387
FXUS61 KBOX 201307
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
907 AM EDT Mon May 20 2024

.SYNOPSIS...
Today is the cornerstone of a pattern change towards more mild
conditions though clouds and cool temperatures will still hang
across the Cape. Much milder conditions Tuesday with heat peaking on
Wednesday. Frontal passage Thursday, with increased thunderstorm
chances, will usher in more seasonable temperatures for the holiday
weekend.

&&

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

905 AM Update...

* Partial sunshine emerges today with significantly warmer
  temps
* Highs 60s immediate coast, 70s inland, near 80 CT River Valley

Previous forecast remains on track with a ridge of high pressure
in control. Visible satellite imagery indicates the low clouds
were beginning to develop breaks in them as of mid morning. This
trend should continue with shallow nature of the moisture in
place combined with the strong May sun angle. Therefore...expect
partial sunshine to emerge in most locations by afternoon. 850T
rising to between +11C/+12C should yield a much warmer afternoon
in most locations that what we saw this weekend. Light onshore
flow will hold highs in the 60s on the immediate coast, but 70s
just a bit further inland, and near 80 in the lower CT River
Valley.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...

Patchy fog development is possible again tonight though developing
SW winds around 5-10kt, and dewpoint depressions around 4F should
preclude widespread fog across the interior. With that said, the
same mechanism of SW flow should encourage some fog/reduced vsbys
along the South Coast and Islands where both temps and dewpoints
hover around 50F and SSTs are in the low to mid 50s.

With surface high still in place and mid level ridge growing
stronger, guidance is still quite poor in diagnosing cloud cover for
Tuesday, with several CAMs, like the HRRR, showing a mostly cloudy
day. Not feeling particularly keen on the "cloudy all day" solutions
with PWATs falling to as low as 0.7" and low RH above the mixed
layer, but early morning fog/stratus, particularly along the south
coast, may take a few hours to burn off. Decent boundary layer
mixing with 850mb temperatures approaching 16C will support highs
well into the 80s across much of the interior, though onshore flow
across the south coast will keep temps much cooler there, in the 60s
to perhaps 70s. Given SW flow, Boston MAY see their first 80F
reading of the season, and while it`s possible the ASOS may not hit
80F, the western suburbs of the city have a high likelihood of doing
so.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...

Highlights:

* Peak of the midweek very warm weather is Wednesday - 80s highs
  with a few locations in the CT and Merrimack Valleys seeing a few
  90 degree readings. Tolerable humidity levels.

* Cold front Thursday afternoon to early evening brings a reprieve
  to the warmth but also could fuel scattered t-storms.

* Drier and with cooler, more seasonable temperatures Friday into
  the Memorial Day Weekend.

Details:

Still looking at Wednesday as the warmest day of the anticipated
midweek spell of very warm temperatures reminiscent of summer. 850
mb temps rising into the +14 to +16C range along with full mixing
and southwest breezes should promote these very warm temperatures.
NBM probs continue to show moderate to high (60 to 90 percent) probs
of temps reaching 90+ degrees in the CT and Merrimack Valleys, with
lower (no greater than 30%) probs of 90+ high temps in central MA,
northeast CT and portions of Metrowest. These are a bit higher than
the prior 13z NBM cycle. The one thing that is a little peculiar in
the NWP is that modeled dewpoints were shown to be in the lower to
mid 60s across a good majority of the guidance; I don`t think values
this high are necessarily realistic given the well mixed PBL and
adiabatic downsloping that should be taking place per model
soundings. That`s also a concern/source of uncertainty as we move
into Thursday as well. Advertised highs upper 80s to low 90s in the
CT and Merrimack Valleys - although those could be somewhat
conservative - and the mid 80s for most of northern RI, central and
eastern MA. Cooler readings near the south coast (upper 60s-mid 70s)
where SW flow will allow for a bit of a maritime breeze. Should be a
pretty mild Wednesday evening too, mid 50s to lower 60s, but did
note some of the guidance showing weakening convection moving in
from eastern NY into our far western counties in MA later in the
overnight. Left a slight chance PoP mention in the forecast for now
for the eastern Berkshires but we`ll have to see if this
materializes in subsequent guidance.

A cold front then trudges eastward through Southern New England on
Thursday. It seems as though in the 00z guidance that the front is
about 6 hours slower than prior model cycles; that may allow for
another very warm day with a slower timing, but also could allow for
slightly more convective instability to help spark showers/t-storms
along the front, particularly near and west of I-95. Similar to
Wednesday, models are now forecasting dewpoints in the lower to mid
60s although those seem a bit high. If we can see dewpoints push
into the lower 60s, we could see CAPE values around 800-1500 J/kg
accompanied by deep-layer southwesterly shear magnitudes around 35
kt (stronger flow mainly to our north). This is really on the
borderline for storms becoming strong to severe and doesn`t really
scream "severe storms", but is something we`ll be needing to monitor
in the coming days. Kept highs in the upper 60s/around 70 near the
south coast, and in the upper 70s to mid 80s further inland.

For Friday, the cold front`s southeastward passage further offshore
could be delayed a bit as the front`s orientation becomes nearly
parallel to the 700-300 mb flow, which could keep clouds and showers
around the south coast for part of the day. However high pressure is
progged to ridge into much of New England, with ridging continuing
into a good part of the Memorial Day Weekend. Temperatures Friday
into the weekend trend cooler and more seasonable than midweek, with
highs in the 70s (60s near the coast). Thus so far, weather
conditions for Memorial Day Weekend seem favorable.

&&

.AVIATION /13Z MONDAY THROUGH FRIDAY/...

Forecast Confidence Levels:

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

12z TAF Update:

Today: High confidence in TAF/trends but moderate on the
timing.

Initially, a mix of MVFR to LIFR from stratus ceilings, with
lowest ceilings further east and south one goes. Stratus should
start to burn off to VFR by ~14-16z for the western and central
airports, and around ~16-18z for BED, BOS and PVD. It may take
until mid afternoon before scattering out to VFR over the Cape
and Islands. One potential fly in the ointment that will need to
be monitored is if stratus returns back landward in coastal
areas as winds shift from NE to E/ESE - it seems unlikely as the
flow is light but is a possible outcome.

Tonight: Moderate confidence.

IFR to MVFR continues for the Cape and Islands, generally VFR
across the interior except IFR where patchy fog.

Tuesday... Low cigs/vsby due to fog burn off quickly to VFR for
all terminals. Winds from the SW will be strong enough to stave
off a seabreeze.

KBOS TAF...High confidence in TAF/trends though some uncertainty
on the timing of eroding stratus. MVFR should trend to VFR by
~16-18z with winds becoming E by this afternoon. Low clouds may
return tonight after 03z as winds shift to light southeast.

KBDL TAF...High confidence in TAF/trends, though some
uncertainty on the timing of eroding stratus. MVFR bases erode to
VFR by ~13-15z. Light NE winds shift to S around 4-6 kt this
afternoon. Could see patchy fog develop tonight but this isn`t
set in stone.

Outlook /Tuesday Night through Friday/...

Tuesday Night: VFR.

Wednesday through Wednesday Night: VFR. Breezy.

Thursday: VFR. Breezy. Slight chance SHRA, slight chance TSRA.

Thursday Night: VFR. Breezy. Chance TSRA, slight chance SHRA.

Friday: VFR.

&&

.MARINE...
Forecaster Confidence Levels:

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

Today... High confidence

Lows clouds/drizzle and patchy early morning fog with vsbys 1-3
miles, slowly burns off with improving conditions mid to late
afternoon. High pressure over SNE results in light NE winds become
east and then southeast late in the day.

Monday night...high confidence.

Ridge of high pressure crest over the waters, yielding light SE
winds becoming SW. Patchy fog may limit vsby at times.

Tuesday... high confidence

AM fog will burn off to pleasant boating conditions. SW winds
with seas less than 5 ft.

Outlook /Tuesday Night through Friday/...

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

Thursday: Winds less than 25 kt. Areas of seas approaching
5 ft.

Thursday Night: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers, slight chance of
thunderstorms.

Friday: Winds less than 25 kt. Slight chance of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 2 PM EDT this afternoon for ANZ254.

&&

$$

SYNOPSIS...Loconto/KS
NEAR TERM...Frank
SHORT TERM...KS
LONG TERM...Loconto
AVIATION...Loconto/KS
MARINE...Loconto/KS