Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service Taunton Massachusetts
1017 PM EDT Tuesday Jul 22 2014

warm and humid weather is expected through Wednesday night ahead of
an approaching cold front. The front will trigger scattered showers
and thunderstorms Wednesday afternoon and evening. A few strong
storms are possible especially inland. Showers may linger into
Thursday along the South Coast however the trend will be for drier
and less humid weather Thursday night into Saturday. The weather
pattern then becomes unsettled and more humid beginning Sunday and
especially early next week.


Near term /until 6 am Wednesday morning/...
10 PM update...

Previous forecast on track with very little in way of changes
needed. Main change was to eliminate a small area of slight chance
probability of precipitation in the west...and have forecast probability of precipitation near zero overnight
night throughout the forecast area. Tweaked temperatures just a
bit to fit latest observations and lowered sky cover some
overnight based on latest satellite imagery.

Will continue to monitor for any development of low clouds and/or
fog. Current thinking is that there may be enough surface gradient
to preclude any dense fog and may be more likely to witness patchy
low clouds along the South Coast later tonight.

7 PM update...

As of 7 PM...daytime cloudiness had diminished except in the
immediate slopes of the Berkshires. At 6 PM...a patch of haze
moved across Nantucket reducing visibility to 5 miles. Visible
satellite imagery showed this to be part of a larger band of haze
to the south of New England...presumably from fires in the western
states. The area was shifting to the south farther away from our
region and as of 7 PM Nantucket was back to 10 mile visibility.
Sea breezes have come to an end along the Eastern Shore of
Massachusetts...with winds now out of the southwest at most locations.
Tweaked temperatures upward in the CT river and Merrimack valleys
for the next few hours per current observations.

Previous discussion...
tonight...high pressure continues to remain well east of the area
with upper level flow becoming more zonal. Southerly flow will
continue across the region increasing the moisture levels allowing
for a very muggy temperatures drop only to the middle to upper
60s. Because of the increase in moisture anticipate patchy valley
fog to develop limiting visibilities.


Short term /6 am Wednesday morning through Wednesday night/...
tomorrow and tomorrow night...

* a few strong to severe storms possible late Wednesday afternoon
into the evening hours *

The weather will become more interesting beginning tomorrow
afternoon into the overnight hours as a cold front begins to
approach the region.

Ahead of the approaching cold front expect a fairly quiet morning
with partly sunny skies to start. There is a good amount of mixing
across the region allowing southwesterly winds to gust between 20-30
miles per hour. This will also allow for temperatures to warm at or above
90...especially since several sites reached 90f today.

Cold front will begin to approach the region by late afternoon.
BUFKIT soundings do indicate a cap across the region but with
falling heights and cooling temperatures aloft...believe the cap will
break. Still some uncertainty on the exacting timing but feel it
will be closer to the late afternoon hours. Because of the moist
unstable airmass across southern New England...once the front
approaches storms will develop...with the coverage being more
scattered to start with perhaps turning into a broken line. The
region will be very unstable with MUCAPE values will be between 1500-
2500 j/kg with shear values very meager...15 kts from 0-6km. The
only downfall for any afternoon convection is the lack of deep layer
moisture. However closer to 00z and into 06z the better moisture
approaches the region and with lingering instability and better jet
dynamics anticipate thunderstorms to continue into the overnight
hours. The best area to see the strong to severe storms will be
north and west of I-95.

The biggest threats for these storms will be strong damaging winds
and localized flash flooding. Held off on Flash Flood Watch as precipitable water
anomalies are only 1 Standard deviation away. Therefore feel that
flooding potential will be localized within the stronger convection.
Shear values are fairly weak across the region...however across the
immediate northwest zones...there is marginal shear values closer to 30 kts
and with some turning in the hodographs cold see updrafts sustain
themselves to produce damaging winds. At this time continued with
enhance wording of gusty winds and heavy rainfall.

Front will move northwest down to the southeast through the
overnight hours. As mentioned before believe conditions will still
be favorable for thunderstorms to continue through the overnight.
However believe the strong to severe thunderstorm threat will
diminish and turn more to a heavy rain threat after 06z.


Long term /Thursday through Tuesday/...

* much cooler Thursday along with rain lingering South Coast
* dry...less humid and seasonable Thursday night through Sat
* pattern becomes unsettled and more humid sun into next week

Synoptic overview and model preferences....

Northern stream middle level trough moves across the area Thursday with its
surface cold front slowly exiting the South Coast. Post frontal
airmass overspreads the region Friday. The middle level mean trough axis
then moves offshore Sat with subtle height rises and anticyclonic
flow overspreading New England. This will maintain dry weather over
the area through Sat. By Sunday and especially early next week all
model guidance including the gefs and ecens ensembles are in good
agreement on a high amplitude /+2 South Dakota/ long wave trough setting up
over southeast Canada southward into the Ohio vlly. Both deterministic
and ensemble guidance are in very good agreement on the large scale
flow across North America this period. Therefore will base this
portion of the forecast on a model blend.

Daily details...


Middle level trough axis still well west of New England at 12z Thursday.
Thus cold front will be slow to exit the South Coast which will
increase the risk for showers and possible a thunderstorm to linger across
this region. The high res deterministic guidance /GFS and NAM/ are
slower with the frontal passage as these models form a stronger frontal wave
than the ensembles. This is possible given the higher res models would
perform better than the course ensembles simulating a small scale
frontal wave. Therefore will go with likely probability of precipitation Thursday morning across
Rhode Island and southeast Massachusetts and trending to chance as the rain shield moves
slowly southeast and eventually offshore later Thursday. Departure of
rain will be critical to temperature forecast as the wetter GFS 2 meter
temperatures never climb out of the 60s! Even the warmer European model (ecmwf) and NAM 2
meter temperatures are only into the l70s. Thus a cooler day on tap Thursday and
if the GFS verifies our temperatures will be too warm.

Elsewhere across interior CT/Massachusetts into southwest New Hampshire mainly dry
conditions should prevail with lots of middle and high clouds along
with north-NE winds yielding a cooler day.

Thursday night through Saturday...

Post frontal airmass overspreads the area Thursday night and Friday with a
850 temperature anomalies around 1 Standard deviation cooler than climatology...
about +10c/+11c. Hence refreshing airmass with dew points in the 50s.
The core of the dry/cooler airmass will be over southern New England
around 12z Friday. This will yield a very comfortable and cool temperatures
Thursday night/early Friday am.

Sunday through Tuesday...

Pattern becomes unsettled as long wave trough sets up over southeast
Canada southward into the Ohio Valley. This provides deep cyclonic
flow across the northeast along with a broad surface low pressure over
New York and New England. Low level flow will be south-southwest off the Atlantic into
New England so expect increasing humidity along with the risk of
scattered showers and thunderstorms.


Aviation /02z Wednesday through Sunday/...
forecaster confidence levels...

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

Short term /through Wednesday night/...

Generally moderate confidence on forecast.

Overnight into Wednesday...conditions remain VFR as of 02z. Anticipate
patchy MVFR fog/ceilings across interior due to the increase of low
level moisture. Stratus and fog may again appear over the cape
and islands. There may be enough surface gradient to keep visibilities
from dropping below MVFR...but possibility of at least patchy IFR
ceilings from stratus. Conditions clearing to VFR by or shortly after daybreak.

Late Wednesday afternoon into Thursday...MVFR ceilings and visibilities in
vicinity of scattered thunderstorms ahead of a cold front. A few thunderstorms may
briefly lower ceilings/visibilities to IFR levels. Also...a few thunderstorms may
produce strong wind gusts. Visibilities may be reduced in patchy fog
ahead of frontal passage as well. Sref probabilities are high for
low cloudiness to form over southeast Massachusetts and Rhode Island middle to late Wednesday
afternoon in the warm muggy airmass... so will need to watch for
this separate area of cloudiness well ahead of any storms.

Kbos taf...high confidence. T-storms possible by Wednesday late

Kbdl taf...high confidence in taf. Low probability on fog potential
tonight. T-storms possible by Wednesday afternoon.

Outlook...Thursday through Sunday...moderate confidence

MVFR possible Boston to Providence in rain...possibly IFR South
Coast with VFR northwest Massachusetts into southwest New Hampshire. Then VFR all
terminals Thursday night into Sat. Low risk of MVFR in scattered
showers/thunderstorms Sun afternoon western New England.


forecaster confidence levels...

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

Short term /through Wednesday/...moderate confidence.

Winds and seas are expected to remain below Small Craft Advisory
levels through tonight. The SW flow will strengthen ahead of a
cold front on Wednesday with a risk of gusts approaching 25 knots. Areas of
fog may develop along the south coastal waters tonight.

Outlook...Thursday through Sunday...moderate confidence.

Thursday...modest north-NE wind along with periods of rain especially south
coastal waters.

Fri/Sat...pleasant boating weather with dry conditions and light
winds becoming a modest south-southwest Sat afternoon.

Sun...modest south-southwest wind along with low risk of a few showers near
shore. Otherwise mainly good visibilities.


kbox radar may be in standby Mode Wednesday morning while
construction takes place nearby. The radar will return to service
by early afternoon prior to start of any convection in our
forecast area.


Box watches/warnings/advisories...
New Hampshire...none.
Rhode Island...none.
Marine...Small Craft Advisory from 10 am to 7 PM EDT Wednesday for
Small Craft Advisory from 2 am Wednesday to 3 am EDT Thursday
for anz250-254.


near term...dunten/gaf/Thompson
short term...dunten
long term...nocera

National Weather Service Glossary of Abbreviations