Area Forecast Discussion Issued by NWS Boston, MA
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000
FXUS61 KBOX 171119
AFDBOX
Area Forecast Discussion
National Weather Service Boston/Norton MA
719 AM EDT Sun Mar 17 2024
.SYNOPSIS...
A cold front will bring scattered showers to the region this morning
with partial sunshine & breezy conditions by afternoon. A few
additional showers are possible this afternoon...but the vast
majority of the time will feature dry weather. Temperatures
trending near to cooler than seasonable for much of the upcoming
week. Isolated showers possible Monday through Wednesday along
with gusty winds. Drier on Thursday and Friday.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
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7 AM Update...
Made some minor adjustments to precip chances to better capture
the first round of showers sliding through. On top of this did
add an isolated thunderstorm mention to the forecast through
roughly 9 AM across portions of the south coast. There really
isn`t a whole lot of instability to work with, but appears the
meager amount at the apex of LLJ was enough for some isolated
activity. Stuck with this, but could be just a bit overdone.
Think best shot will be across Cape/Islands at this point as
showers are fairly progressive. Rest of the forecast on track.
350 AM Update...
* Scattered showers this morning
* Partly sunny & breezy this afternoon with a few showers
* High temperatures in the middle 50s to the lower 60s
Approaching shortwave trough was inducing a modest southwest LLJ
early this morning. This has allowed an area of low clouds to
overspread much of RI/SE MA. In addition...we expect to see
scattered showers move in from the west toward daybreak as the
forcing for ascent increases. These showers should be relatively
short-lived in a given location and rainfall amounts will be on the
light side. Behind the morning shortwave...expect to see partial
sunshine develop and it also will become breezy. Enough diurnal
heating will trigger some marginal instability...which will allow
for a few more showers this afternoon. Not a washout though and most
of the day will be dry in a given location.
Given the mild start and good mixing...expect to see another day
with above normal temperatures. Highs should be in the middle 50s to
the lower 60s with the mildest of those readings in the CT/RI and
portions of southeast MA. A west breeze of 20 to 30 mph will also
develop during the afternoon with deep mixing indicated by Bufkit
soundings.-- End Changed Discussion --
&&
.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
Key Points...
* Gradually diminishing winds tonight with lows mainly in the 30s
* Partly sunny & breezy Mon with highs in the upper 40s to lower 50s
Details...
Tonight...
Low pressure over Quebec moves eastward into the Canadian Maritimes
tonight. This will generate a westerly flow of cooler air into
southern New England. Westerly winds should diminish with the loss
of daytime heating and skies will become mostly clear. Winds will
probably not decouple in most locations given modest winds off the
deck...so expect overnight low temps mainly in the 30s. Some upper
20s are possible across the normally coldest spots of western MA.
Monday...
Low pressure over the Canadian Maritimes will generate a cooler
westerly flow of air across the region on Mon compared to today.
However...we should still have very deep mixing and with 925T near
or just above 0C...expect high temps to end up still a bit above
normal especially in pre-greenup. Thinking highs will mainly be in
the upper 40s to the lower 50s under partial sunshine along with
scattered diurnal CU. Good mixing should yield westerly wind gusts
of 20 to 30 mph.
&&
.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
Highlights
* Dry for most on Tue with temps trending near to below normal.
Isolated showers during the day. Not out of the question there is
some graupel in more vigorous showers.
* A quick moving front/system slides through on Wed bringing
scattered showers.
* Drier on Thu and perhaps through much of Fri. Could see a return
to more unsettled weather late Fri into Sat. Lots of uncertainty
at this point.
Monday Night through Tuesday...
A shortwave ridge builds into New England Mon Night, but will
quickly build into the Gulf of Maine by early Tue. A shortwave
trough digs into New England from Ontario during the day. A surface
trough rotates through on Tue.
Temperatures will be trending cooler on Tue with a fair amount of
cloudiness. This is a result of the anomalously cold airmass
overhead at 500 hPa.
Trending cooler on Tue with more cloudiness. Should have a better
shot for isolated showers across the interior. An anomalously cold
airmass will be overhead at 500 hPa. The NAEFS/EPS situational
awareness tables continue to indicate temps -2 to -2.5 STD below
model climo. Should see those 500 hPa temps range from -25 to -35
degrees Celsius. This combined with the shortwave and diurnal
heating will bring a fair amount of CU. On top of this think there
could be isolated shower activity across the interior. Guidance
continues to keep us dry, but have seen this set up in the past. For
now have capped our chances at slight given the limited moisture.
Could actually see spots where there is graupel.
On top of this the other change made was to lower dew points/RH
values. Looking like another day where the boundary layer will be
well mixed per GFS/NAM Bufkit soundings. Shouldn`t be too difficult
to mix down the -15 to -25 degree dew points at the top of the PBL.
For now went with minimum RH values toward the 10th percentile of
guidance. May actually need to be lowered in future updates as we
tend be a bit drier than guidance indicates in pre-greenup.
Wednesday through Thursday...
Flow remaining somewhat cyclonic through this timeframe. A fast
moving shortwave trough slides through the Great Lakes and New
England on Wed. The trough lifts into the Gulf of Maine by early
Thu. A ridge axis will begin building over the Low/Mid Mississippi
River Valley early on Thu. This builds into the TN Valley by late
Thu. A frontal system swings through on Wed. High pressure begins
nudging in on on Thu.
Confidence increasing in how things evolve on Wed with the frontal
system sliding through. At this point the big question is will the
shower activity be a bit limited due to the little moisture in
place. At this point PWATs are roughly -0.5 to -1 STD below model
climo. Deterministic guidance shows values ranging from 0.2 to 0.4
inches with W to WSW flow. The W to WSW flow could aid in drying
things out a bit more and should have good mixing within the PBL per
model soundings. The other potential hang up is the best upper
forcing is more to our north. For now think that slight chance to
chances of precip are fine with the best opportunity for showers
north of the MA Turnpike. For most will be warm enough for rain
showers, but the higher elevations could see snow showers. There are
some low probs (10-30 percent chance) of 24 hr snow AOA 1 inch per
the GEFS/EPS guidance. Temps may actually near to slightly milder
than normal with the WSW/SW flow.
Appearing drier on Thu as high pressure nudges into southern New
England. Though will be quite breezy with a tightened pressure
gradient in place over the region. Temps trend near to cooler than
normal with highs in the low/mid 40s.
Friday and Saturday...
Still looking like it will be dry and quiet for much of Fri as a
ridge builds into the Mid Atlantic. Our next trough lifts into the
Carolinas/VA late Fri and perhaps into our region on Sat.
Should see those precipitation chances increasing later on Fri into
Sat, which is supported by the latest GEPS/GEFS guidance. This is
actually not the case with the EPS, which is much more muted with
the probs of 0.1 inches over a 24 hr period. Given there are
significant differences at this point have stuck with the NBM.
Temperatures still cooler than seasonable through this period.
&&
.AVIATION /11Z SUNDAY THROUGH THURSDAY/...
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Forecast Confidence Levels:
Low - less than 30 percent.
Moderate - 30 to 60 percent.
High - greater than 60 percent.
Today...Moderate Confidence.
A brief band of scattered showers and a period of mainly MVFR
conditions impact the region through mid to late this morning.
Drier weather this afternoon, but a few brief passing showers
possible. Conditions improve to mainly VFR thresholds by
15-19Z, but this process may be slower near the Cape & Islands.
S winds 5 to 15 knots early this morning gust between 20 and 30
knots. Highest wind gusts across the Cape and Islands. Winds
shift to the W by this afternoon with gusts of 20-25 knots.
Tonight...High Confidence.
VFR. W winds diminishing to 5-10 knots.
Monday...High Confidence.
VFR. W winds increasing to between 10 and 15 knots. Should see
those gusts picking up to 20-25 kts by roughly 15-18Z.
KBOS TAF...High confidence in TAF.
KBDL TAF...High confidence in TAF.
Outlook /Monday Night through Thursday/...
Monday Night: VFR. Breezy.
Tuesday: VFR. Breezy. Isolated SHRA.
Tuesday Night: VFR. Breezy.
Wednesday: VFR. Breezy. Slight chance SHRA.
Wednesday Night: VFR. Windy with gusts up to 30 kt. Slight
chance SHRA, slight chance SHSN.
Thursday: VFR. Windy with gusts up to 30 kt.-- End Changed Discussion --
&&
.MARINE...
Forecaster Confidence Levels:
Low - less than 30 percent.
Medium - 30 to 60 percent.
High - greater than 60 percent.
Today through Monday...High Confidence.
Low pressure moves across Quebec today and into the Canadian
Maritimes tonight and Monday. This will generate a westerly flow of a
colder air aloft into our region into Monday with good mixing over
the waters. Winds should shift from the S to the W by this afternoon
with gusts between 20 and 30 knots today. Small craft headlines are
posted for all our waters. These wind gusts should diminish a bit
tonight...but increase again on Monday with diurnal heating/mixing.
While we extended the small craft headlines for the outer-waters
through Monday...we did not do that for the sounds/bays as the winds
will temporarily die down tonight. But we may need additional
Headlines for those waters on Monday.
Outlook /Monday Night through Thursday/...
Monday Night: Winds less than 25 kt. Areas of seas approaching
5 ft.
Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt. Areas of seas approaching 5 ft.
Tuesday Night: Winds less than 25 kt. Areas of seas approaching
5 ft.
Wednesday: Winds less than 25 kt. Seas locally approaching
5 ft. Slight chance of rain showers.
Wednesday Night: Moderate risk for Small Craft Advisory winds
with gusts up to 30 kt. Local rough seas. Slight chance of rain
showers.
Thursday: Low risk for gale force winds with gusts up to 30 kt.
Local rough seas.
&&
.FIRE WEATHER...
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Monday and Tuesday...
There are some Fire Weather concerns for Monday and Tuesday. We
expect afternoon westerly wind gusts of 20 to 30 mph with minimum
relative humidity values between 20 and 40 percent. Would not be
surprised if RH values ended up on the lower side of those numbers
too. Given we are in pre-greenup something will have to watch.-- End Changed Discussion --
&&
.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 5 PM EDT this afternoon for
ANZ230>237-251.
Small Craft Advisory until 8 PM EDT Monday for ANZ250-254>256.
&&
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SYNOPSIS...Frank/BL
NEAR TERM...Frank
SHORT TERM...Frank
LONG TERM...BL
AVIATION...Frank/BL
MARINE...Frank/BL
FIRE WEATHER...Frank