Area Forecast Discussion
Issued by NWS Boston, MA

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000
FXUS61 KBOX 251754
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
154 PM EDT Fri May 25 2018

.SYNOPSIS...
High pressure centered south and east of New England will bring
a very warm dry day with gusty southwest winds. A cold front
drops south across the region late Saturday into Saturday
evening, bringing scattered showers and thunderstorms followed
by much cooler and unsettled weather Sunday with some
improvement by Monday. Mainly dry and warmer weather returns for
Tuesday through Thursday as high pressure moves in.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
150 PM Update...

Unseasonably warm conditions persist this afternoon. High
pressure is centered well SE of the area, placing us in a return
flow from the W/SW. Sunny/mostly sunny skies continue, temps
climbing into the upper 80s to low 90s for most locales, except
cooler near the south coast/Cape/Islands due to onshore wind
component. Will also have excellent mixing this afternoon,
allowing for wind gusts 25 to around 30 mph. Hot but not humid
away from the south coast with dew points in the 40s to low 50s.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/...
Tonight...

Diminishing winds and quiet, dry weather. S/SW winds prevailing,
breezy along the S/SE-coast, ushering higher dewpoint air into
the region. This will limit temperatures to drop overnight to
their cross-over threshold (the dewpoint that which was observed
at time of max daytime heating). Lows around the lower 60s.

Saturday...

Chance of late-day scattered shower and thunderstorm activity. Yet
signals of an unremarkable convective-supporting environment. Rising
heights through the column, pockets of dry air, somewhat mid-level
confluence, perhaps upper-level speed diffluence. Only thing going,
another N-stream vortlobe, Canadian high pressure following, presses
a sagged cold front S/W as a back-door across S New England, a much
cooler airmass in tow. Certain low-level lift on a mild, somewhat
humid boundary layer airmass, with upper 50s to low 60s dewpoints.

High-res guidance spitting out some scattered activity, convective
with weak mid-level instability, pockets of negative showalters,
total-totals above 50, and K-indices exceeding 35. While noting
precipitable waters upwards of 1.5 inches and indications of mid-
level subsidence, can`t rule out scattered shower and thunderstorm
activity along the back-door cold front, yet feel what goes up will
not sustain and come right back down given the absence of shear /
weak hodograph profile. More likely in regions with surface to H7
lapse rates of 7-8 C/km. Falling cores while noting inverted-V model
forecast soundings, in addition to lightning can`t rule out gusty,
downdraft winds, especially given downdraft CAPE upwards of 2000
J/kg. Airmass thunderstorms overall, perhaps more focused atop high
terrain with any upsloping anabatic flow.

Chance PoPs continue but future forecasts may want to include threats
of heavy rain and gusty winds into the forecast grids. Difficult to
nail down where across the interior, away from the NE coast at least,
but as to when, the back door cold front pushing into afternoon during
peak interior heating per model forecast consensus, a late-day show
seems likely continuing into evening.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
Big Picture...

A southern jet moves over Mexico and the Southern USA. This jet
forms a closed low over the Gulf of Mexico with upper high pressure
over Northern Mexico and the Southwest Atlantic. This southern
pattern remains static through the middle of next week and allows
tropical moisture to feed north into areas south of the Ohio Valley
and south of Mason-Dixon. The question for New England is whether
any northern stream disturbances during the coming week can reach
down and pull this moisture farther north into our area.

Zonal flow is aloft over Canada and parts of the Northern USA. One
shortwave trough moving through this flow crosses the Northeast USA
Sunday and Monday. Closed low over the Western USA this weekend
ejects into the northern stream by midweek and crosses New
England late in the week. Both of these systems have the
potential to pull the tropical moisture north as they moves
through our area, one on Sunday and the other late in the week.
Both will need to be monitored.

Model consensus shows 500 mb contours starting around 576 dm and
building to 582 dm during mid week. This is higher/warmer than
normal during the long term. The surface layer flow favors a cool
period over the weekend but then trending warmer than normal again
Tuesday through Thursday.

Model mass fields are similar over New England through Wednesday
night, but diverge over the Western USA Tuesday night. The
differences then spread to New England by late week. Our thermal
fields also show similarity through Tuesday but then start diverging
mid to late week. This generates moderate confidence in the Saturday
night-to-Monday forecast, then moderate to high confidence midweek,
diminishing a little on Thursday.

Concerns...

Saturday night-Sunday...

The cold front stalls south of our area overnight. High pressure
building over the Maritimes brings increasing east winds trailing
the front and a cool moist marine flow to Southern New England.
Model 1000 mb winds show 20-30 knot easterlies raking the
Massachusetts east coast with strongest winds on Cape Ann and Outer
Cape Cod. Ocean temps are in the mid 50s, except upper 40s and low
50s around the Outer Cape and Nantucket. This would suggest Saturday
night min temps in the 50s and max temps Sunday mid to upper 50s
east and mid 60s west, farthest away from the ocean.

The upper shortwave approaching from the Great Lakes Sunday and its
supporting 70-knot upper jet may induce a wave along the stalled
front. This should keep a chance of showers with us Sunday, with
enough confidence to toss in likely pops.

Monday through Thursday...

Shortwave trough moves east of us by Monday morning. Cross sections
for relative humidity show moisture below 700 mb lingering but
diminishing in magnitude. This suggests clouds will linger, although
some breaks are possible. Also can`t rule out a slight chance of
showers, but the lack of appreciable lift puts a damper on that
possibility.

High pressure builds down from Northern Ontario Tuesday and
Wednesday, then moves offshore Thursday. This should be a period of
sunny weather. Mixing is projected to reach 825 mb Tuesday and
Wednesday, and 900 mb Thursday. Temperatures at those levels support
max sfc temps in the low to mid 80s, while dew points suggest min
temps in the 50s.

&&

.AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
Forecaster Confidence Levels...

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

Short Term /through Saturday/...High confidence thru tonight.
Moderate confidence on timing of back door cold front and
associated impacts during later Saturday.

Today...

VFR all areas through the day. W/SW winds gusting to 25-30 kts
through this afternoon.

Tonight...
SW winds diminish to around 10 kts, but remain brisk over S/SE
coast with gusts up to 20 kts. LLWS potential along the Cape and
Islands. Low chance for development of IFR CIGS on ACK.

Saturday...
Back door cold front produces a wind shift from the W/SW to N/NE
during the afternoon/evening, first near BED/BOS and then later
for points S/SW. SCT SHRA/TSRA towards late-day, producing
TEMPO MVFR CIG/VSBY impacts. Can`t rule out brief, gusty winds
upwards of 30+ kts with any activity. Low chance for development
of IFR CIGS on Cape/Islands.

KBOS Terminal...High confidence in TAF thru tonight, moderate
confidence for timing of back door cold front on Sat. WSW winds
gusting to 25 kts thru this aft`n, can`t rule out some gusts to
30 kts. Winds diminish this evening. WSW winds around 10 kts to
start Saturday then wind shift to NNE with back door cold front
during the afternoon.

KBDL Terminal...High confidence in TAF. WSW winds gusting to 25
kts thru this aft`n, can`t rule out a few gusts to 30 kts. Back
door cold front holds off until later on Sat/Sat eve which would
produce a wind shift from WSW to NNE..

Outlook /Saturday Night through Tuesday/...Moderate confidence.

Saturday Night: Mainly VFR, with areas IFR possible. Breezy.
Chance SHRA, chance TSRA.

Sunday: MVFR/IFR conditions possible. Breezy. Chance SHRA,
slight chance TSRA.

Sunday Night: Mainly IFR, with areas VFR possible. Breezy.
Slight chance SHRA, isolated TSRA, patchy BR.

Memorial Day: Mainly VFR, with local IFR possible. Slight
chance SHRA.the

Monday Night: Mainly VFR, with areas IFR possible. Patchy BR.

Tuesday: VFR.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Saturday/...High confidence.

This afternoon into Tonight...

SMALL CRAFT ADVISORIES for all waters, especially S/SE, with
brisk WSW winds/gusts around 25 kts, thus allowing waves to
build at or above 5 feet. Winds dampening overnight, advisories
allowed to drop off.

Saturday...
SMALL CRAFT ADVISORIES may be needed. Increasing chances of
late day showers and thunderstorms ahthe ead of a NE wind-shift
associated with a back door cold front. Gusty winds immediately
behind the front upwards of around 25 kts building seas upwards
of 5 feet. Greatest threat of SCA conditions over the E waters.

Outlook /Saturday Night through Tuesday/...Moderate confidence.

Saturday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers, chance of
thunderstorms.

Sunday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Local rough seas. Chance of rain showers, chance of
thunderstorms.

Sunday Night: Winds less than 25 kt. Areas of rough seas.
Chance of rain showers, isolated thunderstorms.

Memorial Day: Winds less than 25 kt. Areas of seas approaching
5 ft. Slight chance of rain showers.

Monday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Local visibility 1 to 3 nm.

Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>008.
MARINE...Small Craft Advisory until 2 AM EDT Saturday for ANZ232>234-
     250.
     Small Craft Advisory until 8 PM EDT this evening for ANZ230-
     231-236-251.
     Small Craft Advisory until 11 AM EDT Saturday for ANZ235-237.
     Small Craft Advisory until 8 PM EDT Saturday for ANZ254>256.

&&

$$
SYNOPSIS...WTB/Sipprell
NEAR TERM...WTB/Sipprell/NMB
SHORT TERM...Sipprell
LONG TERM...WTB
AVIATION...WTB/Sipprell/NMB
MARINE...WTB/Sipprell/NMB



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