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Area Forecast Discussion (AFD)

Issued by Boston / Norton, MA (BOX)

FXUS61 KBOX 181054

Area Forecast Discussion
National Weather Service Boston/Norton MA
654 AM EDT Mon Jun 18 2018


Excessive heat and humidity is expected this afternoon away from the 
cooling marine influence of the south coast. A cold front approaching
from the west may result in a few strong thunderstorms this afternoon
and evening...mainly across interior southern New England. A much
nicer day is on tap for Tuesday behind the cold front with plenty
of sunshine and much lower humidity. Skirting wet-weather Wednesday
followed by a period of seasonable, dry weather. Return of warmer,
muggier conditions into the weekend with the chance of showers and
thunderstorms. Perhaps a return of seasonable, dry conditions early
next week.




* Excessive heat/humidity this afternoon northwest of I-95

* Isolated strong-severe t-storms with localized heavy rain possible 
  between 2-10 pm mainly across interior MA/CT


1) Excessive Heat/Humidity: 

Anomalous airmass was moving into southern New England early this 
morning. 850T should rise to between +18C and +20C along with plenty 
of sunshine into mid afternoon. This coupled with excellent mixing 
on gusty southwest winds should allow high temps to exceed guidance 
numbers in many locations. Highs should reach into the middle to 
upper 90s to the northwest of the I-95 corridor. Dewpoints rising 
well into the 60s should result in afternoon heat index values 
around 100 degrees and heat advisories are posted for this region.

Meanwhile...southeast of the I-95 corridor there will be enough of a 
marine influence to hold highs in the upper 80s to near 90 with 
upper 70s to lower 80s on south coast beaches.  In fact...gusty 
southwest winds will result in these high temperatures occurring by 
early afternoon and cooling marine air will be advancing northward 
later in the day.

2) Isolated Strong to Severe Weather Potential:

There is uncertainty in regards to the convective potential in 
southern New England today....but there is the risk for isolated 
strong to severe thunderstorms mainly across interior MA/CT. Most of 
the high resolution guidance develops isolated to scattered 
convection after 2 or 3 pm across interior southern New England in 
association with a pre-frontal trough.  We may see another round of 
scattered convection...perhaps with a bit more areal coverage after 
7 or 8 pm in association with the surface cold front.  

The biggest limiting factors are weak mid level lapse rates which 
will limit updraft strength/potential and also uncertainty in 
regards to the amount of forcing. However...given the anomalous 
airmass in place there will be modest instability along with 0 to 6 
km shear of 30 to 40 knots. This may be enough for isolated strong 
to severe thunderstorms mainly across interior MA/CT with the main 
threat being localized strong-damaging wind gusts.  While the 
tornado threat is low it can not be completely ruled out either, 
because there is decent low level helicity.  Pwats 3+ standard 
deviations above normal will result in the potential for any 
thunderstorm to produce torrential rain and a very localized street 
flooding threat.  The strong to severe weather threat appears rather 
low across eastern MA/RI as forcing will be slower to arrive and a 
stabilizing marine layer will be working up from the south lowering 



Late tonight...

While the isolated severe threat should diminish by late 
evening...the potential for scattered showers and embedded t-storms 
will continue into much of the overnight hours.  This a result of 
some elevated instability coupled with a cold front moving into a 
high Pwat environment.  We should see the scattered shower/isolated 
t-storm threat confine itself to the south coast toward daybreak. 
Low temps will only fall back into the upper 60s to the lower 70s in 
most locations and it will be quite muggy for most of the night. Fog 
may impact the Cape/Islands as well given high dewpoint air in place.


A much more comfortable day is in store for the region on Tuesday 
behind the cold front.  Much drier air should work in from the 
northwest allowing for plenty of sunshine and significantly lower 
humidity.  High temperatures should top out in the lower to middle 
80s...but it will feel refreshing after today/s heat and humidity.



*/ Highlights...

 - Skirting S-coastal rains Sunday night
 - Cool, dry late-week pattern
 - Return of warm, muggy conditions into the weekend
 - Accompanying chances of showers and thunderstorms 

*/ Overview...

Back and forth. Rossby wave train evident. A sharpening Aleutian H5 
trof, a present downstream ridge-trof-ridge pattern up against the 
Baffin Bay gyre. Midweek broadening, pattern breaks, flatter, shifts 
E, with Pacific energy rotating towards the gyre, S-stream energy at 
times drawn N, accompanying warm, muggy air. But with re-affirmation 
of the Aleutian trof, downstream pattern amplifies once again, air-
masses transition. 

So a cooler, drier early week pattern beneath NW flow, accompanying 
H5 trof, energy drawn through amplified via cyclonic digging, parent 
cold front around Wednesday night ahead of which some rains could 
skirt S New England. Pattern broadening, higher heights, bit more S-
stream dominance over Baffin Bay gyre, a warmer and muggier pattern 
towards the weekend. If the upstream Aleutian trof sharpens, down-
stream pattern amplifying, our natural air conditioner could re-
emerge towards late June, otherwise Summer is here. Targets of 
opportunity in the discussion below.

*/ Discussion...

Around Wednesday night...

Weak Mid-Atlantic energy emerging as H5 trof digs around the Baffin 
Bay gyre. Hesitant on PoPs given a weak low up against a dominant 
Canadian high. Fast, flat flow at the base with the N-stream vort-
max digging, left feeling rain will skirt the S-shore, remain mostly 
offshore, S New England between the two waves, prevailing sinking 
air, less favorable environment. Keep chance PoPs confined to S 
waters, Islands. 

Thursday through Saturday...

High pressure, likely sea-breezes. Transitioning over S New England 
Friday, monitoring radiational cooling, interior fog issues. Leaning 
low-end of MOS guidance for nighttime lows. High pressure E, return 
S flow ahead of warm front Sunday, marine stratus may return. 

Sunday into Monday...

Lifting warm front followed by a weak cold frontal passage. Watching 
for showers and thunderstorms. Return of warm, muggy conditions. Too 
early on specifics, a low confidence forecast.



Forecaster Confidence Levels...

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

Short Term /through Tuesday/...Moderate confidence. 

VFR through early afternoon. Low risk SCT MVFR-IFR along S-coast
where SW winds will be gusting upwards of 35 kts. SCT SHRA / TSRA
developing by 18z, mainly interior MA / CT. RA/+RA possible along
with IFR CIGs and VSBYs. W G30-40 kts possible with storms as well.

SCT SHRA / TSRA dissipating, as late as 3z, again mainly interior
MA / CT. Continued RA/+RA threats along with IFR CIGs and VSBYs.
Otherwise VFR. SW winds through early morning, LLWS for S-coast 
along with MVFR-LIFR CIGs and VSBYs, before winds shift NW by 
Tuesday morning.

VFR conditions expected. 

KBOS Terminal...
SHRA / TSRA into terminal roughly 21z. SCT nature, will prevail
with VCTS. 

KBDL Terminal...
SCT SHRA / TSRA into the terminal roughly 20z. Will prevail
accordingly with VCTS. 

Outlook /Tuesday Night through Friday/...Moderate confidence.

Tuesday: VFR. Breezy. Patchy FG.

Tuesday Night: VFR. 

Wednesday: Breezy. 

Wednesday Night: Slight chance SHRA, slight chance TSRA.

Thursday through Friday: 



Forecaster Confidence Levels...

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

Short Term /through Tuesday/...High confidence. 

Southwest winds increase to between 20 and 30 knots by afternoon.
Strongest of those winds will be near shore with excellent mixing
given daytime very choppy waters to develop in Harbors
and Bays. Long southwest fetch will also build sees to between 3
and 6 feet.

Near shore southwest wind gusts should diminish a bit by late 
evening...but marginal small craft wind gusts and 3 to 6 foot 
seas should persist longer over the open waters. An isolated 
t-storm or two is possible tonight.

Lingering small craft swell across our southern most outer-waters
diminishes by late morning. Otherwise...winds/seas should remain
below small craft advisory thresholds.

Outlook /Tuesday Night through Friday/...Moderate confidence.

Tuesday: Winds less than 25 kt. Seas locally approaching 5 ft.
Patchy fog. 

Tuesday Night through Wednesday: Winds less than 25 kt. 

Wednesday Night: Winds less than 25 kt. Slight chance of rain
showers, slight chance of thunderstorms. 

Thursday through Friday: Winds less than 25 kt. 



Here are the record and forecast highs for Monday June 18...

BOS ... 94 (1907 & 1929) ... Forecast High 95
BDL ... 95 (1957 & 1994) ... Forecast High 96 
PVD ... 94 (1929)        ... Forecast High 88
ORH ... 93 (1907 & 1929) ... Forecast High 91



CT...Heat Advisory from 1 PM this afternoon to 6 PM EDT this 
     evening for CTZ002.
     Air Quality Alert from 11 AM this morning to 10 PM EDT this 
     evening for CTZ002>004.
MA...Air Quality Alert from 11 AM this morning to 11 PM EDT this 
     evening for MAZ003>007-009>016-026.
     Heat Advisory from 1 PM this afternoon to 6 PM EDT this 
     evening for MAZ003-005-006-010-011-013>016.
MARINE...Small Craft Advisory from 10 AM this morning to 10 PM EDT this 
     evening for ANZ230>237-251.
     Small Craft Advisory from 11 AM this morning to 4 AM EDT 
     Tuesday for ANZ250.
     Small Craft Advisory from 11 AM this morning to 10 AM EDT 
     Tuesday for ANZ254>256.



NEAR TERM...Frank/Sipprell
LONG TERM...Sipprell