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Weather Center


Area Forecast Discussion

Forecast Discussion for BOX NWS Office
FXUS61 KBOX 201811

Area Forecast Discussion
National Weather Service Boston/Norton MA
211 PM EDT Mon May 20 2019


Warm, summer-like day ahead of a sweeping cold front this afternoon
into evening with isolated to scattered showers and thunderstorms,
some of which may be strong to severe. Dry, cool air building in
overnight. Seasonable weather with windy conditions Tuesday. Dry
weather continues Wednesday into Thursday with less wind, then
a period of showers will likely impact the region sometime late
Thursday into early Friday. Summer like warmth may return by the
end of next weekend.



2 pm update ...

*/ Highlights ...

 - Severe thunderstorm watch 198 posted for N/W Massachusetts

 - Isolated to scattered showers and thunderstorms, some of
   which may be strong to severe

 - Main threats: Gusty winds, possibly damaging. Small hail,
   heavy rain perhaps leading to localized flooding, and frequent

 - Timing: 2 to 9 pm mainly W/NW of the Boston to Providence

*/ Discussion ...


Showers beginning to pop. Scattered to broken cloud decks have allowed
2m temperatures to over-achieve. Pushing with warmest guidance just
meets highs already observed today, in some places highs have already
exceeded. Surface based instability building over 2k J/kg, effective
shear in excess of 40 kts. Steep low level lapse rates parent, showers
have begun to develop. But it`s dry above H7 where lapse rates are
poor. K-indices and total-totals reflect this. Updrafts getting choked
on the dry air, we are still expect isolated to scattered showers
and thunderstorms, some of those storms possibly becoming strong
to severe with the main threat being damaging winds. Heavy rain and
frequent lightning.

Severe thunderstorm watch 198 posted in collaboration with SPC and
neighboring weather forecast offices to highlight the threat of strong
to damaging winds with any thunderstorm activity.

Previous discussion:

Broadly speaking, Great Lakes shortwave ejecting northeast through
cyclonic flow with attendant H3 jet streak diffluence and subsequent
height falls. Surface reflection cold front ahead of which a fairly
moist, unstable airmass resides with daytime heating. Sweeping thru
this afternoon, isolated to scattered showers and thunderstorms are
expected to develop with potential threats of strong to damaging
winds, small hail, heavy rain perhaps leading to localized flooding,
and frequent lightning.

Small-scale, instability building to +2k J/kg CAPE within fairly uni-
directional SW-NE shear already exhibited by vertical wind profilers.
Both 0-6 km bulk shear (40-50 kts) and mean wind parallel to one
another as well as along the cold front. Corfidi vectors also in a
SW-NE manner parallel 0-6 km winds / shear. Destabilizing boundary
layer, well-mixed to H8 with subsequent inverted-V profiles, but above
which are poor along with indications of drier air which can already
be seen in the latest water vapor satellite in wake of morning short-
wave activity / overnight thunderstorms. Forecast model H5 dewpoint
depressions around 15-20C. K-indices just above 30 threshold but total-
totals below 50. Localized theta-E along the cold front, low-level
storm development with freezing levels to 10 kft, precipitable waters
to 1.5 inches, heavy rain is a threat and think any flooding will
be very localized as these storms will be moving quick.

Overall, thinking multi-cellular storm mode which has the potential
for line-echo wave patterns towards the northeast as updrafts develop
and both deep-layer shear and mean wind are parallel SW-NE, that in
addition to squall-like structures perhaps along the cold front with
attendant lift of the warm-moist airmass. Uni-directional, will aid
in the strong to damaging wind threat. Small hail possible if strong
updrafts develop, but so much dry air, not incredibly concerned. With
bulk shear, always the threat of rotating storms, low risk of a
tornado. Timing roughly 2 pm to 9 pm for thunderstorm activity.

Following HREF trends closely, favor the NSSL WRF with the morning
convection. There does appear to be a tone-down of outcomes to which
we have to agree given the environment noted above. For what it`s
worth, in closing, HREF probabilities seem to hint on activity over
N New England perhaps with better synoptics and low-level forcing,
more moisture aloft, stronger updraft helicity ... and off the Mid-
Atlantic coast with greater instability out ahead of lift associated
with the cold front. S New England in the go-between, that`s why
a lean to more isolated to scattered activity.



Tonight ...

Isolated to scattered shower activity pushing offshore with the
sweeping cold front. Will keep brisk SW winds in advance, but
also as winds back behind the front it`ll be NW gusty as well,
especially along the high terrain. Cooler, drier air working in,
lows getting down around 50.

Tuesday into Tuesday night ...

Cool, dry airmass in place. Scattered cloud decks developing as
the boundary layer becomes well mixed with highs getting up to
around 70 degrees. Momentum mix down along with drier air,
looking at dewpoints in the 30s and NW wind gusts up around 20
to 25 mph. Should winds subside during the evening and overnight
periods, then perhaps radiational cooling could proceed,
especially over N/W MA and CT sheltered valleys allowing
temperatures to drop into the upper 30s.




* Continued dry weather Wed into Thu with less wind
* A period of showers possible late Thu into early Fri
* Summer like warmth may return next Sunday


Closed mid level low moving SE across Maine Tue then lifting NE into
the Maritimes Wed will lower heights across SNE with NW flow aloft
bringing temps back to near seasonable normals. Anomalous
subtropical ridge builds across SE CONUS through the end of the week
and into the weekend bringing major heat to SE US. SNE will be on
northern periphery of the ridge and vulnerable to shortwave passages
given nearby polar jet. A fairly robust shortwave riding over the
ridge into New Eng Thu night/Fri will amplify trough to the east
and keep heat suppressed to the south, and also bring increased risk
of showers. Northern periphery of the SE CONUS ridge builds back
into New Eng next weekend with another weak shortwave approaching.
Warmer temps return by Sunday.

Wednesday into Thursday...

High pres builds into the region resulting in less wind and temps
remaining near or slightly below normal. Lots of sunshine Wed then
increasing clouds Thu as mid level shortwave approaches from the NW
with warm advection developing. It should remain dry Thu, but if
shortwave is faster than models indicate, a few showers could spill
into the region in the afternoon.

Thursday night into Friday...

Fairly robust shortwave dives SE across New Eng with attending
frontal wave moving across the region. This will lead to increasing
risk of showers Thu night which may linger into Fri morning, then
improving conditions as shortwave exits. Cooler Fri with easterly
flow bringing below normal temps. May end up cooler than forecast
with potential for 50s in eastern New Eng.

Saturday into Sunday...

Looks mainly dry Sat with seasonably mild temps as sfc ridge in
control. Another shortwave and approaching cold front may bring some
showers Sat night into Sun but this is low confidence. It does
appear that considerably warmer temps will return Sun with SW flow.
Temps should reach well into 70s with some 80s possible in the



Forecaster Confidence Levels...

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

Short Term /through Tonight/...High confidence.

Today and Tonight...
Low-end VFR SCT-BKN CIGs. FEW-SCT SHRA and TSRA roughly through 1z-
3z sweeping NW-SE across the terminals. TSRA mostly NW of PVD-GHG.
+RA with 24020G30KT possible with any TSRA, MVFR as well with 010-030CB.
Otherwise, winds around 20015G20KT Improving 1z-6z especially along
the S/SE-coast. W/NW winds at around 28015G25KT, strongest along
the high terrain.

Tuesday into Tuesday night ...
VFR. SCT 040. NW sustained 15 to 20 kts, highest gusts around 30
kts, strongest along the high terrain. Winds diminishing evening
and overnight.

KBOS Terminal...
FEW-SCT TSRA threat 21z-1z.

KBDL Terminal...
FEW-SCT TSRA threat 19z-0z.

Outlook /Tuesday through Friday/...

Wednesday: VFR. Breezy.

Wednesday Night: VFR.

Thursday: Mainly VFR, with local MVFR possible. Chance SHRA.

Thursday Night: Mainly MVFR, with local IFR possible. Breezy.
Chance SHRA, isolated TSRA.

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



Forecaster Confidence Levels...

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

Short Term /through Tonight/...High confidence.

SW winds sustained around 15 kts with gusts around 20 kts. Seas of
3 to 6 feet. Isolated to scattered showers and thunderstorms
approaching the waters towards evening.

Isolated to scattered showers and thunderstorms along a sweeping
cold front. Some could be strong, especially for the E-waters off
of MA. Frequent lightning. SW winds continuing with sustained around
15 kts, gusts around 20 kts, backing out of the NW behind the cold
front pushing through this evening. Seas around 3 to 6 feet. Maybe
some renewed fog along the S/SE coast before the cold front sweeps

Tuesday into Tuesday night ...
Breezy NW winds. Gusts up to 30 kts possible over the E waters.
Can`t rule out attendant wave heights 5-7 feet.

Outlook /Tuesday through Friday/...

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

Tuesday Night: Low risk for Small Craft Advisory winds with
gusts up to 30 kt. Seas up to 5 ft.

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 30 kt. Local rough seas.

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: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of rough seas. Chance of rain showers.



MARINE...Small Craft Advisory until 8 AM EDT Tuesday for ANZ230>237-250-



NEAR TERM...KJC/Frank/Sipprell
SHORT TERM...Frank/Sipprell

NWS BOX Office Area Forecast Discussion



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