Area Forecast Discussion
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FXUS61 KBOX 221845
Area Forecast Discussion
National Weather Service Taunton MA
245 PM EDT FRI JUL 22 2016
Heat and humidity increase today along with the risk for
scattered showers and thunderstorms during the afternoon and
evening, some of which may be severe. There may be a few leftover
thunderstorms Saturday. Hot weather is expected to continue this
weekend. An approaching cold front will bring another round of
scattered showers and thunderstorms Monday night and Tuesday. More
warm weather follows midweek.
.NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
2 pm Update...
Getting heat index values in the upper 90s and just shy of heat
index criteria. Did add a short paragraph in the updated SPS for
convection. WSW wind is quite favorable for high temperatures
along the eastern MA coastal plain given some downsloping in
addition to an air mass with 850 mb temperatures pushing 20C.
There remains below average confidence on convection for later
this afternoon and evening. Looks like enough instability to
initiate convection after about 3 or 4 PM in the western zones.
Vertical wind shear is marginal and still think convection is
most likely to take the form of individual pulse cells or small
clusters, although some guidance hinted at the possibility of a
mini-MCS trying to form near NW CT and SW MA. Not really buying
into the mini-MCS solution at this point but does look like SW MA
and CT may be a somewhat more favored region for scattered
thunderstorms mid to late afternoon and then possibly shifting to
RI and SE MA during the evening. Confidence is quite low how this
might evolve. There are signals of drier air attempting to work
its way into northern MA and have lowered POPs some in that region
but not confidence enough to go below chance.
Low level jet across SE MA resulting in significant surface wind
gusts, especially along portions of the Cape. This will likely
continue into early evening and then gradually diminish.
1015 AM Update...
Heat indices are expected to reach mid to upper 90s across much of
the area for a few hours this afternoon. This remains below Heat
Advisory criteria but still high enough to have some impact. May
make a mention in the SPS for convection that will be updating
Most of the forecast area remains in a slight risk from SPC.
Vertical shear looks to be marginally high enough to support a
strong storm or two combined with sufficient instability. NAM may
be a little overdone in its convective robustness but still looks
like enough ingredients in place for pulse storms and perhaps
small clusters with the potential for localized damaging winds and
large hail. Precipitable waters of 1.5+ inches suggest heavy
downpours could also be an issue. It is difficult to pinpoint
exactly where these storms will fire. There is a suggestion of a
weak boundary in eastern NY along the I-90 corridor and some
activity may initially fire over the higher terrain. Then, may see
outflow boundaries from one storm or cluster be the initiating
mechanism for future convection. The surface instability looks to
remain high well into the evening and think may see individual
storms or small clusters develop off and on through at least mid
evening. Believe we may start to see thunderstorm development
initiate around 3 PM.
Have adjusted temperatures a little upwards in portions of the
area based on current trends and consistent model 850 mb
temperatures of 19C to 20C mid to late afternoon across eastern
half of area.
7 AM Discussion...
No major changes to the ongoing forecast this morning. A few
showers developing in the Berkshires already. All varieties of
CAPE were less than 300 J/kg, so not expecting thunderstorms just
yet. Tweaked the forecast to bring conditions back in line with
Previous discussion for 3 PM to 10 PM time frame...
***Risk for strong to severe thunderstorms late today into the
early overnight hours***
Late today and tonight...
At this time it appears the marine layer under SW flow will be
the primary mitigating factor for destabilization as overnight
cloud cover remains mostly CI which will do little to impact the
diabatic heating. All guidance, including meso-scale guidance
continues to indicate ML CAPE values reaching nearly 1500j/kg by
21Z-00Z thanks to remnant EML with mid lvl lapse rates 6.0-7.0C/km
peaking late evening. Therefore, still plenty of instability above
the marine layer to work with, supporting CAPE values near the
-20C layer around 300.
At odds is timing and how widespread the convection will be,
initial pre-frontal trof looks to trigger scattered convection
earlier in the day mainly across N New England, closer to the
poleward exit region of the mean upper lvl jet streak. Later, with
convection firing upstream in NY state (less of a marine influence
and therefore better sfc based instability). What this means for S
New England will take some time to fully flesh out, but it appears
that the upstream convection will need to be watched as it will
most likely define the overall convective mode, should an MCS form
there it could track mostly SW of the region with more sct pulse
storms across NE or much less activity altogether, but any shift
would bring it through our region instead. WRF/NAM support this
possibility. Either case it/s later, more with the core of the
remnant EML and upper lvl shortwave passage (mainly after 21Z and
continuing through about 04Z or midnight local). The risk remains,
mainly later in the day, so this morning`s update will reflect
Primary risk with 0-3 and 0-6km shear reaching about 35 kt with
modest turning in the hodograph. Plenty of CIN supporting high
downdraft CAPE as well. Therefore, regardless of the mode,
straight-line damaging winds remain the highest threat, especially
if an MCS or bowing reflectivity patterns are observed.
Secondary risk due to relatively high hail-CAPE as
mentioned above. The shear mentioned above also supports some
rotation in the updrafts of organizes storm which will enhance
hail-growth. Mitigation is the very high WBZ temps, mainly at/or
above 10kft. Will continue the use of small hail with this update.
Warm clouds layers exceeding 10kft combined with PWATs nearly
1.75 (1 std deviation above normal) will allow any storm to
produce localized heavy rain. Corfidi vectors suggest some back-
building potential, so will need to monitor for localized urban
flooding in particular.
Low risk, but non-zero. Very high LCLs is not overly supportive.
However should an isolated supercell structured storm develop
especially in the vicinity of the southern sea breeze or other
localized low-lvl vorticity source, this will need to be watched.
EHI values already progged to be between 1.0-2.0 and near sfc
lapse rates are nearly dry-adiabatic thanks to mixing.
As mentioned above, this lingers into the early overnight hours
until cold front itself stalls near the south coast. Will continue
to monitor the risk, although it`s likely to gradually dissipate
during the morning as drier air begins to move in.
.SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/...
Updated at 2 PM...
* Risk of strong to severe thunderstorms Saturday afternoon into
early evening across northern and eastern Massachusetts...
Increasing confidence of a severe thunderstorm threat across
northern and eastern Massachusetts Saturday mid afternoon to early
evening. Main threats look to be locally damaging wind gusts and
The 12Z GFS and NAM continue to paint an area of instability
across northern MA and eastern MA Saturday afternoon/early
evening. This being helped by an impressive 500 mb cold pool of
temperatures lowering to -12C to -13C in the 21Z to 00Z time
frame. In addition, the vertical wind shear looks more impressive
Saturday afternoon/evening than this afternoon with 500 mb winds
near 40 knots. Other considerations include low level moisture
convergence signal from what is left of the surface frontal
boundary and the presence of the left front quad of a 90+ KT upper
jet over NE MA. Have updated the HWO to communicate this
increaseing concern of the risk for severe thunderstorms during
Saturday afternoon/evening, mainly northern and eastern MA.
Another hot day as H85 temps will struggle to drop lower than
+18C, so expect another round in the low 90s. Dwpts will be
slightly lower however thanks to influence of low-lvl drier air,
so expect heat indices to still remain below advisory thresholds.
Otherwise, am concerned about two factors. The first is the cold
front itself which may remained stalled very close to the S coast
and provide a secondary focal point for afternoon convective
activity. The other is a secondary shortwave which could ignite
convection across Quebec which may slide south into New England
late day as the northeast CONUS remains along the outer edge of
the ring-of-fire. Convective mode from today and final frontal
placement will all need to be watched as a secondary risk for
thunderstorms looms for Saturday, which could once again contain
strong winds. Should conditions not be favorable, the highest risk
would remain in northern New England.
.LONG TERM /SUNDAY THROUGH FRIDAY/...
High pressure covers the Southern and Central USA much of the week.
A zonal flow extends across Canada and the USA Northern Tier.
Shortwaves in that zonal flow will move through New England Monday
night and again on Friday. In between, parts of the large high will
push up over our area. Forecast contour height fields remain
higher/warmer than normal levels through the period, so expect
temperatures to run warmer than normal most of the time.
High pressure in place through the day. Some signs on the GFS and
ECMWF that a twist may develop in the flow in Eastern MA that could
generate an eastern coast sea breeze during the afternoon, but low
confidence at this time. Solar mixing to near 850 mb with temps at
850 mb forecast at 16-17C. This would support max sfc temps inland
at 87-91F, with ocean buffering keeping levels near 80 Cape and
Islands. Northwest flow will bring lower/drier dew points of 55 to
60 inland and lower 60s along the South Coast.
Light wind Sunday night. Some increase in high clouds and slowly
increasing dew points should keep min sfc temps in the 60s, although
upper 50s not out of the question in some normal cool spots.
Shortwave drives low pressure across Eastern Canada with a cold
front sweeping across NY during the day and New England Monday
night. Most of the day should be rain-free. Some signs of marginal
instability crossing during the morning which may be a warm front
with a couple of light showers. Better instability moves in from the
west during the afternoon and evening. Totals forecast in the upper
40s and mid-level lapse rates 6.2 to 6.5 C/Km. Precip Water values
climb to near 2 inches. This would support thunderstorms, some with
heavy downpours. Winds of 20-25 kt at 850 mb and 35 kt at 500 mb
suggest potential for strong gusts in convection. Expect scattered
afternoon/early night convection with local downpours. Temps of
18C at 850 mb supports another day of max temps in the low 90s.
High pressure takes control and brings fair weather. Temps aloft
will again be 16-18C which will support upper 80s and lower 90s.
Next upper shortwave moves through during this time. Both GFS
and ECMWF seem inconsistant with their surface features but indicate
at least one cold front moving through during this time. We will show
chance pops in western sections Thursday afternoon and all areas
Friday. Low confidence in details at this time.
.AVIATION /19Z 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 Night/...
Rest of this afternoon...High confidence except for placement of
scattered late afternoon/evening thunderstorms. VFR conditions
expected to prevail except briefly lower in vicinity of scattered
late afternoon and evening thunderstorms. Some hints that the
thunderstorms would be more likely across SW MA and CT late this
afternoon but confidence no more than moderate on this. A few
thunderstorms may contain strong wind gusts and hail. Surface
wind gusts to 35 KT across Cape terminals due to presence of low
level jet aloft.
Tonight...Moderate confidence. A short period of low level wind
shear is also possible around mid/late evening as surface winds
diminish. Areas of MVFR cigs/vsbys and even pockets of IFR tonight
in areas of stratus and fog, mainly along the south coast and a
few interior valley locations that receive rain. Winds shift to
the NW overnight.
Saturday...Moderate confidence. After any early fog dissipates.
VFR dominates again most of the day. Afternoon/early evening
thunderstorms are possible, mainly across northern and eastern MA.
A few of these thunderstorms may contain strong wind gusts and
hail. Winds mainly W.
KBOS TAF...High confidence. WSW flow should dominate during the
afternoon and think a sea breeze is unlikely. There remains a
chance of an evening thunderstorm but thinking it is a low chance
at this point. There looks to be a better chance of thunderstorms
in the vicinity of the terminal Saturday late afternoon to early
evening. A few of those thunderstorms could contain strong wind
KBDL TAF...High confidence of VFR prevailing. There is a risk of
thunderstorms in the vicinity of the terminal late this afternoon
and early evening. Any thunderstorms could produce a brief strong
wind gust. If it rains over or near the airfield, there would a
possibility of MVFR visibilities in overnight fog patches.
Outlook /Sunday through Wednesday/... Moderate-high confidence.
Sunday...VFR. Possible areas of IFR in early morning fog.
Monday... VFR. Possible IFR in early morning fog. Brief MVFR/IFR
cigs/vsbys in afternoon and early night showers/tstms.
Forecaster Confidence Levels.
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Short Term /through Saturday Night/...High confidence.
Today and tonight...
2 PM update...
Have continued with the SCA in place. Seeing some gusts of 35+
knots over land but generally in the 25 to 30 knots category over
the adjacent coastal waters.
SW winds continue and increase especially near the southeast
coastline today. Gusts 25-30 kt expected. This will also allow
winds to increase to near 7 ft by the overnight hours. Small craft
advisories will continue. Will also need to watch for strong
thunderstorms with localized higher winds, hail and heavy
Winds and seas generally slacken such that small craft advisories
may be dropped through the early half of the day. Will still need
to monitor for some isolated showers and thunderstorms.
Outlook /Sunday through Wednesday/... Moderate confidence.
Winds less than 20 knots and seas less than 5 feet through the
period. A cold front will bring scattered showers/thunderstorms
Monday afternoon and night, with potential for strong wind gusts
and brief poor vsbys in heavy downpours.
CT...Air Quality Alert until 11 PM EDT this evening for CTZ002>004.
MA...Air Quality Alert until 11 PM EDT this evening for MAZ005>007-
RI...Air Quality Alert until 11 PM EDT this evening for RIZ001>008.
MARINE...Small Craft Advisory until 8 AM EDT Saturday for ANZ231>234-
Small Craft Advisory until 2 PM EDT Saturday for ANZ235-237-