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Area forecast discussion
National Weather Service Boston/Norton Massachusetts
947 am EDT sun Apr 22 2018

high pressure remains in control through Tuesday bringing dry
weather with moderating temperatures. Wet weather returns
Wednesday into Thursday as low pressure moves through our
region. Unsettled weather continues into the weekend, but not a


Near term /until 6 PM this evening/...
945 am update...

Just a few tweaks to forecast for this afternoon on an otherwise
quiet weather day.

Forecast soundings show potential for rather deep mixing away
from coast which will bring drier air down to surface, so
lowered dewpoints into teens and 20s which is lower than model
guidance (and a typical bias in the spring). This may also
affect sea breeze potential, mainly along east Massachusetts coast, but for
now we still think it will develop early in afternoon.

Highs in 50s/60s are on track.


Short term /6 PM this evening through 6 PM Monday/...
high pressure remains overhead tonight and Monday. Expect
radiational cooling tonight, bringing another chilly night with
areas of frost, even some temps below freezing. Sunny skies on
Monday with light wind, which will again allow developing sea
breezes. Similar deep mixing to today, but with temperatures a
few degrees warmer and supporting sfc Max temps in the low to
mid 60s away from the cooler coast.


Long term /Monday night through Saturday/...
*/ highlights...

- wet and breezy midweek
- perhaps a reprieve Friday
- more showers possible Saturday

Overview and model preferences...
phasing S stream and Pacific waves will lead to gradual height
falls and reduction of the ridge across NE throughout the day on
Tue and into Wed. Another longwave trof gradually develops
across the Continental U.S. As a result during the latter half of the week.
Following the initial development, a second stronger wave
connected to the Aleutian vortex will likely deepen the trof by
next weekend, yielding a return to cooler, and maintaining the
unsettled and wet conditions that look to begin on Wed. Noting a
better agreement between deterministic runs in the phasing
process and deepening of the wave for mid week than previous
runs. Therefore, although ensemble means will still be used,
will blend in more operational guidance with this forecast


high pres slips east but remains in control through the day and
much of the evening. However, a combination of developing return
flow, and increased moisture loading aloft will likely yield
increasing clouds. Dry weather prevails. 850 mb temps near +4c should be
mixed too, so expecting another day in the 60s in spite of the
clouds. S shore sea breezes likely, with a risk on the East Coast
as well, but increase pres gradient may preclude this.

Wed and Thu...
low pres develops out of the remnant cold pool from convection
across the se, as S stream wave develops. Guidance in fair
agreement on track, east of The Spine of the Appalachians as low
pres deepens enough to draw it more north of east. With it, pwats
increase to nearly 200 percent of normal. As the low deepens, an
low level jet of 40 kt also develops through the morning Wed. This will
lead to a period of rain, potentially heavy at times as k
indices exceed 30 and soundings become conditionally unstable.
Initial overrunning followed by mid low and mid lvl convergence
will lead to rain most of the day. With the wave continuing to
deepen, the low pres will deepen and slow, leading to wet
conditions persisting into the overnight hours, with wrap around
rain showers possible into Thu, but with some improvement. Probs of quantitative precipitation forecast
exceeding 1.00 inches rather high. Clouds along with a dip in
850 mb temps will yield cooler temps each day, with highs mainly in
the 50s and lows in the 40s.

Fri and Sat...
although trof remains in place, a brief period of rising
heights looks possible ahead of a secondary/acute wave
traversing the Continental U.S. From the Aleutian low and northwest territory
origins. Therefore, a period of more dry-than-wet conditions
looking more likely per ensemble/operational guidance. Still
with cyclonic flow and some moisture clouds and a spot shower
can be ruled out. Temps near seasonal normals, although
potentially cooler if more cloud than sun is observed.

Early next week...
the aforementioned wave reinforces the trof across the E, with
a return to cooler and wetter wx possible. Still several details
to Iron out, but confidence rises based on ensemble
means/operational runs showing at least some synoptic agreement.


Aviation /14z Sunday through Thursday/...
forecaster confidence levels...

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

Short term /through Monday/...high confidence.

Some question as to sea breeze potential along east Massachusetts coast this
afternoon as winds aloft may be just strong enough to prevent it
from developing, so confidence is moderate, but at very least
winds will veer to north during afternoon. Higher confidence
along South Coast.

Otherwise VFR with light winds and another round of sea breezes

Kbos terminal...moderate confidence in taf. Per above
discussion, not as confident that east/southeast sea breeze will develop.
At very least, winds veer to north this afternoon.

Kbdl terminal...high confidence in taf.

Outlook /Monday night through Thursday/...high confidence.

Monday through tuesday: VFR.

Tuesday night: MVFR/IFR conditions possible. Breezy. Chance rain.

Wednesday: mainly MVFR, with areas IFR possible. Breezy. Rain.

Wednesday night: mainly MVFR, with areas IFR possible. Breezy.
Chance shra, chance rain.

Thursday: mainly MVFR, with areas VFR possible. Breezy. Chance
rain showers.


forecaster confidence levels...

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

Short term /through Monday/...high confidence.

Sea breezes develop along South Coast this afternoon with
15-20kt gusts expected by mid afternoon, which could result in
choppy seas near shore. Similar conditions Monday. Not as
confident along East Coast where north winds may persist through
day, but better potential for sea breeze Monday.

Otherwise, high pressure maintains light winds and flat seas.

Outlook /Monday night through Thursday/...high confidence.

Monday through tuesday: winds less than 25 kt.

Tuesday night: winds less than 25 kt. Seas locally approaching
5 ft. Chance of rain.

Wednesday: low risk for Small Craft Advisory winds with gusts
up to 25 kt. Areas of rough seas. Rain.

Wednesday night: winds less than 25 kt. Rough seas up to 12 ft.
Chance of rain showers, chance of rain.

Thursday: winds less than 25 kt. Rough seas up to 9 ft. Slight
chance of rain showers.


Box watches/warnings/advisories...
Rhode Island...none.


near term...jwd

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