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Area forecast discussion
National Weather Service Taunton Massachusetts
729 am EDT Tue Apr 25 2017

Synopsis...
low pressure will slowly track up the East Coast today and
Wednesday, bringing periods of rain, gusty winds, and the risk
of minor coastal flooding. Drier and milder conditions will briefly
return Thursday. A cold front may bring scattered showers and
possibly a few thunderstorms on Friday and stall across the region
on Saturday. A backdoor cold front is possible on Sunday.

&&

Near term /until 6 PM this evening/...

730 am update...

Overall trend in the forecast remains on track this morning.
Southerly plume of moisture combined with easterly flow has
resulted in widespread showers across southern New England.
Anticipate these on and off showers to continue through most of
the day. Looking downstream low clouds/stratus and fog will
continue to advect northward into the region. This will result
in conditions dropping throughout the day.

Pressure gradient from approaching system has resulted
in gusty winds across the cape and anticipate this to
continue.So far only aligned forecast up with current
conditions.

Previous discussion...

High confidence in trends, moderate confidence in timing.

A low pressure off the mid Atlantic coast will try to move
north today. However, a weak surface ridge over southern New
England will prevent it from making much northward progress
today. Despite that, moisture will stream north into our region,
resulting in plenty of clouds, and eventually periods of rain,
across all of our region. Precipitable water values approach
1.25-1.50", which is about 2-3 Standard deviations above normal.
Rainfall could be heavy at times, especially late this
afternoon. Not thinking this rainfall will lead to river
flooding, but will need to monitor the poor drainage areas,
especially the urban centers.

Dew point depressions across northern Massachusetts remain large early this
morning, which means it will be a while longer before
significant rainfall reaches the ground. Previous forecast had
this covered well, so only tweaked the onset timing by an hour
or so to reflect observed trends. Still thinking at least light
rainfall will develop over the region by 9-10 am.

East winds will mean another raw Spring day, especially along
the coasts. Max temperatures today are expected to be around 10
degrees below normal.

&&

Short term /6 PM this evening through 6 PM Wednesday/...
tonight...high confidence.

Widespread rainfall will fall tonight as weak ridge over
southern New England breaks down. A strong southeast low level
jet will also approach, especially after midnight. Not a lot of
instability, but marginal mid level lapse rates could result in
a few thunderstorms tonight into Wednesday morning. The most
likely area where this could occur remains the cape and islands,
due to their proximity with the low level jet.

Temperatures should remain steady or slowly rise tonight,
especially across Rhode Island and southeast Massachusetts. Gusty easterly winds will
continue tonight along the coast.

Wednesday...moderate confidence.

Low pressure continues to move slowly north along the coast.
Still expecting this low to remain south of our region through
Wednesday evening. Despite that, the mid level low associated
with this system should be overhead, opening the door for mid
level drying to bring an end to the widespread rainfall.
Thinking this is more likely to happen during the afternoon
hours, but do not have a lot of confidence in the exact timing.
Even so, scattered areas of rain, or perhaps drizzle, may
persist for much of the afternoon.

Low risk for thunderstorms continuing, mainly across the cape
and islands, and the adjacent coastal waters.

&&

Long term /Wednesday night through Monday/...
highlights...

* active weather pattern through the period
* moderating temperatures towards the end of the work week
* possible back door cold front on Sunday

Pattern overview...

00z guidance is in good agreement for the synoptic scale, does
have issues at the mesoscale level that will need to be
resolved. An active weather pattern will continue into the
extended as cut-off low pressure will begin to move towards the
Maritimes Thursday morning. Long wave trough over the central
Continental U.S. Will result in a narrow short wave ridge axis which will
move across the region during the later half of Thursday.
Approaching cold front on Friday appears to be stalled/weaken as
Bermuda high builds into the region. This puts the flow/mid-
level boundary right over the region with several waves moving
out of the plains, this could result in on/off showery weather
Saturday and into Sunday. There is the potential for a backdoor
cold front due to passing wave over the Maritimes on Sunday
keeping things a bit cooler Sunday into Monday. Potent cold
front moving across the Great Lakes late Sunday will result in
strong ridging for Monday with cold frontal passage on Tuesday.

Details...

Thursday into Friday...high confidence.

Upper level low still lingers early Thursday before moving
towards the Maritimes during the later half of the day. Both
low and mid- levels are still fairly moist so anticipate clouds
to stick around as well as hit or miss showers thanks to onshore
flow. Thursday will start on the cooler side as 850 mb temps
fall to about 8-10 c thanks to weak cold air advection. However later in the
day, strong warm air advection returns from narrow short wave ridge which will
dry out the mid-levels and 850 mb temps warm to near 13c. Still
anticipate a few rogue showers if not sprinkles during the day.
Soupy conditions for the overnight with fog as dewpoints remain
high.

Approaching cold front on Friday will keep clouds around the
area as well as strong southwesterly flow. Dewpoints could
reach into the mid to upper 50s during the day, and depending on
the amount of sunshine, we could see enough cape for a rumble
of thunder. Still some uncertainty with this system as mid-level
ridge from Bermuda high will be increasing heights ahead of
approaching cold front. This will stall and/or weaken the front
as it arrives. There is also some timing issues with the ec/CMC
a bit more progressive with this frontal system versus the GFS.
Still cannot rule out some showery activity with a rumble of
thunder. Some of these showers could contain heavy downpours as
precipitable water values increase to above an inch to near 1.5 inches.

The weekend...moderate confidence.

Bermuda high continues to build with ridging over the eastern
Continental U.S.. potent Southern Plains trough will dig and slowly move
eastward through the period. This will keep the region in a
continuous southwesterly flow as Gulf moisture continues to
stream into the area thanks to Bermuda high. A few pieces of
shortwave will move through the flow which could trigger a few
showers on Saturday. Temperatures will be warm with Saturday
appearing to be the pick of the weekend with highs in the 70s to
low 80s, despite the rain/cloud cover. Cannot rule out a rumble
of thunder.

Upper level ridge over the region, will flatten overnight and
into Sunday as northern wave shortwave across Canada drops a
back door cold front. Still some uncertainty on timing and how
strong the front will be but both GFS and ec dropped surface
temps be 15-20f compared to previous runs. Thus have a lower
confidence in this forecast.

Next week...moderate confidence in trends, low confidence on
specifics.

Potent trough across the central Continental U.S. Will approach the region
by early next week. High precipitable water airmass will continue to stream
into the region resulting in showers well ahead of cold frontal
passage. Still some timing with the ec still a bit more
progressive versus the GFS and its 12z run. But overall good
model consensus that system will moving into the region
Monday/Tuesday.

&&

Aviation /12z Tuesday through Saturday/...
forecaster confidence levels...

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

Short term /through Wednesday/...

Today and tonight...high confidence in trends. Moderate
confidence in timing. Conditions lowering to IFR/LIFR later this
morning with widespread rain and patchy fog. East wind gusts to 30
kt developing over cape/islands with winds veering to southeast
tonight with similar gusts. Low level wind shear developing cape/islands this
evening as southeast/S low level jet at 50+ kt develops, especially
after midnight.

Wednesday...moderate confidence. MVFR-IFR ceilings continue with
areas of -ra/rain and patchy fog, especially across the eastern
half of southern New England. Reduced visibilities likely, down to IFR
in spots. May see areas of LIFR ceilings across the higher inland
terrain and along the coast. Southeast winds gusting up to around 25 kt
across Cape Cod and the islands. Low risk of tsra, especially
the S coastal terminals. Low level wind shear impacts for the cape and islands
during the morning hours.

Kbos taf...moderate confidence.

Kbdl taf...moderate confidence.

Outlook /Wednesday night through Saturday/...

Wednesday night into Thursday...moderate confidence. Showers
continue with MVFR/IFR conditions. Cannot rule out LIFR in fog
and dz during the overnight hours. Improving conditions later
Thursday.

Friday...moderate confidence. MVFR/IFR possible during the
overnight hours in fog. Showers moving from west to east
resulting in possible MVFR conditions. Iso thunderstorms and rain is possible with
heavy downpours.

Saturday...moderate confidence. VFR with possible MVFR in
passing showers.

&&

Marine...
forecaster confidence levels...

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

Short term /through Wednesday/...high confidence.

Increasing easterly flow across the coastal waters today into
tonight. Gusts up to 30 kt possible, especially across the
southern coastal waters. Widespread rain will also develop from
south to north this morning, and be heaviest tonight into
Wednesday morning. Plenty of visibility restrictions in both
rain and fog expected. Low risk of thunderstorms, mainly on the
southern waters.

Extended small craft advisories across most waters through
Wednesday.

Outlook /Wednesday night through Saturday/...

Thursday...high confidence. Lingering southeast swell will help
keep seas up on Wed PM into Thursday. Conditions begin to
relax/improve, but Small Craft Advisory will still be needed.

Friday...moderate confidence. Southwesterly flow with outer seas
near 5 feet. Moderate confidence on if Small Craft Advisory will
be needed or not, as Showers Pass over later in the day.

Saturday...moderate confidence. Southwesterly flow continues with
hit or miss showers. Seas lingering near 4-5 feet. Low confidence on
if Small Craft Advisory will be needed.

&&

Tides/coastal flooding...
* potential minor flooding impacts during astronomically high
tides tonight through Wednesday along the east and south
coasts

Not anticipating any flooding impacts for the high tide later
today, as winds, surge and seas will not have built up enough.

Minor flooding possible during the high tide cycles tonight and
Wednesday along both coastlines. A storm surge around 1 ft with
low risk of 1.5 ft surge on top of high astronomical tides and
building seas may lead to minor inundation of vulnerable
shoreline roads. Some beach erosion is also possible, especially
for east and southeast facing beaches.

Boston high tides (flood stage 12.5 feet)...

11.47 feet / Tuesday 11:20 PM
11.15 feet / Wednesday 11:50 am

Providence high tides (flood stage 7 feet)...

5.98 feet / Tuesday 8:11 PM
5.53 feet / Wednesday 841 am

&&

Box watches/warnings/advisories...
CT...none.
Massachusetts...none.
Rhode Island...none.
Marine...Small Craft Advisory until 3 PM EDT Wednesday for anz232>234.
Small Craft Advisory until 3 PM EDT Wednesday for anz231.
Small Craft Advisory until 8 PM EDT Wednesday for anz235-237-
255-256.
Small Craft Advisory until 8 PM EDT Wednesday for anz250-251-
254.

&&

$$
Synopsis...Belk/evt
near term...Belk/dunten

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