Lexington Weather

Area Forecast Discussion
for Boston / Norton, MA

        
000
FXUS61 KBOX 181121
AFDBOX

Area Forecast Discussion
National Weather Service Boston/Norton MA
721 AM EDT Thu Apr 18 2024

.SYNOPSIS...

Cloudy, much cooler, with scattered showers at times today with the 
majority of them across interior southern New England. Dry and 
seasonable for the daytime hours on Friday. A cold front brings 
another round of showers Friday Night into early Saturday. Dry 
for the rest of Saturday with gusty winds. The dry stretch of 
weather continues as we head into early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...

715 AM Update...

Have made some minor adjustments to our temps for this AM as we
were running a bit too cool compared to latest obs. Rest of
forecast still on track though as onshore flow will keep our
temps suppressed today with little sunshine. See previous
discussion for more details. 

305 AM Update...

* Cloudy & much cooler today with scattered showers...focused inland
* High temps only in the upper 40s to the lower 50s

The mid level warm front has resulted in showers along with a few 
downpours early this morning...generally southwest of an ORE-ORH-PVD 
line. The mid level ridge axis across southern New England was 
keeping the forcing/deeper moisture for the shower activity mainly 
across the west and southwest portion of our county warning area. 
This shortwave will continue to deamplify as it moves eastward into 
the mid level ridge axis. Therefore...we expect the bulk of the 
scattered showers to remain across interior southern New England 
today. Even in those locations...not expecting the entire day to be 
a washout. A few brief light showers will probably survive all the 
way to the coast...but regardless the majority of the day will 
feature dry weather in this region. 

High pressure across Quebec will generate easterly flow level flow 
today and this will combined with mainly cloudy skies. The result 
will be cool high temperatures only reaching into the upper 40s to 
the lower 50s.

&&

.SHORT TERM /FRIDAY/...

Key Points...

* A few lingering showers dissipate this evening
* Partial sunshine returns Fri with highs in the 50s to near 60

Details...

Tonight...

A few lingering showers will be exiting the coast this evening as 
the deamplifying shortwave will be moving east of the region. 
Otherwise...clouds will probably be slow to dissipate given there is 
not any real push of drier air behind the shortwave. Overnight low 
temps will mainly be in the upper 30s to the lower 40s.

Friday...

Some modest upper level ridging moves over the region on Friday. 
This should allow for some partial sunshine to return along with 
milder temps then today. We should see more clouds return by later 
in the day/early evening out ahead of an approaching cold front. 
Highs on Friday should recover well into the 50s to near 60 in most 
locations. Surface high pressure off the coast will still result in 
onshore flow which will hold highs along the immediate coast to 
between 50 and 55.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...

Highlights

* Scattered showers Fri Night into early Sat. This is followed by 
  drier, mild and breezy conditions Sat afternoon. 

* Dry, breezy and cooler on Sun. Still dry and quiet Mon/Tue, but 
  milder. Shower chances return late Tue into Wed.

Friday Night through Saturday...

Flow turning more cyclonic during this timeframe. Will have a cutoff 
rotating from Ontario northward through this period. A shortwave 
embedded within the cutoff will lift from the central/eastern Great 
Lakes Fri Night into Quebec by early Sat. Another shortwave/trough 
will dig across the Great Lakes into New England by late Sat. A cold 
front crosses the region Fri Night through Sat. Could have a 
secondary cold front swing through Sat afternoon.

Main change to the forecast at this point was more focused on 
temperatures, dew points and relative humidities overachieving in a 
well mixed boundary layer on Sat. At this point both the GFS and NAM 
Bufkit soundings showing an extremely well mixed boundary layer. The 
GFS mixes roughly to 750 hPa, while the NAM up to 650 hPa! Given the 
W to WNW flow have bumped our temps up to the 75th percentile of 
guidance as the NBM performs poorly in these situations. The result 
are fairly widespread 60 degree readings and wouldn't be surprised 
if we get some low 70s across either the CT or Merrimack Valley. 
This will also bring down much drier dew points, so went toward the 
10th percentile of guidance for dew points and RH values. At this 
point have min RH values ranging from 30 to 40 percent away from the 
south coast. Bumped up the wind speeds/gusts toward the NBM90th for 
now to increase things a bit, but if the NAM is correct then we will 
need to increase things further in future updates. Given this setup 
and we are still in pre-greenup there could be elevated fire weather 
concerns, though this could be mitigated for portions of the region 
depending on how much precip falls as the front slides through.

A bit of a backtrack here time wise, but think the fire weather 
concerns will be more impactful than the showers Fri Night into 
early Sat. At this point think that there won't be a whole lot of 
precip that is realized other than across portions of the south 
coast, but particularly the Cape/Islands. Ensembles show moderate to 
high probs (30 to 80 percent) of 24 hr precip AOA 0.1 inches across 
the Cape/Islands and even some low probs (10 to 20 percent) of AOA 
0.5 inches. It is in this area where there is the best shot for a 
wetting rain (totals AOA 0.25 inches), whereas totals elsewhere 
generally will total less than 0.1 inches. This makes sense given 
the best PWAT moisture plume of up to 1 inch impinges on the 
Cape/Islands.  This PWAT plume advects offshore late Sat AM/early 
afternoon. There is potential for the secondary cold front to bring 
the interior some spotty showers, but moisture appears quite limited 
as we dry out in the low/mid levels (1000-500 hPa layer) in wake of 
the first front. Given this have dialed back our chances during the 
afternoon given the limited moisture.

Sunday...

Cyclonic flow still in place through this timeframe with a 
deamplifying shortwave lifting through. A mid level ridge will build 
from the Mississippi River Valley into the Great Lakes region. High 
pressure nudges into our region from the Mid Mississippi River 
Valley. 

Dry and quiet weather anticipated through this timeframe. Could be 
another day where there may be elevated fire weather concerns. 
Appears that will have a similar set up to Sunday with a very well 
mixed boundary layer per GFS BUFKIT soundings. At this point think 
we could mix up to 750 hPa with once again W to WNW flow. Will be a 
bit cooler as 850 hPa temps fall to 0 to -3 degrees Celsius. Despite 
this did increase our temps to the 75th percentile of guidance given 
the well mixed boundary layer and W to WNW flow. Highs top out in 
the upper 50s to the low to perhaps mid 60s. Have also lowered dew 
points and relative humidities to the 10th percentile of guidance. 
At this point have min RH values of 25 to 45 percent with the driest 
readings across the interior. Could be another day of elevated fire 
weather concerns. 

Monday through Wednesday...

Cyclonic flow remains persistent through this timeframe. A mid level 
ridge builds from the Great Lakes/OH Valley region early on Mon into 
the Mid Atlantic/offshore by late Mon. Aloft a trough/shortwave 
slides well to our S Mon Night into Tue. A trough digs from the 
Great Lakes region early on Tue into the eastern Great Lakes/New 
England by late Tue or Wed. High pressure nudges in from the 
Mississippi River and TN Valley on Mon and Tue. Could see a system 
slide in late on Tue or Wed, but still uncertain on the timing and 
intensity at this point.

Dry and quiet weather on Mon and Tue with mild temps and lighter 
winds given the pressure gradient relaxes a bit. High temps getting 
into the low to mid 60s. Could have some showers spreading in late 
on Tue into Wed or just Wed, but there are significant differences 
at this point amongst guidance. Due to this have just stuck with the 
NBM. Temps near to perhaps cooler than seasonable levels by Wed.

&&

.AVIATION /11Z THURSDAY THROUGH MONDAY/...

Forecast Confidence Levels:

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

Today...High confidence.

VFR conditions prevail east of the CT River Valley with an
abundance of mid level clouds. Will have marginal MVFR
conditions near the CT River Valley along with scattered
showers. Could see the shower activity shift further east as 
the day progresses, but anticipate most across eastern MA to
remain dry. ENE winds 5 to 15 knots with gusts of 20-25 knots 
developing near the south coast and Islands.

Tonight and Friday...High confidence.

VFR conditions with pesky mid level clouds lingering. NE winds 
5 to 15 knots tonight becoming SE on Fri.

KBOS TAF...High confidence in TAF. 

Low prob of a spot shower late this afternoon, generally after
21Z. Per collab with ZBW have added a TEMPO group for this risk. 

KBDL TAF...High confidence in TAF. 

Marginal MVFR ceilings this morning with rain showers as we head
into the afternoon. Should see ceilings lift to VFR roughly
18-21Z and remain for the rest of the TAF.

Outlook /Friday Night through Monday/... 

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

Saturday: VFR. Breezy. Slight chance SHRA.

Saturday Night through Sunday Night: VFR. Breezy. 

Monday: VFR. 

&&

.MARINE...

Forecaster Confidence Levels:

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

Today...High Confidence. 

A wave of low pressure dropping southeast across the mid Atlantic 
coast will combined with high pressure across eastern Quebec. This 
will be enough to generate ENE wind gusts of 25 knots across many of 
our southern waters...where we have hoisted small craft headlines. 

Tonight and Friday...High Confidence. 

The pressure gradient weakens as the wave of low pressure continues 
to move further southeast and away from the region. Seas will also 
diminish...but expect some 5 foot seas to linger across our southern 
outer-waters so will continue small craft headlines in those 
locations. By Friday...winds and seas should have dropped below 
small craft thresholds across all waters...but still expect easterly 
wind gusts to around 20 knots. 

Outlook /Friday Night through Monday/...  

Friday Night: Winds less than 25 kt. Areas of seas approaching
5 ft. Chance of rain showers. 

Saturday: Winds less than 25 kt. Chance of rain showers. 

Saturday Night: Winds less than 25 kt. Seas locally approaching
5 ft. 

Sunday through Monday: Winds less than 25 kt. 

&&

.BOX WATCHES/WARNINGS/ADVISORIES...
CT...None.
MA...None.
RI...None.
MARINE...Small Craft Advisory until 5 PM EDT this afternoon for 
     ANZ232>235-237.
     Small Craft Advisory until 5 AM EDT Friday for ANZ255-256.

&&

$$

SYNOPSIS...Frank/BL
NEAR TERM...Frank/BL
SHORT TERM...Frank
LONG TERM...BL
AVIATION...Frank/BL
MARINE...Frank/BL