LexMAWeather

Area Forecast Discussion
for Boston, MA

        
000
FXUS61 KBOX 191958
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
358 PM EDT Sun May 19 2019

.SYNOPSIS...
Additional scattered showers and thunderstorms are possible 
again tonight. Summer-like heat and humidity is forecast for 
Monday. A cold front will trigger scattered strong thunderstorms
Monday afternoon and early evening. This will be followed by 
dry, windy and cooler weather Tuesday. Dry weather continues 
Wednesday into Thursday afternoon with less wind and 
temperatures moderating back to seasonably mild levels. A period
or two of unsettled weather may return late Thursday into 
Saturday, but a washout is not expected.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...

For some it may have seemed as if clouds and showers would never 
move out today, but as of about 2PM the last of this morning's 
showers finally exited the coast. Behind it we enjoy mostly 
sunny skies and warm temperatures, though areas beneath the 
clouds and rain much of the day have underperformed on 
temperatures. Additionally, the lingering clouds kept a damper 
on the higher wind gusts this morning and early afternoon for 
many inland areas. As we've cleared out more locations are 
experiencing southerly gusts generally 20-25 mph, and this will 
continue into the evening, coming down overnight. 

To our west regional radar shows showers and strong to severe 
thunderstorms have popped up in western New York where skies have 
been clear much of the day and instability is maximized (CAPE values 
~2500). This instability decreases rapidly over the next 6 hours and 
storms are expected to weaken as the remnants approach New
England late this evening. A Severe Thunderstorm Watch has been
issued for areas to our west including Berkshire county but our
CWA has not been included given weak instability parameters.
Another round of showers and scattered thunderstorms makes its
way through overnight, but instability is weak (CAPE of a few
hundred J/kg) and elevated, so any thunderstorms aren't expected
to be severe. Some locally heavy downpours are possible
overnight, with PWATs reaching >1.5". Low temperatures will be
noticeably warmer tonight, in the 50s and 60s, ~10F above
average. Areas of dense fog are likely to develop, mainly along
the south coast.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/...

The main focus of the short term period will of course be the 
potential for our first severe weather event of the Spring. Monday 
southern New England will feel positively summer-like in the warm 
sector of a surface low moving across the Great Lakes region. Return 
flow tapping into subtropical moisture will bring temperatures in 
the 70s-80s and dewpoints in the low 60s. Winds are breezy out 
of the southwest. Model soundings indicate potential to mix down
gusts of 25-30kts, peaking in the afternoon. 

Things continue to come together for a severe weather potential 
by the afternoon. First we'll start with the good, then mention
a few inhibitors that are keeping some uncertainty in the 
forecast as to how intense and widespread any storms will be. 
Lift is there, if marginal. First and foremost, a cold front 
sweeps through from the west as our primary source of lift, but 
at 300 mb we're also placed beneath the RRQ of an upper level 
jet which will assist in some synoptic lift, though moreso to 
our north. Looking at 850-500mb omega Monday afternoon, values 
are decent, but again stronger over VT/NH. We'll have ample 
instability to work with by afternoon/evening. MUCAPE values on 
the order of 1000-2000 J/kg and low level lapse rates near 8 
C/km are centered over western/central MA/CT, where the best 
chance of severe storms lies. For strong to severe storms we 
want sustained updrafts, and the necessary shear values are 
there. 0-6km bulk shear approaches 50kts with 0-1 km shear 
>20kts. That being said, HREF guidance indicates that the best 
updraft helicity will be centered over central NH, with lesser 
values extending over northern and northwest MA. This, together 
with the westward placement of greatest instability means those 
in western MA/CT would stand the best chance of seeing 
significant storms tomorrow. Low level lapse rates indicate a 
threat of strong damaging winds along with large hail. Though a 
low probability, the chance of a tornado exists, given said low 
level shear and 0-1 SRH values near 150. Finally, moisture is 
the biggest question for tomorrow. A necessary ingredient for 
widespread storms, it may be limited at the mid levels by the 
time the best instability moves in. The NAM has now joined the 
GFS and EC in showing drying aloft coincident with the best 
surface based instability. This could have the effect of keeping
storms low topped and limiting intensity and areal coverage. 
That being said, even if coverage is sparse, with these 
convective parameters, any storms that to strengthen have the 
potential to be very damaging. Another thing we'll have to watch
overnight into the morning hours will be how quickly the 
morning rain/clouds clear out. If we get another situation like 
today where showers linger longer than expected, it could limit 
our destabilization. Southeast of the BOS-PVD corridor, 
southwest winds off the 50 degree ocean will keep things more 
stable and storms weaker. The cold front passes Monday night 
bringing gusty winds out of the northwest and cooler lows in the
40s and 50s. 


&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
Big Picture...

The upper pattern through next weekend becomes mostly high-
amplitude with a ridge over the Eastern USA and a trough/closed 
low over the Western USA, and another building ridge over the 
East Pacific. Shortwaves in such a flow will take the scenic 
route across North America...south along the Pacific Coast, then
turning northeast across Northwest Mexico and the Plains/Great 
Lakes, then turning southeast across New England. 

Model consensus shows one shortwave moving across New England 
Tuesday, then off to the east. A second shortwave moves over the
top of the ridge and then across New England Thursday night. A 
third shortwave moves over the top of the ridge and across 
Eastern Canada Saturday. Timing on these last two shortwaves is 
approximate. 

Upper heights/temperatures trend a little cooler Tuesday, but 
return to higher/milder levels through the weekend. This would 
suggest seasonably mild temperatures. 

There have been differences run-to-run on shortwave timing, 
which is not surprising. But the general pattern looks 
consistant through the week with differences among models 
developing next weekend. Confidence is high Tuesday-Wednesday, 
then moderate due to shortwave timing Thursday to Sunday.

Concerns... 

Tuesday-Wednesday...

Upper low and cold pool...-23C at 500 mb...move across Northern
New England Tuesday with a supporting 150-kt jet moving across 
Southern New England. Cold pool moisture is concentrated in the 
north, but some extends south into at least areas north of the 
Mass Pike. Low level lapse rates over Srn New England become 
inpressive, but mid level rates show little of interest. Perhaps
no surprise that model rainfall is limited to ME/NH/VT with 
nothing for our area. We continue to expect diurnal clouds 
Tuesday, heaviest over the hills and near the NH border. Temps 
aloft support max sfc temps in the upper 50s and 60s. Min temps 
should be in the 40s and low 50s.

Winds will be a concern for Tuesday. Cold advection aloft and 
daytime heating will promote the strong low-level lapse rates 
and promote mixing to 800 mb...which should bring northwest wind
gusts around 35-kt to the surface. 

High pressure builds over the region Wednesday, so expect a 
fair-weather day with north winds gusting 15-20 kt. Temps aloft 
4-6C should support max sfc temps in the 60s to around 70, 
cooler Cape adn Islands. Min temps should repeat in the 40s and 
low 50s.

Thursday to Sunday...

As noted above, two shortwaves move through the New England 
area, and each is sufficient to support a period of showers. 
Current model timing would suggest one period is Thursday 
afternoon and night, the second period is Friday night and 
Saturday. Given this is in the late part of the forecast, the 
timing will likely shift a little and so should not be 
interpreted literally. Temperatures should be milder during this
time with highs in the upper 60s and 70s and lows mostly in the
50s.

&&

.AVIATION /20Z SUNDAY THROUGH FRIDAY/...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...

Today...High confidence. VFR. Showers have moved offshore. A mix
of sun and clouds with breezy south winds. 

Tonight...Moderate confidence. MVFR ceilings lowering to IFR 
and LIFR overnight. Showers and scattered thunderstorms move in 
from the west and southwest in the evening. Convection should 
be elevated and not severe. Areas of fog, especially along the 
south coast, where it may become dense. 

KBOS Terminal...Moderate to high confidence in TAF. 

KBDL Terminal...Moderate to high confidence in TAF.

Outlook /Tuesday through Friday/...

Tuesday: VFR. Windy with gusts up to 30 kt. 

Tuesday Night: VFR. Windy with local gusts up to 30 kt. 

Wednesday: VFR. Breezy. 

Wednesday Night: VFR. 

Thursday: Mainly VFR, with local MVFR possible. Slight chance
SHRA.

Thursday Night: Mainly MVFR, with local IFR possible. Chance
SHRA, patchy BR.

Friday: Mainly MVFR, with areas VFR possible. Slight chance
SHRA.

&&

.MARINE...
Forecaster Confidence Levels...

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

Short Term /through Monday Night/...

Tonight...High confidence. South to southwest winds will gust 
to 20 kt. Scattered showers and isolated thunderstorms. Small 
Craft Advisories end along the nearshore, but are in effect for 
all outer waters as seas build to 5 ft. Areas of dense fog may 
reduce visibility to below 1 nm, especially south coast. 

Monday...High confidence. Small Craft Advisories continue for 
the outer waters with southwest winds gusting to 25 kts and seas
building up to 6 ft in the southern outer waters. Scattered 
thunderstorms possible toward evening. Visibility reduced to 1 
to 3 nm in fog, locally below 1 nm in the morning.

Monday night...High confidence. Winds becoming NW gusting to
20kts. Seas 3-6

Outlook /Tuesday through Friday/...

Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 30 kt. Areas of seas approaching 5 ft. 

Tuesday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Areas of seas approaching 5 ft. 

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 25 kt. Seas locally approaching 5 ft. 

Wednesday Night through Thursday: Winds less than 25 kt. 

Thursday Night: Winds less than 25 kt. Areas of seas
approaching 5 ft. Chance of rain showers. 

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

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 8 PM EDT this evening for 
     ANZ230>236-251.
     Small Craft Advisory until 11 PM EDT Monday for ANZ250.
     Small Craft Advisory until 8 AM EDT Tuesday for ANZ254.
     Small Craft Advisory from 2 AM Monday to 8 AM EDT Tuesday for 
     ANZ255-256.

&&

$$
SYNOPSIS...WTB/BW
NEAR TERM...BW
SHORT TERM...BW
LONG TERM...WTB
AVIATION...WTB/BW
MARINE...WTB/BW