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000
FXUS61 KBOX 201339
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
939 AM EDT Sat Apr 20 2024

.SYNOPSIS...
A round of showers mainly this morning will persist into early
afternoon across parts of Rhode Island and southeast
Massachusetts, but partial sunshine should develop across the
interior later today. A ridge of high pressure will result in
dry weather Sunday through Tuesday with mild days and cool
nights along with elevated fire weather potential. Unsettled,
wet and cool on Wednesday. Turning drier late in the week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Last several runs of the HRRR and RAP looked to have a good
handle on the showers this morning, and the possibility for a
second round later this afternoon into this evening. Thus, used
the consensus of that guidance for rainfall chances and timing
with this update.

First round of showers was moving east, and should be largely
offshore by noon. Could be as late as 3-4 PM before the risk for
showers from this band clears the Cape and islands. By that time
however, thinking a second round of showers will be underway
across the western half of southern New England. Eventually,
these showers should move across the eastern half of our region,
before moving offshore later this evening.

Only minor tweaks to temperatures. Tricky forecast, as current
forecast is predicated on seeing a brief period of sunshine.
Starting to see hints of some breaks developing across NE PA
into eastern NY state. Will continue to monitor. Latest near
term guidance has trended slightly cooler, but thinking even a
little bit of sunshine will have a quick impact on temperatures.

Previous discussion...

* Showers develop this morning with a few downpours too
* Bulk of the showers come to an end late morning into mid
  afternoon
* High temps this afternoon in the upper 50s to the middle 60s

A shortwave and its associated cold front was approaching from
the west this morning. In advance of this front, increasing low
level moisture has allowed low clouds to overspread much of the
region very early this morning. A few showers were starting to
develop across western MA very early this morning. We expect the
activity to increase in areal coverage & intensity as the front
pushes east into an environment where PWATS will exceed 1 inch.
So expect showers to blossom across the region through daybreak
with a few brief downpours too.

This front is progressive and we expect the showers to pretty
much come to an end from northwest to southeast by mid afternoon
in most locations and earlier across the interior. Partial
sunshine will develop by mid to late afternoon northwest of I-95
and towards evening along the I-95 corridor. The partial
sunshine coupled with the upper trough will result in enough
diurnal instability to trigger a few additional showers later
today, but the vast majority of this time will feature dry
weather in a given locations. Given the cold temps aloft its
not out of the question that a few of the late day showers
contain some graupel.

The relatively mild start coupled with some partial sunshine
later today should allow areas northwest of I-95 to reach the
lower to middle 60s. Southeast of I-95 where clouds/showers
linger a bit longer...highs will be in the middle 50s to the
lower 60s. The coolest of those readings will be across the
Cape and Islands.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/...
Key Points...

* Clearing tonight with lows in the 30s to the lower 40s
* Partly sunny a bit breezy Sunday with highs upper 50s/near 60

Details...

Tonight...

Lingering clouds across eastern New England should push east of
the region this evening as drier air works in from the west.
This should yield mostly clear skies and diminishing wind will
allow for a good night of radiational cooling. Some locations
will see low temps bottom out well down into the 30s. If the
winds diminish enough there also could be areas of frost,
particularly in the typical low-lying locations. The mild spot
will be the urban heat Island of Boston where overnight lows
should bottom out in the lower 40s.

Sunday...

A weak ridge of high pressure will be nosing in from the
southwest, resulting in a mixture of clouds and sunshine. 850T
are fairly chilly around -3C...but good mixing on westerly flow
should allow for high temps to reach the upper 50s to near 60
in most locations. Westerly winds will gust to between 20 and 25
mph by afternoon.

&&

.LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
Highlights

* Dry, breezy and mild on Mon and Tue. Elevated fire weather
  concerns possible.

* Shower chances return Wed and could linger into Thu. Temps
  trending colder.

* Temps trending milder late in the week into early next weekend.
  Dry on Fri, but we may turn unsettled over the weekend.

Sunday Night through Tuesday...

Stuck under persistent cyclonic flow throughout this period. A
ridge axis builds from the Mid/Upper Mississippi River Valley
Sun Night into the Great Lakes by early Mon. The ridge builds
into the Mid Atlantic by late Mon and offshore on Tue. Another
trough digs into the Great Lakes Region by late Tue. High
pressure nudges into our region through this period. The high
builds overhead Mon Night into early Tue before shifting
offshore later on Tue.

Not a lot of change from the past couple of days. Dry and quiet
weather expected through this timeframe. Main concern will be
elevated fire weather concerns. For both days the boundary layer
will be well mixed, but on Mon flow will be out of the W to NW
and on Tue winds turn southerly as the high shifts offshore.
Both the NAM and GFS continuing to show the boundary layer
mixing to approximately 800 hPa per Bufkit soundings. This
should mix down the drier air aloft fairly easily. In these
situations guidance tends to perform poorly as temps, min RH and
dew points overperform. Opted to increase temps to the 75th
percentile of guidance for both days. High top out in the mid
50s to the low/mid 60s. Though Tue appears to be the milder of
the two days given the southerly flow.

Lowered dew points/RH values during the daytime due to the
excellent mixing to the 10th percentile of guidance for Mon and
25th for Tue. Not as confident in being much drier on Tue given
the wind shift to southerly flow, which tends to pump a bit more
moisture in. At this point have minimum relative humidities of
15 to 35 percent on Mon and 25 to 45 percent on Tue. Could see
some 20-25 mph gusts during the day.

Tuesday Night through Thursday...

Still stuck in cyclonic flow. A trough over the Great Lakes Tue
Night digs into the eastern Great Lakes/New England on Wed. The
trough may lift out of our region by Thu. This will be our next
shot for unsettled wet weather as a system slides into our
region.

A cold frontal system swings through New England mostly on Wed
bringing widespread rain showers. The PWAT plume not appearing
overly impressive with PWATs below 1 inch per EPS/GEFS and GEPS
nil probs of values AOA 1 inch. Deterministic guidance showing
we generally we be under roughly 0.75 to 0.85 inches. This is
around the 75th percentile for this time of year per SPC
Sounding Climo for CHH. We do have a deeper S/SWly low level jet
at 850 hPa, which could help squeeze out the moisture
available. At this point deterministic guidance shows roughly
30-40 kt jet in place. So we are trending a bit weaker of a
system compared to 24 hrs ago. Should remain mild/seasonable on
Wed. Temps will be above 0 C as the system is initially moving
in, so should stay all rain. However, as things are winding down
much colder air may filter in late Wed into early Thu. This is
especially if the GFS is correct with a deep cutoff parking
itself over the area. At this point the GFS is the outlier with
the ICON/UKMET/ECMWF and GEM guidance all milder. So, have
backed away from snow mixing in as things are winding down.

At this juncture we`ve still got mod to high probs (30-80
percent) of totals AOA 0.1 inches. Whereas the probs of have
lowered for 0.5 inches to low (around 10 percent). The best shot
is across the interior per GEFS/EPS and GEPS guidance. Still
just sticking with WPC for now.

High temperatures on Wed range from the mid 50s to the low 60s.
As for Thu we will be cooler in wake of the system with highs
in the low to mid 50s. There could be isolated showers across
the interior.

Friday through Saturday...

Ridge axis builds into the Great Lakes region on Fri and into
Sat, but a shortwave may ride the ridge as we head into Sat.
High pressure generally in control through this period, but a
frontal boundary may lift toward us as we head into the weekend.

Dry and quiet weather anticipated with temperatures rebounding
nicely as we head into the weekend. Do have some uncertainty as
we head into the weekend with that shortwave lifting toward us.
Given it is spring time there could be some convective
influences. Have just stuck with the NBM for now. Highs around
seasonable levels on Fri and slightly warmer than normal on Sat.

&&

.AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
Forecast Confidence Levels:

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

Today...High confidence in trends, moderate in timing.

MVFR to IFR with localized LIFR as showers slide through.
Anticipate activity ending NW of I-95 by 15-17Z with improvement
to VFR. Southeast of I-95 the lower conditions persist until
roughly 21-03Z with the latest across the Cape/Islands. Could
see a second round of hit/miss showers this afternoon mainly
ORH/PVD/BED and BOS. Low prob there is some thunder as well
along with graupel in these showers, but given the coverage have
stuck with VCSH for now. Winds out of the SW becoming more W at
5-15 kts as day progresses. Could see some 20-25 kt gusts during
the afternoon across the interior.

Tonight and Sunday...High confidence.

Any lingering MVFR-IFR conditions across the Cape/Islands
clear to VFR this evening. VFR conditions expected into Sunday.
WNW generally 5-10 knots shifting to the WSW Sunday at 10 to 15
knots with some gusts of 20 to 25 knots.

KBOS TAF...High Confidence in TAF.

IFR conditions this morning with rain. Will gradually improve to
MVFR by 15-17Z and showers end. Could have another round of
hit/miss activity around 20Z. Could see some graupel or a rumble
of thunder, but given coverage have kept VCSH for now. Winds out
of SW shift to the WSW/W late with gusts of 15-20 kts.

KBDL TAF...High Confidence in TAF.

MVFR/IFR to start, but showers will slide through by roughly
14Z. Should improve to VFR around 16-18Z. Winds out of the S
shifting to the SW/W late and eventually the NW this evening.
Will become gusty around 18-20Z.

Outlook /Sunday Night through Wednesday/...

Sunday Night through Monday: VFR. Breezy.

Monday Night: VFR.

Tuesday: VFR. Breezy.

Tuesday Night: VFR. Breezy. Slight chance SHRA.

Wednesday: Mainly VFR, with local IFR possible. Breezy. Chance
SHRA.

&&

.MARINE...
Forecaster Confidence Levels:

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

Today through Sunday...High Confidence.

A relatively weak pressure gradient should keep winds and seas
generally below Small Craft Advisory thresholds through Sunday.
However, good mixing on westerly flow Sunday may yield a few
afternoon nearshore wind gusts of 20-25 knots creating some
chop.

Outlook /Sunday Night through Wednesday/...

Sunday Night through Monday Night: Winds less than 25 kt.

Tuesday: Low risk for Small Craft Advisory winds with gusts up
to 25 kt.

Tuesday Night: Low risk for Small Craft Advisory winds with
gusts up to 25 kt. Seas locally approaching 5 ft.

Wednesday: Low risk for Small Craft Advisory winds with gusts
up to 25 kt. Areas of seas approaching 5 ft. Chance of rain
showers.

&&

.FIRE WEATHER...
Sunday...

Dry westerly flow in pre-greenup should allow for elevated fire
weather concerns on Sunday. The airmass will mix deeply and
allow for a large temp/dewpoint spread. Highs should reach the
upper 50s to near 60 with dewpoints dropping into the lower to
middle 20s. This should result in minimum afternoon relative
humidities between 20 and 30 percent. The deep mixing should
also produce some 20 to 25 mph wind gusts with perhaps a few
locales approaching 30 mph briefly.

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...None.

&&

$$

SYNOPSIS...Frank/BL
NEAR TERM...Belk/Frank/BL
SHORT TERM...Frank
LONG TERM...BL
AVIATION...Frank/BL
MARINE...Frank/BL
FIRE WEATHER...Frank