Area Forecast Discussion
Issued by NWS Boston, MA

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

859
FXUS61 KBOX 181730
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
130 PM EDT Tue May 18 2021

.SYNOPSIS...
High pressure south of the region will be the main controller of our
weather for much of the week, resulting in warmer than normal
temperatures. A weak disturbance aloft will trigger a few spot
showers this afternoon mainly towards the southeast New England
coast. Otherwise, dry weather will prevail until Friday. An
unsettled weather pattern is expected to continue into the
weekend.

&&

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

Not too many updated to the forecast at this time. A few showers
have developed across SE MA and have moved southeast across the
mid Cape. Chance for more pop-up showers could blossom later
this afternoon into the early evening mainly across SE MA, Cape
Cod, and Islands. Otherwise, it`s WARM, but nothing record
breaking. For Boston, today is the first 80F or higher
afternoon... Reaching 84F as of 119 PM.

Otherwise we`ve bumped the POPs for SE Mass given the earlier
showers.

Previous discussion follows below.


1025 AM Update...

Temperatures are quickly climbing well into the 70s across
southern New England this morning. One of the more noticeable
changes made to our near term forecast was to bump the
temperatures. 850mb temperatures over ALY are nearing +10C and
with mixing heights well above that we feel confident that areas
of southern New England could reach well into the low-80s this
afternoon. For this update we blended the 75 percentile of the
NBM with the original forecast - this gave us temperatures in
the low 80s across much of southern New England - the exception
being southern RI and the south coast of MA/Cape Cod/Islands.

This will put places like Boston, Providence, and Worcester in
reach of their first 80 degree day of 2021. Out of our four
climate sites, only Hartford (BDL) has hit 80F this year.

Otherwise the forecast remains on track.

PREVIOUS DISCUSSION...

345 AM Update...

* Warm this afternoon with mainly just a few brief spot showers

A shortwave crosses much of our region during the first part of the
day. It will be associated with some mid/high level cloudiness, but
partial sunshine is still expected. Height fields rise a bit from
yesterday as the upper trough slowly pushes further east. 925T
rising to between +16C and +17C along with the strong late May sun
angle should allow afternoon high temperatures to reach near 80 in
many locations. This includes Boston too given winds will be
offshore today.

The shortwave will bring the risk for a few brief spot showers and
perhaps a thunderstorm, but the areal coverage will be more limited
than yesterday. This a result of the upper level disturbance
crossing the region before peak heating & west northwest surface
winds limiting low level convergence/forcing. That being said, it is
cold enough aloft to generate a few hundred J/KG of Cape and modest
mid level lapse rates. So a few brief spot showers are possible this
afternoon, but the main focus will be towards the southeast New
England coast especially the Plymouth to Taunton to Falmouth area.
This is where the best low level convergence will be and the
greatest risk for a few showers and perhaps an isolated t-storm or
two. Even in this region though the vast majority of the day will
feature dry weather.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM WEDNESDAY/...
* Quite Warm Wednesday inland from the coast

Tonight...

Any isolated to scattered shower activity near the southeast New
England coast will dissipate this evening with the loss of daytime
heating. Otherwise, upper level ridging building in from the west
will result in light winds/dry weather tonight. Low temperatures
should bottom only in the 50s across most of the region.

Wednesday...

Upper level ridging will continue to build into the region from the
west. This will result in rising height fields and rather warm
temperatures inland from the coast. Plenty of that strong late May
sunshine coupled with a mild start should push Wednesday afternoon
highs into the middle 80s across interior MA & CT. The weak gradient
will result in cooling sea breezes along the coast. This should hold
high temperatures in the 70s along the immediate coast; while
interior eastern MA reaches near 80. The upper level ridging should
keep our weather generally dry on Wednesday, although an isolated
afternoon spot shower can not be ruled out across the interior.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
Highlights...

* Above normal temperatures and below normal precipitation with
  taste of early summer continuing into Thursday and possibly
  Friday.
* Possibly becoming more unsettled this weekend but by no means a
  washout.
* Early indications of a cold front pushing through early next week,
  bringing an end to the early summer regime. But confidence is low
  this far out.

Overall Pattern:

A H5 ridge builds into Southern New England, giving us a taste of
early summer. A series of weak disturbances tries to erode the heat
dome into the weekend. Finally, early indications are that a cold
front pushes through early next week.

Details:

Wednesday night into Thursday...

H5 ridge expands eastward from the Ohio River valley into the
Atlantic seaboard, becoming more amplified between a closed low
diving south from the Pacific Northwest and another one diving south
from the Canadian Maritimes. Surface high pressure looks to be in
control and along with 1000-500mb relative humidity in the 40 to 50
percent range, it looks like mostly dry weather mid to late in the
work week. Given the weak pressure gradient and SSTs still in the
50s, however, sea breeze could develop late morning into the
afternoon hours and perhaps provide sufficient forcing for scattered
diurnally driven showers and maybe even a thunderstorm. Most
locations would remain dry and this means temperatures well into the
80s away from the immediate coast and perhaps a few locations
pushing 90 degrees. This would be more like July than middle of May!
Thankfully, humidity levels are relatively low with dew points in
the 50s. If you want to cool off, head to the Cape and Islands.

Friday into the weekend...

Friday could prove to be another toasty day but there are signs that
a series of disturbances would ride along the periphery of the heat
dome that would see increased precipitation chances and consequently
help to keep temperatures in check. In addition, models suggest an
inverted surface trough developing which would indicate a backdoor
cold front. But the strength of the cold front remains in question
and that could mean the difference between highs in the 60s vs. 80s.
Taking a blended approach means forecast highs in the 70s.
Climatological highs are in the upper 60s to low 70s. A shortwave
trough nearby would provide the focus for a few showers and
thunderstorms, but no day appears to be washouts. Besides the
anomalous warmth, also expect a gradual uptick in humidity as the
upper ridge remains parked over Eastern US, which means no change in
air mass.

Early next week...

If you are not a fan of heat and humidity (the latter becomes true
by the weekend as mentioned above), reprieve could (emphasis on
`could`) arrive early next week when a cold front finally sweeps
through. If this happens, expect a return to more seasonable
temperatures for mid to late May. But confidence is low this far out
and once again, a blended approach is used for the forecast. Did opt
to lower the dew points and PoPs on Monday upon collaboration with
neighboring offices due to somewhat increased confidence of a more
refreshing air mass knocking on our doorstep.

&&

.AVIATION /18Z TUESDAY THROUGH SATURDAY/...
Forecaster Confidence Levels.

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

18z TAF Update:

This Afternoon...High Confidence. VFR conditions outside a few
afternoon/early evening spot showers and perhaps an isolated
t-storm. The vast majority of the region will remain dry with
the best chance for this isolated to scattered afternoon
activity near the southeast New England coast. Winds becoming
WNW at 10 to 15 knots across most of the region with a few gusts
between 20 to 25 knots possible this afternoon/evening.

Tonight...High Confidence in VFR conditions. Winds becoming
light/calm.

Wednesday...High Confidence in VFR conditions. Winds becoming NW
at 5 to 10 knots, but sea breezes are expected to develop by
late morning/early afternoon along much of the coast.

KBOS TAF...High confidence in TAF. A few spot showers possible
this afternoon/early evening in the vicinity of the terminal.
Gradient should prevent any sea breezy today, but we do expect
an ESE sea breeze to develop during the day Wednesday.

KBDL TAF...High Confidence in TAF.

Outlook /Wednesday Night through Saturday/...

Wednesday Night through Thursday Night: VFR.

Friday: Mainly VFR, with local MVFR possible.

Friday Night: Mainly VFR, with local IFR possible.

Saturday: MVFR/IFR conditions possible. Slight chance SHRA.

&&

.MARINE...
Short Term /through Wednesday/...

Today and tonight...High Confidence. Winds and seas should generally
remain below small craft advisory thresholds. Winds will be
strongest across the southern waters into this evening, where they
will gust into the lower 20 knots but seas should remain in the 3 to
4 foot range given the inversion over the relatively cool ocean.
These winds should shift to the NW with gusts diminishing overnight
behind the surface trough.

Wednesday...High Confidence. A weak pressure gradient will keep
winds and seas below small craft advisory thresholds.

Outlook /Wednesday Night through Saturday/...

Wednesday Night through Thursday Night: Winds less than 25 kt.

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

Friday Night: Winds less than 25 kt. Seas up to 5 ft.

Saturday: Winds less than 25 kt. Seas up to 5 ft. Slight chance
of rain showers.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$
SYNOPSIS...Frank/Chai
NEAR TERM...Frank/Gaucher
SHORT TERM...Frank
LONG TERM...Chai
AVIATION...Chai/Gaucher
MARINE...Frank/Chai/Nash



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.