BOX issues Area Forecast Discussion (AFD) at Sep 7, 7:36 AM EDT764
FXUS61 KBOX 071136
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
736 AM EDT Sat Sep 7 2024
.SYNOPSIS...
A cold front moving through southern New England late today
into the evening will bring a period of showers to most areas,
along with a few embedded thunderstorms. Clearing and cooler
behind the front for late tonight and Sunday. High pressure
will be in control through the upcoming week with temperatures
trending upward leading to unseasonably warm conditions toward
the end of the week.
&&
.NEAR TERM /THROUGH TONIGHT/...
730 AM Update...
Only change in the latest update was to remove fog from the
immediate near term other than along the higher terrain given
the onshore flow. Otherwise forecast looks on track at this
point with stratus gradually eroding this AM.
Previous discussion...
Areas of stratus will linger into the morning to start the day, but
mixing of the boundary layer and some drying will result in lifting
cloud bases and partly sunny skies developing. Highs will reach mid-
upper 70s, warmest in eastern MA although developing sea-breezes
will keep the immediate coastline a few degrees cooler.
SNE will start the day with ridging and subsidence between offshore
low pres and approaching cold front from the west. This will ensure
dry weather lasting into the afternoon, but cold front assocd with
robust upper trough and shortwave will be approaching from the west
with narrow but deep pre-frontal moisture plume leading to
increasing risk of showers moving into western MA/CT late in the day
into early evening. CAMs are suggesting potential for a fine line
developing given decent forcing for ascent from strong low level
convergence. While instability is limited, strong height falls and
cooling temps aloft moving in from the west indicate a few embedded
t-storms can't be ruled out, along with potential for brief
downpours.
High surf conditions and risk for dangerous rip currents will peak
today along the exposed eastern MA coastline as easterly swell
builds to 6-9 ft just offshore. High Surf Advisory continues.
&&
.SHORT TERM /SUNDAY/...
Tonight...
Cold front will be moving into the CT valley 6-8 pm then sweep
across the rest of SNE during the evening and moving off the coast
around of just after midnight. Shortwave energy will be lifting NE
from the eastern Lakes this evening so forcing for ascent will be
weakening as the front moves east and becomes further removed from
best forcing. As a result, expect line of showers to weaken and
become more scattered as they approach the coast during the first
half of tonight. However, area of marginal instability over the
ocean, fueled by higher dewpoints, surges northward this evening
along the south coast and Cape/Islands where an isolated t-storm is
possible. Deep moisture plume will be quickly pushing to the east 03-
06z so any showers will be brief, followed by rapid drying and
clearing skies overnight as PWATs fall to near 0.3". Post-frontal
cold advection on modest NW winds will bring cooling temps overnight
with lows ranging from mid-upper 40s interior to low-mid 50s near
the coast.
Sunday...
Upper low will be lifting northward through eastern Canada. While
trough axis will be west of New Eng, rather dry air in the column
will result in abundant sunshine with excellent mixing bringing
dewpoints down to the upper 30s and lower 40s. Cooler post-frontal
airmass with highs ranging from upper 60s to lower 70s, mildest
across SE MA, but low-mid 60s over higher terrain. The cooler temps
will be accompanied by NW winds 10-20 mph.
&&
.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Highlights
* Dry weather through the week with high pressure in control.
Temperatures trending above normal and moderating as the week
progresses. Will also see a couple of cool nights due to
radiational cooling.
Sunday Night through Tuesday Night...
A ridge axis will initially be over the High Plains Sun Night, while
a cutoff is over Quebec keeping the Great Lakes region and New
England under cyclonic flow. Cyclonic flow persists roughly into Tue
as the cutoff gradually lifts into northern Quebec and Newfoundland.
The ridge builds into the Upper Mississippi River Valley/western
Great Lakes by late Mon and the Great Lakes Region by Tue. High
pressure nudges in from the OH Valley Sun Night before building (and
continuing to nudge in) from the Mid Atlantic/eastern Great Lakes by
Tue.
Tranquil weather expected through this timeframe with high pressure
in control. Lowered night time lows Sun Night toward the 10th
percentile of guidance as am anticipating strong radiational cooling
with clear skies and relatively light winds. The result are
widespread lows in 40s (away from the immediate coast). Only lowered
to the 25th for Mon Night as a weak shortwave may bring some
mid/high clouds overhead, which will limit the radiational cooling.
Should have another strong radiational cooling night Tue Night with
the high essentially overhead, so went with 15th percentile of
guidance.
During the daytime will have a well mixed boundary layer with WNW to
NW flow at 925 and 850 hPa. Should see the 15-25 kts of wind mixing
down fairly easily given the setup, so increased wind speeds/gusts
during the day. Given this have also increased high temps (75th
percentile of guidance) and lowered dew points/RH values (25th
percentile). High temps around seasonable values on Mon, but we
climb slightly warmer than seasonable for Tue.
Wednesday through Friday...
Will have continued ridging in place over the Great Lakes region
through this timeframe. The result is high pressure overhead on Wed,
but gradually shifting offshore to the SE/E by Thu/Fri.
The tranquil stretch of weather persists through the end of the
extended. Temperatures expected to continue to moderate and climb
further above normal as we head late into the week. Highs topping
out in the 70s to low 80s on Wed and solidly into the 80s for much
of the region on Thu/Fri (outside of the immediate south coast).
Should note that 925 hPa temps on the Thu/Fri timeframe are 20 to 25
degrees Celsius with W to NW flow. Could see temps overachieving
given this setup (did bump up to 75th percentile of guidance for
now). There are even early indications of low to mod probs (10-40%)
of 2M temps AOA 90 in the Merrimack and CT River Valley. This is
only indicated by the latest and prev GEFS, whereas international
ensemble guidance (GEPS/EPS) is nil at this point.
&&
.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Today...High confidence in trends, moderate in exact timing.
MVFR-IFR improving to VFR by 14-16Z. Showers and MVFR to
localized IFR cigs moving into western MA/CT later in the day.
Isolated thunder possible. N wind becoming S 8-15 kt in the
afternoon.
Tonight...High confidence in trends.
A brief period of showers move through SNE during the evening
with low end VFR-MVFR cigs. Showers move offshore 03-06Z
followed by clearing skies overnight. Cold front with wind
shift to NW moves W to E across SNE 23-04z, closer to 06z at
ACK. Brief G20 kt possible with FROPA.
Sunday...High confidence.
VFR. NW wind 10-20 kt.
KBOS TAF...High confidence in trends, lower confidence in
timing.
MVFR stratus to start should scatter out by roughly 14-16Z. This
is also right around when the seabreeze materializes until this
evening. Winds turn more southerly as cold front approaches
along with a brief shower and return of MVFR ceilings this
evening (00-03Z). Improving rapidly to VFR afterword with winds
shifting to the NW.
KBDL TAF...High confidence in trends, lower confidence in
timing.
MVFR cigs this morning improving to VFR 15-16Z. Showers and
lower cigs develop after 21Z. Isolated thunder possible but
confidence is low. Winds abruptly shift from S to NW as front
slides through roughly 23-01Z.
Outlook /Sunday Night through Wednesday/...
Sunday Night through Monday Night: VFR. Breezy.
Tuesday through Wednesday: VFR.
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Today through Sunday...High confidence.
Easterly swell from offshore storm peaks today with 6-9 ft seas over
eastern waters and 5-7 ft over southern waters. Seas will gradually
subside tonight into Sunday with seas falling below 5 ft on Sunday.
N winds this morning will shift to S this afternoon, then post-
frontal NW winds will increase to 15-20 kt late tonight into Sunday
morning before gradually diminishing in the afternoon. Low risk for
a few 25 kt gusts tonight behind the cold front.
Outlook /Sunday Night through Wednesday/...
Sunday Night: Winds less than 25 kt. Areas of seas approaching
5 ft.
Monday through Monday Night: Winds less than 25 kt. Seas
locally approaching 5 ft.
Tuesday through Wednesday: Winds less than 25 kt.
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...High Surf Advisory until 8 PM EDT this evening for MAZ007-019-
022-024.
High Risk for Rip Currents through this evening for MAZ015-016-020-
021-023.
RI...High Risk for Rip Currents through this evening for RIZ006>008.
MARINE...Small Craft Advisory until 2 PM EDT this afternoon for ANZ232.
Small Craft Advisory until 2 PM EDT Sunday for ANZ250-254>256.
Small Craft Advisory until 8 AM EDT Sunday for ANZ251.
&&
$$
SYNOPSIS...KJC/BL
NEAR TERM...KJC/BL
SHORT TERM...KJC
LONG TERM...BL
AVIATION...KJC/BL
MARINE...KJC/BL
Source:
BOX issues Area Forecast Discussion (AFD) at Sep 7, 7:36 AM EDT----------------
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