Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 200749
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
349 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/...

345 AM Update...

* 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 
on Sunday...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 /08Z SATURDAY THROUGH WEDNESDAY/...

Forecast Confidence Levels:

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

06z TAF Update...

Today...High Confidence. 

Conditions deteriorate to low end MVFR-IFR with localized brief
LIFR conditions through 12z as showers blossom across the
region. The showers should end northwest of I-95 by lunchtime
with conditions improving to VFR. Southeast of I-95...the lower
conditions will persist until late afternoon and early evening 
across the Cape/Islands. SW winds becoming more W at 5 to 15 
knots as the day wears along with some 20-25 knot gusts 
developing during the afternoon across the interior.

Tonight and Sunday...High Confidence.

Any lingering MVFR-IFR conditions across the Cape/Islands
dissipate this evening. Otherwise...VFR conditions expected
tonight and 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.

KBDL TAF...High Confidence in TAF. 

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. 

06z TAF Update...

Today...High Confidence. 

Conditions deteriorate to low end MVFR-IFR with localized brief
LIFR conditions through 12z as showers blossom across the
region. The showers should end northwest of I-95 by lunchtime with 
conditions gradually improving to VFR. Southeast of I-95...the lower 
conditions will persist until late afternoon and early evening 
across the Cape/Islands. SW winds becoming more W at 5 to 15 knots 
as the day wears along with some 20-25 knot gusts developing during 
the afternoon across the interior.

Tonight and Sunday...High Confidence.

Any lingering MVFR-IFR conditions across the Cape/Islands
dissipate this evening. Otherwise...VFR conditions expected
tonight and 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.

KBDL TAF...High Confidence in TAF. 

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...Frank
SHORT TERM...Frank
LONG TERM...BL
AVIATION...Frank/BL
MARINE...Frank/BL
FIRE WEATHER...Frank