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Area forecast discussion 
National Weather Service Taunton Massachusetts
747 PM EDT Wednesday Jul 23 2014

Synopsis...

Line of thunderstorms along and ahead of a cold front will sweep
across the region tonight. Thunderstorms will be strong to severe
mainly north and west of the Worcester Hills transitioning to
showers southeastward into Thursday with embedded rumbles of
thunder. High pressure builds back in with drier weather Friday
into Saturday. Unsettled weather returns for Sunday into early
next week.

&&

Near term /until 6 am Thursday morning/...

* Severe Thunderstorm Watch #432 continues until 10 PM EDT this
evening for western Massachusetts and southern New Hampshire *

* line of strong to severe storms continues across north and west
New England into this evening. *

* Severe threats are expected to diminish with time as the line
sweeps southeast *

730 PM update...

Line of strong to severe thunderstorms continues to March east/southeast
with the main threats of strong to damaging winds...along with
heavy rain / downpours resulting in the risk of urban and poor
drainage flash flooding.

Per Storm Prediction Center mesoanalysis...instability on the order of 2k j/kg west/north a
region of nearly 2+ inch precipitable waters prevails. Yet the line encroaches
on a region of very weak / negligble shear. Expecting the line to
weaken as it continues to slide east/southeast into the evening...becoming
diffuse with just widespread rain and embedded thunderstorms. This
a consequence of weaker shear and conclusion of daytime heating.

Have modified the forecast with the latest trends. Kept close to
the near-term guidance by incorporating the rap/hrrr into the
previous forecast which gave a good blend of expected weather.
Severe threats of damaging wind gusts and heavy rain remain over
areas incorporated in the watch. Upon conclusion of the watch...
have continued with heavy rain / gusty wind wording into the early
morning hours with the expectation that there will be some
embedded thunder in a very saturated environment.

Line will March into central-southern New England west/north the next
couple of hours...reaching the Boston-Providence corridor prior to
midnight. Will see the line slow with its approach to the S/se...
so impacts from Providence to the southeast will linger into the early
morning hours and then on into morning before concluding roughly
prior to midday.

&&

Short term /6 am Thursday morning through 6 PM Thursday/...

Thursday...

Cold front will slow down during its passage across southern New
England Thursday morning. Both the ec and NAM show a wave developing
along the front allowing for the slow down while the GFS seems to be
the outlier on pushing the front though by 12z. In fact the upper
level trough axis is still across upstate New York by 12/15z resulting in
clouds and the confidence that precipitation will linger especially for areas
south and east of the bos-pvd corridor. Believe that by 00z the
front and the bulk of the precipitation will be offshore but cannot rule
out a few lingering showers especially for the cape and the islands.
Because of the slow down and the potential wave development...
cant rule out the potential for embedded thunderstorms during the
day.

Drier air will work its way in behind the front resulting in a
cooler day with highs well below average and close to 15 degrees
cooler then today. Aside from the clouds and lingering precipitation
chances...should be a pleasant day.

&&

Long term /Thursday night through Wednesday/...

*/ highlights...

- dry and seasonable into Friday
- return of warm and muggier conditions for Saturday
- significant weather Saturday night into early next week
- a return of dry and seasonable weather by midweek

*/ overview and model preference...

Surprising pattern as we close out the month of July! As present northwest
Pacific impulse ejects east it couples with an upper-level low parked
over NE Canada resulting in trough enhancement through the Great
Lakes region. Subsequent downstream ridging over the North-Atlantic and
ridge strengthening over the west-Continental U.S. Ahead of a Pacific impulse
bottled in the Gulf of Alaska...the atmospheric pattern buckles.
With the stagnant pattern in place...anomalous conditions prevail as
decent agreement among ensemble members signal a -3 to -4 South Dakota in
heights/temperature/humidity.

But a note of caution: similar ensemble members out to d8 and into
early August are exhibiting considerable spread within nao/pna
trends. It is with no certainty as to how this pattern will evolve.
This makes the forecast over the NE-Continental U.S. Difficult further out in
time as New England especially finds itself within the east-periphery
of the aforementioned trough. Just a slight wobble of the overall
buckled pattern east or west can result in considerable changes in the
forecast.

Feel the decent agreement between deterministic and ensemble members
warrants a blend of model guidance towards the forecast. Will hit on
the main points along with any preferred guidance in the dailies
below.

*/ Day-to-day details...

Thursday night through Saturday...

Thinking a dry forecast period with the rrq of the upper-level jet
shifting southeast. Feel the cold front and attendant wave low will remain
over the waters. Showers dissipating /if any/ towards midnight as
dynamics are shunted out of the region.

Otherwise...high pressure and light northwest-flow sweeps across New
England Friday /sea-breezes along the shores/ followed by return
breezy S/SW-flow for the remainder of the period as the high slides
east/southeast and low pressure begins to build in from the west. Seasonable weather
initially...but expect the return of warmer/muggier conditions by
Saturday. Looking clear for Friday...diurnal cumulus for Saturday.

Saturday night into early next week...

The evolution of the trough and the proximity of its east-periphery are
Keys to the forecast /as is the offshore enhacing ridge across the North-
Atlantic/. The track timing and morphology of individual impulses
through the cyclonic flow will ultimately determine the potential
outcomes for the region.

For Saturday night into Sunday...seemingly an initial impulse is a
focus for convective activity /perhaps a mesoscale convective system/ where thermal/moist
axes and subsequent instability converge at the surface and along
the nose of a low level jet rear of an attendant warm-front. The more zonal
regime advects this activity W-to-E. Nearly Uni-directional shear
presents a threat of damaging wind. But early in time and likely
abundance of cloud cover...threat may be limited. Hints of middle-level
drying and wake subsidence? Just a huge mess. May be looking at more
widespread stratiform with embedded thunderstorms. Who knows...just
a first guess. Biggest concern is the plume of precipitable waters exceeding 1.5
inches and a possible risk of flash flooding. Some confidence of
that at least.

Into Monday...the low deepens as it occludes over the Great Lakes.
Along and ahead of the cold front within warm/muggy/unstable
conditions...convection is likely within an environment of deep-
layer forcing...but it is just too early to go into detail. Many
questions as to whether the environment will destabilize beneath
strong SW-NE Uni-directional shear. Appears activity may occur in
the morning once again followed by middle-level drying and subsidence.
Yeesh. Again...lots of uncertainty. Only confidence is with surging
precipitable waters in excess of 2-inches continuing the possible risk of flash
flooding.

A final note...evaluating the 23.0z and 23.12z cips analogs...both
ping upon potential convective threats Sunday and Monday. The
probabilities of severe weather impacts have slightly increased with
the 23.12z analogs /up to 70-percent/. So while the above discussion
contains a fair amount of uncertainty...the forecast still warrants
close monitoring.

At some point the cold front should push east behind which drier and
cooler air wrapping into the occlusion should prevail resulting
in seasonable conditions for the region. Whether this occurs on
Tuesday or Wednesday...just too uncertain.

&&

Aviation /00z Thursday through Monday/...

Forecaster confidence levels...

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

Short term /through Thursday/...moderate confidence.

Line of thunderstorms and rain/+ra sweeping S/se. Strongest mainly west of lwm-bed-ijd.
Mix of MVFR-IFR impacts with passage along with northwest gusts of around
20 kts. Stronger to damaging gusts possible for west and central New
England terminals. Line will weaken further S/southeast with time. Tafs
trended accordingly. For those tafs east of mht-orh-ijd...will hold
off on thunderstorms and rain mention till activity gets reasonably closer.

Towards morning...widespread rain west/ embedded thunderstorms and rain lingers for the
S/southeast terminals exiting out to sea by about midday. Coastal fog
along with vlifr-IFR ceilings along the immediate S-coast /especially
ack/ is anticipated. Conditions improving towards VFR northwest-southeast with
time during the day Thursday.

Kbos taf...moderate confidence. Will hold west/ thunderstorms in the vicinity at 2z. Hold off
on thunderstorms and rain mention until activity closes in. Trend is for thunderstorms and rain to
weaken below the Point of consideration prior to the terminal.

Kbdl taf...moderate confidence. Thunderstorms and rain on the door step should get
in shortly after 0z. Prevailed accordingly. West/north 0-1z...strong line
of storms with frequent lightning and strong to damaging winds
anticipated.

Outlook...Thursday night through Monday...

Thursday night through Saturday...high confidence.

VFR. -Ra moves out Thursday night under northwest flow. Light winds and
mostly clear on Friday /sea-breezes along the shores/. Scattered-broken 5 kft
ceilings Saturday with breezy S/SW winds.

Saturday night through Monday...low confidence.

Periods of ra/tsra. Mix of MVFR-IFR conditions...worst of which
anticipated during overnight periods and +ra. Continued blustery
S/SW winds.

&&

Marine...

Forecaster confidence levels...

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

Short term /through Thursday/...moderate confidence.

The SW flow will strengthen ahead of a cold front to produce gusts
approaching 25 knots across many of the near shore waters...therefore Small Craft Advisory
is still in effect. Seas may approach 5 feet east of Cape Ann this
evening. Otherwise will need to monitor for areas of fog that may
develop along the south coastal waters tonight.

Outlook...Thursday night through Monday...

Thursday night through Saturday...high confidence.

Tranquil boating weather. Some showers may linger along the southeast outer-
waters Thursday night...but should be good after midnight. High
pressure and light northwest-flow Friday /sea-breezes along the shores/.
Winds veer S/SW overnight and become breezy during the day Saturday.

Saturday night through Monday...low confidence.

Weather turning sour. Increase S/SW winds ahead of a strong low over
the Great Lakes region will result in significant wave heights over
the S-waters especially. Showers/thunderstorms anticipated some of
which could be strong.

&&

Box watches/warnings/advisories...

CT...none.
Massachusetts...none.
New Hampshire...none.
Rhode Island...none.
Marine...Small Craft Advisory until 8 PM EDT this evening for
anz230>237-251.
Small Craft Advisory until 8 am EDT Thursday for anz250-254.

&&

$$

Synopsis...dunten/sipprell
near term...dunten/sipprell
short term...dunten
long term...sipprell
aviation...dunten/sipprell
marine...dunten/sipprell

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