continues to
rotate eastward into the eastern Great Lakes by late Sun with
another shortwave rotating into New England on Sun. The setup is a
bit complex with the coastal storm still east of us on Sat, but
lifting toward Nova Scotia by late in the day. A cold front slides
into the eastern Great Lakes on Sat and perhaps into southern New
England by late Sat or Sun.
As mentioned yesterday this is the next potential opportunity for
more widespread shower activity across southern New England. Should
note that with the setup it appears that a good portion of Sat could
be dry as high pressure nudges in from the north, but there are
differences in timing the front moving in from the west.
Deterministic and ensemble guidance has been trending later and
later from run to run, so have followed this trend in the latest
forecast. Current thinking is most of southern New England dry until
Sat afternoon/evening, so have dialed back NBM precip chances. This
matches up fairly well with latest EPS/GEFS guidance decreasing
probs from run to run of QPF AOA 0.10 inches (was roughly 40-80% in
previous and has lowered to 30-70% in current). Still wondering if
western portions of the region can tap into some higher moisture
values due to prolonged southerly flow, which could bring a brief
heavy rain risk. Though PWATs in area are still only 1-1.5 STD above
model climo per NAEFS/EPS.
Early next week...
Cyclonic flow persists with the cutoff low rotating into Quebec by
Mon and perhaps still in place over portions of Quebec on Tue. Given
the position of the cutoff it still looks like the dry slot punches
in/through, which should keep our weather dry. Confidence in the
specific details still lower than would like as how quickly the
cutoff lifts in/through is always tricky to time. For now the NBM
seems reasonable with a dry forecast and near seasonable temps.
&&
.AVIATION /17Z WEDNESDAY THROUGH SUNDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
18z TAF Update:
Today through Thursday...High confidence.
VFR, except patchy late night fog in the CT valley. Wind
becoming south 5 to 10 knots this afternoon with sea-breezes
along the coast. Light to calm wind tonight, then East to
Southeast wind 5 to 12 knots Thursday.
Thursday Night... Moderate confidence.
VFR, becoming MVFR with localized IFR. Mainly dry, through there
could be areas of light rain or drizzle mainly across eastern
MA. East winds 10 to 15 knots along the coast, light and
variable across the interior.
KBOS TAF...High confidence in TAF.
VFR with sea-breeze through 00z, becoming south/southwest
overnight, will have another sea-breeze develop on Thursday
around 15z.
KBDL TAF...High confidence in TAF.
VFR with light south winds for the afternoon and then light and
variable tonight. East/southeast winds Thursday 5 to 8 knots.
Outlook /Thursday Night through Sunday/...
Thursday through Thursday Night: VFR.
Friday: VFR. Breezy.
Friday Night: Mainly VFR, with local IFR possible. Breezy.
Saturday: Mainly VFR, with local MVFR possible. Slight chance
SHRA.
Saturday Night through Sunday: Mainly VFR, with local MVFR
possible. Chance SHRA.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Today through Thursday... High confidence.
Tranquil boating conditions through tonight with light winds and
seas. Easterly winds increase to 10-15 kt Thu with G20 kt
developing over southern waters leading to seas building to 4
ft.
Outlook /Thursday Night through Sunday/...
Thursday: Winds less than 25 kt. Seas locally approaching 5 ft.
Thursday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft.
Friday: Winds less than 25 kt. Seas up to 5 ft.
Friday Night: Winds less than 25 kt. Areas of rough seas.
Saturday: Winds less than 25 kt. Areas of rough seas. Slight
chance of rain showers.
Saturday Night through Sunday: Winds less than 25 kt. Seas up
to 5 ft. Chance of rain showers.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.
&&
$$
SYNOPSIS...KJC/BL
NEAR TERM...KJC/Dooley/BL
SHORT TERM...KJC
LONG TERM...BL
AVIATION...KJC/Dooley/BL
MARINE...KJC/BL
Source:
BOX issues Area Forecast Discussion (AFD) at Sep 4, 1:25 PM EDT----------------
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